1 '.VOLUTION 
THE WAY OF MAN 
VERNON KELLOGG EVOLUTION EVOLUTION 
BY 
VERNON KELLOGG 
AUTHOR OF "DARWINISM TO-DAY," "EVOLUTION 
AND ANIMAL LIFE" (WITH D. S. JORDAN), 
"INSECT STORIES," ETC, 
D. APPLETON AND COMPANY 
NEW YORK :: LONDON :: MCMXXIV COPYRIGHT, 1924, BY 
D. APPLETON AND COMPANY 
FEINTED IN THE UNITED STATES OF AMERICA TO 
MY WIFE 
CHARLOTTE KELLOGG 
WHO HAS DONE MUCH TO 
MAKE THIS BOOK READABLE PREFATORY NOTE 
I have made this book untechnical, and tried to 
make it truthful. I have written much of it out- 
of-doors, and have drawn my examples as much as 
possible from familiar instead of foreign plants 
and animals. Also I have tried not to forget that 
we are all more interested in the evolution of man 
than we are in that of any or all other creatures. 
I have tried to make this book human and per- 
sonal. Too much of the evolution written about 
has been too far away from most of us. There is 
plenty of evolution all around and in all of us. 
V. K. CONTENTS 
CHAPTER 
I. 
Introduction : 
Evolution: What Is It? ., ., 
PAGE 
1 
II. 
Growth and Triumph of the Evolution 
Idea 
11 
III. 
What Evolution Must Explain . 
24 
IV. 
Evidences of Evolution : 
Comparative Anatomy and Embry- 
ology 
47 
V. 
Evidences of Evolution: 
Paleontology and Geographical Dis- 
tribution 
65 
VI. 
Causal Explanations of Evolution . 
94 
VII. 
Fundamental Factors in Evolution: 
Reproduction and Development, Vari- 
ation, Heredity 
109 
VIII. 
Fundamental Factors in Evolution : 
Selection, Segregation, Response to 
Environment ........ 
132 
IX. 
The Evolution of the Plants 
147 
ix X 
CONTENTS 
CHAPTER 
X. 
The Evolution of the Animals: 
The Invertebrates 
PAGE 
170 
XI. 
The Evolution of the Animals: 
The Vertebrates 
195 
XII. 
The Evolution of Man . . . . 
217 
XIII. 
The Evolution of Mind ....... 
238 
XIV. 
Social Inheritance and Societal Evolu- 
tion 
257 
XV. 
The Human Future 
269 EVOLUTION EVOLUTION 
THE WAY OF LIFE 
CHAPTER I 
EVOLUTION; WHAT IS IT? 
Evolution is again an exciting word. It has 
been at other times; most notably in the years just 
after the publication of Darwin's Origin of Species. 
One hundred years before that, too, when Lamarck 
and St. Hilaire were being attacked by Cuvier and 
the clergy for their transformation ideas, it, or its 
French equivalent, was a word to stir men to wrath. 
And now again it creates excitement. Its utter- 
ance is the stimulus to much discussion: keenly 
scientific discussion, absurdly uninformed discussion, 
bitter discussion, trivial discussion; discussion of a 
single problem, as the origin of man; discussion of 
all things of earth and heaven, from the color pat- 
tern of a butterfly to the whence and whither of 
man's soul. 
Evolution is defined in a score of ways, but not 
1 EVOLUTION 
2 
clearly in any way. Each one defines it for himself, 
and no two define it alike. It is used in the titles of 
hundreds of books, and each book covers what it 
will. We need a general treaty of understanding. 
How can there be evolutionists and antievolutionists 
when there is no agreement between them as to what 
is meant by evolution? When each one means what 
he pleases to mean, with little or no regard to what 
his colleague or antagonist means. At least, every 
one who talks or writes about evolution should try to 
explain, at the very beginning, what he proposes to 
talk or write about. Let me try to do that for this 
book in these very first pages of it. 
I am seated, as I write, in a pine wood near the 
ocean side. There are insects and birds and a few 
squirrels in the trees, and in the tide pools of the 
shore a rich variety of beautiful and strange salt- 
water creatures. Each beetle and woodpecker, each 
starfish and sea anemone, has a body form and char- 
acteristic appearance of its own, a way of breathing 
and feeding, of protecting itself, of producing and 
caring for its young, all different in detail from the 
forms and ways of the others, but all fundamentally 
similar in purpose and achievement. The same is to 
be said of the trees and the bushes and the grasses 
and the tender little flowers of this hillside, and of 
the giant kelp and the red-and-green seaweeds of the 
near-by ocean shore. All these plants and animals EVOLUTION; WHAT IS IT? 
3 
are alive and trying, even if unconsciously, to keep 
alive, at least until the young or the seeds or the 
spores shall be produced which are to assure the 
continuity of the life chain. 
And among the birds and butterflies and the pine 
trees and the lupines, watching the gulls and the sea- 
lions and catching the glint of the washing seaweeds 
as the tide runs out, am I; I, also alive and of a 
given form and habit of body and body parts, and 
unconsciously breathing and circulating my life-pre- 
serving blood, and consciously careful to feed myself, 
and to protect myself from beasts of prey or mur- 
dering bandits, and also taking care, the very best 
care I can take, of a little girl who shall, if all goes 
well, be a link between us and our grandchildren; 
that is, "assure," as with the sea anemones and the 
blue lupines, "the continuity of the life chain." 
Now evolution means to me, first, something of 
an explanation of why and how there are so many 
kinds of living creatures with all their varied forms 
and manners, yet all striving for similar ends and 
with much commonness of method. It means some- 
thing of an explanation of the likenesses and the 
differences and the relationships among these animals 
and plants. And, finally-and I say this without 
misgivings and without shame-it means to me some- 
thing of an explanation of the likenesses and differ- 4 
EVOLUTION 
ences and the relationships between myself and all 
these other living things. 
But evolution means to me only part and not all 
of an explanation of these things. It is no ulti- 
mate explanation of any of these things; that is, of 
life itself and the final cause of the variety and yet 
identity of all life, including my own life. Evolu- 
tion can be only a more or less immediate or detailed 
explanation of how, granted life, granted matter, 
granted energy, granted any existence of anything 
at all, and granted an ultimate cause or causes, the 
form and behavior of living things can be and are as 
they are. It is an explanation of process, not primi- 
tive cause. It explains much that I seek explana- 
tion for as I study the amazing variety and the 
astonishing fitness to their surroundings of the host 
of living creatures, including myself and my kind of 
creature, on this earth. But it does not explain to 
me, in any ultimate way, the fact that there are liv- 
ing creatures or an earth. Or that I have a con- 
sciousness of myself and of my relations to the other 
living things and the earth. Or that I have visions 
and aspirations of a kind not referred to in the 
books of biology, and recognize that other human 
beings have them, and feel that the most important 
things we live for are things of which I get no 
glimpse in studying the sea anemones or the pine 
trees. EVOLUTION; WHAT IS IT? 
5 
But evolution, even thus limited, means so much, 
and so much more than I can ever make apparent 
by words; it means so much about nature and so 
much about man and so much in connection with 
our eager, ceaseless attempt to know the how and 
why of all life, that when I deliberately set myself 
to try to explain what it is to me and how much 
it means to me, I am like Christopher Morley's 
impotent Gissing, who "felt like a clumsy strummer 
seated at a dark, shining grand piano, which he 
knows is capable of every glory of rolling music, 
yet from which he can only elicit a few haphazard 
chords." 
Evolution means outrolling, unfolding. It means 
a reasonable, satisfying, ennobling conception of life, 
a conception that gives life infinite promise. Or- 
ganic evolution is the outrolling of the plan of life, 
the unfolding of the possibilities of life. It runs 
naturally and logically from simple to complex, from 
the general to the special, from the lowly to the 
high, from amoeba-and simpler-to man. Will it, 
some time, be to something higher? That would 
be hard for us to admit. And we need not admit 
it, for we simply know nothing about it. All that we 
know is what has been and what is. The future 
suggests itself, but rarely really reveals itself in 
advance of its time. For the present we need only 
consider man, and our type of man, as the highest 6 
EVOLUTION 
round in evolution's ladder, the apex of evolutionary 
achievement. But, for myself, I see nothing impos- 
sible in a higher man. Nature seems so infinitely 
fecund, evolution so unlimitedly possible, and time 
so interminable and hence so generous to evolu- 
tion. 
Evolution means continuity, means transmutation, 
the origin of the new from the old; means change, 
continuous movement, gradatory development. It 
means genetic relationship, blood cousinhood, an all- 
embracing genealogy of life. It means the funda- 
mental unity of all life, however varied the appear- 
ance and manner of it in different living kinds and 
individuals. It means a continuous living stream 
varying in appearance in its different parts, but 
never really broken or with its parts really sep- 
arated. Every living creature, be it monstrous whale 
or microscopic phosphorescent animalcule in the 
ocean, free-roaming tiger in the jungle or minute 
parasite that crawls about over the tiger's skin, 
wheeling eagle surveying its broad domain of air 
and land over a life span of many years, or swarm 
of fluttering May flies dancing an evening's life 
away about an electric light by the lake shore, giant 
Sequoia holding its proud place in a Sierran forest 
through thirty centuries, or tenderest bit of transi- 
tory moss that nestles at its base-every living crea- 
ture, large or small, long-lived or ephemeral, active EVOLUTION; WHAT IS IT? 
7 
or quiescent, myriad-celled or single-celled, is, in 
certain fundamental structure and behavior, like 
every other living creature. It has need, to remain 
alive, of certain physical and chemical surroundings 
which every other living creature has to have, and 
it does certain things which every other creature 
does. As varied as life seems to be it has its rig- 
orous limitations. But these limitations not trans- 
gressed, their requirements met, it can play its vari- 
ations, it can embroider and adorn itself almost 
endlessly. It can change itself from one form into 
another. It can throw off one branch after another. 
But it is all of one piece. 
The study of evolution is especially the study of 
the significance of this variety of life, arising out 
of identity, and of how it comes about. This variety 
has its major expression in the different great 
branches of plants and animals, then in the differing 
classes within each of these branches, then, grad- 
ually lessening in degree, in the orders and families 
and genera and differing species or kinds, and, finally, 
has its least expression in the differences among the 
individuals belonging to a single species. There are 
no two animal or plant individuals exactly alike in 
the world; not even offspring of the same parents, 
not twins, not even "identical twins." The extent 
and the limitation of this variety are determined by EVOLUTION 
8 
degrees of blood relationship and by relation to 
environment. These two influences are what de- 
termine the form and manner of each individual, 
its size and shape and color and habit and life 
span. 
Evolution is the reasonable explanation of this 
abundance of kinds of animals and plants and of the 
amazing adaptations of these many kinds to their 
environment. We have found and described and 
classified and named about 500,000 living kinds of 
animals and 250,000 living kinds of plants. There 
are certainly many more kinds of both animals and 
plants still to discover and catalogue. Not one of 
these but presents its own problem of adaptation- 
adaptation in form of body, of legs, of wings, of 
sense organs, of roots and stems and leaves and 
flowers and seeds, in habits of food-getting, of 
escaping or conquering enemies, of home-building, 
of production and care of young, of manner of 
growth, of fertilization of flowers, of distribution 
of seeds. 
Much of the beauty and glory of nature is its 
variety. And much of the interest of nature springs 
from the significance of this variety and from the 
significance of the fundamental likenesses underlying 
this variety. How? Why? Those are the ques- 
tions of the naturalist. Evolution is the most rea- EVOLUTION; WHAT IS IT? 
9 
sonable answer. And as we ourselves are in and of 
nature, evolution is the answer to many of our 
questions about ourselves. Why do we have so many 
things in our bodies that are much like correspond- 
ing things in the bodies of other animals? Why do 
we pass through-in the development of each of us 
from fertilized egg cell to mature individual-so 
many stages that are like stages passed through in 
the development of other vertebrates? Why do the 
human fossils of early glacial time show that man 
of that time had a smaller brain, heavier jaws, and 
a skeleton that indicates a less erect posture than 
has man of to-day? Why are there living races of 
men of much more primitive make-up than others 
also living to-day? Evolution is the reasonable 
answer to these questions. 
But, again, I do not claim that evolution is the 
answer to all the questions we can ask about our- 
selves. Even were there none about consciousness 
and charity and imagination and soul, there are 
always those about the primal origin of life, of which 
we are a part, and about the ultimate whence of it. 
But in our attempt to understand ourselves we can 
look too much at ourselves alone; we can too easily 
forget that we are but part and parcel of nature. 
About any part of nature every other part teaches 
something. Hence, if we would understand as much 
as we can about our own evolution, we must under- 10 
EVOLUTION 
stand as much as we can about the evolution of all 
of nature. 
And hark! how blithe the throstle sings! 
He, too, is no mean preacher: 
Come forth into the light of things, 
Let Nature be your teacher. 
• ••••• 
One impulse from a vernal wood 
May teach you more of man, 
Of moral evil and of good, 
Than all the sages can. CHAPTER II 
GROWTH AND TRIUMPH OF THE EVOLU- 
TION IDEA 
The conception and formulation of the evolution 
idea, as we know it to-day, were not the achievement 
of any one man; not even of Darwin. And if not 
of Darwin, then certainly of no other. The idea 
of evolution and its expression have been the result 
of many men's thinking through many years. It is 
the old story of the slow progress of human under- 
standing. 
As usual in tracing the history of a great philo- 
sophic conception, we go back at least to the Greeks. 
Empedocles has been called the "Father of Evo- 
lution." So has Aristotle. Others might be. If one 
searches the writings of the Greek philosophers and 
naturalists, as Henry Fairfield Osborn has done to 
write his From the Greeks to Darwin, sentences here 
and there reveal ideas, hypotheses, guesses, about 
the origin and relations of the kinds of creatures 
that can easily be construed to mean, and, perhaps, 
really mean, that this one and that one had glimpses 
of the Great Answer. Empedocles thought organ- 
isms were created in separate parts of a great 
11 EVOLUTION 
12 
variety which were subject to the attracting and 
repelling forces of love and hate-the two great 
forces in nature-and if the right parts managed 
to get together a viable creature was produced. If 
the wrong parts got joined, the creature could not 
persist. One can call this, if he wishes to, and as 
some have called it, an early expression of natural 
selection. Empedocles believed in spontaneous gen- 
eration, and assumed that nature did not produce 
lower and higher forms simultaneously, but that 
plants came first and animal life only after a long 
series of trials. 
Aristotle, a hundred years later, opposed Em- 
pedocles' ideas of fortuitous origin of characters, 
favoring the idea of intelligent design in the origin 
of organisms. He thought the variety of life origi- 
nated from a primordial soft mass of living matter. 
He had an idea of an ascending series, presumably 
a genetic series, beginning with plants, then "plant 
animals," such as sponges and sea anemones, then 
animals with locomotion and sensibility, and thence, 
by graded stages, up to man. He recognized the 
influence of heredity and at the same time believed 
in the modifying influence of environment, and in 
the inheritance of acquired characters. 
After Aristotle, who was assuredly the outstand- 
ing naturalist and natural philosopher of his day 
and, indeed, of the whole stretch of time from the GROWTH OF EVOLUTION IDEA 
13 
beginnings of recorded human history up to the re- 
vival of science in the sixteenth century, the evolution 
idea languished. This stretch of time included that 
long period dominated completely by theology and 
appropriately, at least from the point of view of 
science, called the Dark Ages. But the evolution 
idea was not, however, entirely obliterated from 
men's minds. It simply could not be. There are 
always, even in the darkest of dark ages, inquiring 
minds, minds that do not accept, without question- 
ing, the asseverations and dogmas of the pundits 
whether in science, philosophy, politics or religion. 
And there must have been men through those cen- 
turies between the coming of Christ and the so- 
called revival of learning in the Middle Ages who 
asked questions about nature and about the teeming 
life all about them. They must have asked ques- 
tions of themselves and of others. 
But it was not an auspicious time to ask these 
questions too loudly, much less to utter too boldly 
as answers any guesses or ideas that might irritate 
the dominating theologians. A few records exist, 
however, which indicate that some of the more open- 
minded theologians of sufficiently high place to af- 
ford to speak up were not able to forego utterance 
of their feeling that something besides the dogma 
of specific creation according to biblical legend was 
necessary to explain the abundant and various forms 14 
EVOLUTION 
of matter and living creatures. Gregory and Augus- 
tine, in the fourth century after Christ, both at- 
tempted certain forms of explanation of the variety 
of life, which have, in their essentials, ever since 
figured largely in attempts at reconciling the declara- 
tions of Genesis with the perceived facts of science. 
Gregory taught that creation was potential. God 
imparted to matter its fundamental properties and 
laws. The objects and completed forms of the uni- 
verse developed gradually out of chaotic material. 
And Augustine ventured the idea that the biblical 
account of creation is allegorical. He expounded 
the declaration, "In the beginning God created 
heaven and earth," to mean that "in the beginning 
God made the heaven and earth, as if this were the 
seed of the heaven and the earth, although as yet 
all the matter of heaven and earth was in con- 
fusion, but, because it was certain that from this 
material the heaven and earth would be, therefore, 
the material itself is called by that name." And in 
the thirteenth century Thomas Aquinas expounded 
Augustine's ideas without attempting any refutation 
of them, which was equivalent to admitting a kindly 
interest in them. 
This was not getting very far in a thousand years. 
It shows what authority in the hands of men dom- 
inated by one idea, even in the realm of the intellect, 
can do. But such a state of things cannot last for- GROWTH OF EVOLUTION IDEA 
15 
ever. The revolt is sure to come. And it is likely 
to come more violently the longer and more nearly 
completely it has been withstood. It did not come, 
in this case of the struggle of science against theol- 
ogy, until the sixteenth century when, with Francis 
Bacon as chief protagonist, science made itself loudly 
heard and demanded recognition. Bacon picked up 
the evolution conception where Aristotle had left 
it, and expanded it and made it more specific. He 
proclaimed the mutability of species and explained 
this mutability as the result of the accumulation 
of variations. He declared that variations of suf- 
ficiently pronounced character to produce new species 
could and did sometimes occur, and that old species 
might change retrogressively or degeneratively to 
such an extent as to be transformed into new species. 
He stirred men to a new examination of animal 
and plant kinds and to their behavior and relation- 
ships as species. By the eighteenth century enough 
had been learned about nature to warrant naturalists 
and natural philosophers like Buffon and Bonnet and 
Erasmus Darwin and Oken, and finally Lamarck 
and Geoffrey St. Hilaire, to formulate and clearly 
announce the concept of organic evolution as an 
explanation of species forming and adaptation. This 
explanation declares that new species come from 
old by transmutation; and denounces, at least by 
implication, the theological dogma of special crea- 16 
EVOLUTION 
tion. Adaptation comes by plastic response to en- 
vironmental conditions; hence, is not specially 
planned. 
This challenge to theology was promptly accepted. 
And the response and attack came not alone from 
the theologians but from naturalists as well. All of 
the great influence of Cuvier, dean of French anato- 
mists and scientific favorite of the French court, was 
thrown against his colleagues Lamarck and St. 
Hilaire and their special theory of evolution, as well 
as against evolution in general. The great Swedish 
botanist and father of biological taxonomy, Lin- 
naeus, clung to the dogma of the special creation 
of species, and was a formidable obstacle to the 
acceptance of the evolution idea. 
But greater than the influence of the reactionary 
biologists in preventing any popular acceptance of 
the evolution idea was that of the theologians. They 
denounced it as impious and heretic. They did not 
debate it; they simply banned it. And, for the time, 
they held it thoroughly in leash. However clearly 
formulated and clearly stated, however specifically 
applied and worked out in detail by the few rebelling 
and ardent naturalists and natural philosophers cry- 
ing in the wilderness in those days of the last half 
of the eighteenth and first half of the nineteenth 
centuries, the real entrance-and for all time-of 
the evolution idea into the general heritage of human GROWTH OF EVOLUTION IDEA 
17 
understanding did not come until the publication of 
Darwin's Origin of Species in 1859. 
From Empedocles and Aristotle, four and three 
hundred years before Christ, to Lamarck and Dar- 
win, eighteen hundred years after Christ, the evo- 
lution idea had been slowly growing, spreading, 
ripening, but with great resting stages and with the 
heavy restraining, sometimes violently repelling, 
hand of dogmatic theology always holding or thrust- 
ing it back. With the Origin of Species it burst, 
apparently suddenly, into full bloom. Darwin made 
it real and vivid to the world. He set it forth so 
clearly, he brought to bear on it such a wealth of 
detailed observation, he gave it such a reasonable 
and plausible basis of causo-mechanical explanation, 
and he found at once such valiant champions to 
make the necessary fight for it, that it is not sur- 
prising that the triumph of evolution has come to 
be generally spoken of as the triumph of "Dar- 
winism." The triumph of evolution is at least the 
triumph of Darwin. 
But the triumph, speedy and brilliant as it was, 
after the publication of the Origin of Species, did 
not come without a sharp struggle, and there have 
been, even as there are at this very present moment, 
recrudescences of that struggle. But one element of 
opposition is entirely gone. That is the element of 
opposition from scientific men, from biologists, 18 
EVOLUTION 
geologists, naturalists in the widest sense. While in 
Lamarck's day the evolution idea was vigorously 
fought by the greatest anatomist and the greatest 
biological systematist of the time, and by other lesser 
but reputable and influential biologists, and even in 
Darwin's day had its notable and active antagonists 
within the ranks of the scientific men, to-day there 
is practically no naturalist of known achievement 
who does not accept organic evolution as a proved 
natural phenomenon. Almost all of the still exist- 
ing denial of evolution, and especially of the active 
opposition to it, comes from theologians and mystics, 
lay or professional, and from a number of impres- 
sionable individuals influenced by them. 
Around Darwin, in England, gathered a notable 
band of coadjutors and champions. In foreign 
countries other men lifted his banner. Evolution, 
the struggle for existence, and natural selection be- 
came the debated subjects of the day in scientific 
gatherings and churches, in books, magazines and 
newspapers. Dispute was loud and sharp. An 
Anglican bishop publicly taunted Huxley by asking 
him if it were through his grandmother or his grand- 
father that he claimed his descent from a monkey, 
and was answered by the quick-tongued champion 
of evolution to the effect that he would be more 
ashamed of having to recall as an ancestor a man GROWTH OF EVOLUTION IDEA 
19 
who could behave as the bishop did than of having 
an ape for an ancestor. 
Huxley was, indeed, the fighting Darwinian. Ad- 
mitting that polemics are always more or less an 
evil, he believed, nevertheless, that the lukewarm- 
ness which lets error and corruption have their un- 
disputed baneful way is a greater evil. To Huxley 
the questions at issue between the dogmatic asser- 
tions of clericalism and theology on the one side and 
the facts of nature which revealed and proved evo- 
lution on the other, admitted no indifference or com- 
promise. In a letter to his wife, written at Baden 
in 1873, Huxley says: 
"We are in the midst of a gigantic movement 
greater than that which preceded and produced the 
Reformation, and really only the continuation of 
that movement. But there is nothing new in the 
ideas which lie at the bottom of the movement, nor 
is any reconcilement possible between free thought 
and traditional authority." 
It was well that there were Huxley, Herbert 
Spencer, and Haeckel to fight for evolution. For 
Darwin was not a fighter, and he was in no hurry. 
He believed he saw the truth, and if it was the truth 
it would prevail. The generous-minded Alfred Rus- 
sel Wallace, whose name should always be associ- 20 
EVOLUTION 
ated with that of Darwin as coauthor of the theory 
of natural selection-for the setting out of the 
major lines of this theory occurred by the simul- 
taneous presentation and publication of brief papers 
by both of these men-once wrote : "I was then ( and 
often since) the 'young man in a hurry,' he [Dar- 
win] the painstaking student, seeking ever the full 
demonstration of the truth he had discovered, rather 
than to achieve immediate personal fame." 
Lamarck, a hundred years earlier, had developed 
and presented to the world a detailed account of 
the evolution idea, together with an explanatory 
theory of natural causation of the transmutation of 
species. But he had no such militant champions as 
Darwin's to sponsor his cause, and the world hardly 
even heard of his claims, let alone being won by them. 
True, the time was not auspicious. Other great 
matters claimed the world's attention then. And 
there was more of speculation and less of observed 
fact in Lamarck's account of evolution than in Dar- 
win's. Lamarck also had injured his personal pres- 
tige by some earlier wild speculations in the field 
of chemistry. But if he had had a Huxley by his 
side the history of the triumph of the evolution idea 
might have been a different one. 
One must always distinguish in Darwin's con- 
tributions to the evolution idea between his over- 
whelming accumulation of evidence for the general GROWTH OF EVOLUTION IDEA 
21 
phenomenon of evolution and the consequent wide 
acceptance of evolution as a fact, and his formu- 
lation and presentation of a causal explanation of 
evolution, namely, the theory of natural selection 
(together with the auxiliary theory of sexual selec- 
tion). When biologists speak or write to-day of 
Darwinism, they are referring -specifically to Dar- 
win's selection theories as explanations or causal 
factors of evolution. In popular writing and speak- 
ing Darwinism is often, perhaps usually, used as 
synonymous with evolution itself. Sometimes it is 
meant to denote simply the origin of man from 
apes, or from the lower animals in general. Sim- 
ilarly "Lamarckism," in the mouths of biologists, 
means the causal explanations of evolution advanced 
by Lamarck, namely, the modifications of individ- 
uals by use and disuse of organs, and by the in- 
fluence of environment, and the direct inheritance of 
these modifications, or "acquired characters," so that 
the species also becomes modified. 
I shall try, in a later chapter, to explain, in some 
detail, these contributions of Darwin and Lamarck, 
as well as those of other naturalists, to the problem 
of the cause or causes, of evolution. But all that 
we need to note just now is the fact that the evi- 
dences of the reality of evolution adduced by Dar- 
win and made so available to common knowledge 
by him and by his eminent champions, most notably 22 
EVOLUTION 
Huxley, were so overwhelming, so irrefutable and 
hence so convincing, that, without regard to the 
satisfactoriness or unsatisfactoriness of the sug- 
gested causes for this evolution, the fact of evolu- 
tion was established for all time. It was established 
by the quiet, modest master whose name will ever 
stand at the head of the roll of the world's greatest 
naturalists. 
One cannot help, in closing this brief account 
of the origin, struggle, rise and final triumph of the 
evolution idea, from uttering a word of comment 
on the example it furnishes of a usual story in the 
development of human understanding and the win- 
ning of the wTay to truth. It almost seems, as we 
survey the history of the human conquering of 
knowledge, as if mere ignorance were the least of 
the obstacles that have to be overcome. Human 
nature seems to indulge in a perverse and malicious 
pleasure in setting up unnecessary difficulties in our 
way to the light. We put up barriers of inertia, we 
encourage active antagonisms to advance, we tol- 
erate and bow to traditional authority, we brutalize 
the spirits of independence and cry out against the 
man of vision, in such perverseness as to magnify 
inconceivably the difficulties, serious enough in any 
case, of wresting truth from nature. And this, de- 
spite the fact that, after all, we mostly recognize 
this wresting of truth from nature to be the great- GROWTH OF EVOLUTION IDEA 
23 
est and most useful task to which we can set our 
hands and minds. 
Before Darwin and Huxley could make the final 
winning in the struggle for the evolution idea, La- 
marck and many others had to be sacrificed. If the 
naturalists and natural philosophers could have 
worked in the light of evolution since Lamarck's 
time instead of since Darwin's time, one hundred 
years later, how wonderfully much farther would 
our beneficent knowledge of nature stand to-day. 
Lamarck saw evolution almost, if not, indeed, quite, 
as clearly as Darwin did. But the world would 
not see it until it had exhausted its brutal pleasure 
of martyrizing the too forward minds. The story 
of the struggle and triumph of the evolution idea 
is but an example of the usual story of any great 
advance in human understanding. But each example 
saddens one. CHAPTER III 
WHAT EVOLUTION MUST EXPLAIN 
One does not have to make a long voyage on the 
ship Beagle, or to penetrate tropical forests, or to 
live in a laboratory filled with microscopes to learn 
of evolution. Darwin saw artificial selection going 
on in the barnyard; Mendel did his epoch-making 
work on heredity, one of the chief factors of evolu- 
tion, with peas in a cloister garden; variation, an- 
other major evolution factor, can be seen by closely 
comparing any two individuals of any plant or ani- 
mal kind; adaptation is apparent in the teeth and 
claws of the house cat; the struggle for existence and 
survival of the fittest is going on actively in any neg- 
lected corner of your garden. Matters that evolu- 
tion must explain, the evidences of evolution and the 
results of evolution are all obvious anywhere where 
plants and animals are. 
The wood in which the little green cottage, where 
I am writing, nestles so inconspicuously-because of 
its protective coloration, to use the naturalist's 
phrase-is composed mostly of Monterey pines, in- 
teresting Californian near relatives of the wide- 
spread familiar yellow pine, itself abundant in the 
24 WHAT EVOLUTION MUST EXPLAIN 
state. The Monterey pine has an extremely re- 
stricted natural distribution, being found only in a 
very narrow north-and-south stretch along the 
middle California coast, a region embracing alto- 
gether not more than a thousand square miles. Yet 
this tree when artificially distributed grows readily 
in many places outside of its natural range. 
Another strictly Californian conifer, the Mon- 
terey cypress, of which I can see specimens from my 
cottage, has an even more restricted natural range, 
existing, at present, to the number of only a few 
hundred, or, at most, a thousand or so weather- 
beaten individuals on two exposed rocky points pro- 
jecting into the Pacific Ocean near Monterey. Yet 
this tree is one of the most abundant and flourishing 
of planted ornamental trees in the whole West Coast 
region and is familiar in European gardens. I 
stumble here, at once, then, even as I merely glance 
out of my cottage windows, on one of the most inter- 
esting of evolutionary problems, that of the geo- 
graphic and topographic distribution of plant and 
animal kinds. 
To face another problem-in these open, sun- 
lighted woods there are many birds of numerous 
kinds: hawks, owls, jays, flickers and woodpeckers; 
titmice and chickadees; flycatchers, sparrows, vireos, 
russetback thrushes, warblers and humming birds; 
each with its characteristic size and shape and color 
25 26 
EVOLUTION 
pattern. On the edges of the wood are quail and 
meadow larks and along the seashore pelicans, 
grebes, cormorants, gulls and terns, all differing from 
the woods birds and all differing among themselves. 
I hear all day their various cries and songs, from the 
scream of the hunting hawks and the harsh loud 
calls of jays and woodpeckers to the liquid trilling 
of the thrushes and the staccato cheeping of the 
sparrows. I see them busy with food-getting, each, 
in its own way, seeking for its special dishes. Their 
nests, on ground or in bushes or trees, are, for each, 
of a particular kind. I can recognize each species 
by its particular mode of flight, or, at least, I can 
say-"that is a woodpecker, that a flycatcher, that 
a humming bird." 
There are these differences, but also there are like- 
nesses among them. The various woodpeckers not 
only have a common manner of flight, but also are 
similar in food habits, in nesting, have similar cries, 
and cling to tree trunk or branch in a characteristi- 
cally similar way. If we examine their bodies, we 
note a commonness of general form and special parts, 
of bill, tongue, wings, tail, feet. Even if we don't 
know the different species of woodpeckers, we can 
know any of them as a woodpecker; just as we can 
know the various flycatchers as a group, or the 
hawks, or the gulls. And finally we can know any 
bird as a bird, not to be confused with the toads and WHAT EVOLUTION MUST EXPLAIN 
27 
frogs, the lizards or snakes, or rabbits or gophers 
or foxes that are also all here in the woods; or the 
fishes that are in the streams and ocean near by. 
But all these groups of animals that I have men- 
tioned have something in common, namely, a back- 
bone and certain other correlated parts, which gives 
them a certain likeness in make-up and readily dis- 
tinguishes them from the earthworms and insects 
and spiders of the land and from the sponges, sea 
anemones, and starfishes of the ocean tide pools. 
There are two great groups of animals, the verte- 
brates and invertebrates, and within them there are 
distinct major branches and then within each branch 
a number of classes, in each class a number of orders, 
in each order a number of families, in each family a 
number of genera, and in each genus one or more 
species or kinds. 
There is something recognizably common in all 
the species of a genus, all the genera of a family, all 
the families of an order, and so on up the scale of 
classification. Although every kind of animal differs 
from every other kind-each individual differs even 
from every other individual of the same species-yet 
there are likenesses which group them together- 
obvious likenesses among the members of the more 
immediate groups, less obvious but more fundamen- 
tal likenesses among the superficially very different- 
appearing members of the major groups. These 28 
EVOLUTION 
likenesses are facts that evolution must explain, that 
evolution does, in fact, explain. At the same time 
they are proofs of evolution. But we shall come to 
this in a later chapter. I want to consider further, 
just now, the differences, especially the adaptive 
differences, among the birds around my cottage, 
which also are facts that evolution must explain. 
All the birds belong to a single class of a single 
great animal branch, the vertebrates. They are all 
distinguished by certain common structural and 
physiological characters which make them differ 
from the fishes, the amphibians, the reptiles and the 
mammals, which are the other great vertebrate 
classes. About 10,000 living kinds of birds are 
known in the world, of which about 1,000 occur in 
the United States. Almost 500 of these can be 
found here in California. California has within its 
political boundaries such a great geographical, espe- 
cially north and south, extent, and such a variety of 
topography, including plains, valleys, mountains and 
desert, streams, lakes and ocean shore, and is so 
favorably situated with regard to the great coast-line 
migratory routes, that there occur within its bor- 
ders an unusually large number of bird kinds, includ- 
ing all-year residents, summer residents, winter resi- 
dents, occasional visitants and regular migrants of 
spring and autumn. Kansas, a typical inland plains 
state, with little diversity of topography, but in the WHAT EVOLUTION MUST EXPLAIN 
29 
line of the great Mississippi Valley migratory route, 
has 350 kinds, of which nearly one third are mi- 
grants. Only fifty species are permanent, or all-year, 
residents. Variety of environment seems to mean 
variety of life. 
It is not difficult to learn to know all, or most, of 
the birds of a given region; for that matter to know 
all the birds of the whole of such a well-explored and 
readily accessible land as ours. There is no limit, 
except the natural one of the period of life, to the 
years that one might devote to studying birds, their 
structure and habits, their likenesses and differences, 
and their relations and adaptations to their envi- 
ronment, especially, in working out the significance 
or meaning of all these observed facts. That would 
be to study the evolution of the birds, which would 
be, in effect, to study evolution as a whole, using 
bird kinds and bird adaptations as a basis. 
I could put in a long time studying the birds which 
may be found about and within easy walking dis- 
tance of my cottage. But a short time will reveal 
much. Let us confine our attention to just a few 
things about these birds. Let us look first at their 
feet. These feet tell a story, a story of adaptation, 
a story of evolution. Note the foot of a sparrow, a 
warbler, or a thrush. It has three unwebbed toes in 
front and a long hind toe perfectly opposable to the 
middle front one. This is the perching foot. These 30 
EVOLUTION 
birds when not in flight or on the nest, perch on 
branches. Note the foot of a woodpecker. Two 
toes partly yoked together project in front and two 
similarly yoked project behind. The woodpeckers 
can perch, as thrushes do, but they can also cling 
firmly to the rough bark of tree trunk or large 
branches. Note the webbed swimming foot of the 
aquatic birds; note the different degrees of webbing, 
totipalmate where all four toes are completely 
webbed, palmate where the three front toes only are 
bound together but the web runs out to the claws, 
semipalmate, where the web runs out only about half- 
way. Note the unwebbed but lobate swimming foot 
of the coots and phalaropes where there are simply 
expanded separate weblike pads on each toe. Note 
the long, slender, wading legs of the sandpipers, 
snipe and other shore birds; the short, heavy, strong 
legs, set far back, of the divers; the small weak legs 
of the swifts and humming birds, almost always 
on the wing; the stout, heavily mailed foot of the 
scratchers, such as the hens, grouse, quail and tur- 
keys; and the long grasping talons, with their sharp, 
curving nails, of the hawks and owls-birds of prey. 
In all these cases the adaptation or fitness of the 
foot and leg to the special habits of the bird is 
apparent. 
Or we may examine the bill. Note the strong 
hooked and dentate bill of the birds of prey; they WHAT EVOLUTION MUST EXPLAIN 
31 
tear their victim to pieces. Note the long, slender, 
sensitive bill of the sandpipers; they probe the wet 
sand for worms. Note the short weak bill and wide 
mouth of the nighthawk and whippoorwill and of the 
swifts and swallows; they catch insects in this wide 
mouth while on the wing. Note the firm chisel-like 
bill of the woodpeckers; they drill into hard wood 
for insects. Note the long, sharp, slender bill of 
the humming birds; they get small insects from the 
bottom of flower cups. Note the peculiarly crossed 
mandibles of the crossbills; they tear open pine cones 
for seeds. Note the long, hook-end bill of the 
pelican with the large pouch on the under side; they 
scoop up fish from the water. One could go on tire- 
somely but always suggestively. 
These differences in the bills of birds are related 
intimately and advantageously to differing special 
ways of feeding; just as the differences in feet and 
legs have their plain relation to differing special uses. 
One might also thus catalogue the varying types of 
wings and show their relation to needs and habits 
of flight; the long, narrow, perfect wings of the 
great albatrosses which spend most of their time in 
the air and take only an occasional rest on the un- 
easy surface of the ocean; the broad, soaring and 
hovering wings of the eagles and larger hawks; the 
short but strong wings of the swiftly flying ducks 
that make long, thousand-mile flights of migration; EVOLUTION 
32 
the flipperlike wings of the penguins which use them 
for swimming under water. And then there invites 
us the fascinating study of the color and pattern of 
the plumage, with their obvious relation to protective 
coloration and camouflage; although the extraor- 
dinary display of the peacocks and male pheasants 
and others, as well as the brilliantly colored crests 
and long plumes of the herons, ostriches and birds 
of paradise demand another explanation, which Dar- 
win tried to provide by his theory of sexual selection. 
Or the manner of study might change to an inten- 
sive consideration of a single kind or group of birds. 
Take the woodpeckers, searching for all the various 
adaptations of external parts to the whole manner 
of their life; the short, broad wings sufficient for the 
limited flights from tree to tree; the strong clutching 
feet and the stiff, pointed tail feathers applied to the 
tree trunk or branch as a support when the bird is 
drilling its holes with the hard, sharp, chisel-like bill; 
the heavy neck muscles to give the bill driving power; 
and the usually black-and-white or gray color pattern 
that merges concealingly into the color of tree trunk 
or branch. This kind of study leads inevitably, or 
used so to lead, to the question: Have the wood- 
pecker's habits led to the gradual development of its 
adaptively specialized structure from a more gener- 
alized bird form, or is the manner of the wood- 
pecker's life determined by its originally having such WHAT EVOLUTION MUST EXPLAIN 
33 
structural characters ? Evolution answers this ques- 
tion one way; special creation the other. 
But let us leave the woods and the birds for a 
swift glance at the life in the near-by tide pools of 
the ocean shore. The creatures here, except the 
little fishes we may see, belong to other branches of 
the animal kingdom, lower branches, we are accus- 
tomed to call them. They are not vertebrate but 
are representatives of half a dozen invertebrate 
branches. The fundamental differences in body 
make-up among these various lowly animals are 
radical and important, and these differences depend 
primarily on the wide genetic separation of the 
various kinds. But, inside of a single group belong- 
ing to any one branch, there may exist a wide variety 
of forms, and we can recognize in this variety much 
that is plainly adaptive. 
As we approach the rocks at low tide we hear a 
lively scratching and catch glimpses of a host of 
crabs, mostly small, scurrying into hiding places. 
They are equally at home on the rocks or in the 
water and despite their apparently awkward move- 
ments they get quickly into narrow crevices or hide 
under seaweed. If followed to their hiding places 
they face the enemy, with their strong pincer claws 
brandished threateningly in front of them. Their 
bodies are incased in a strong spiny covering, which, 
in some species, is of such color and general appear- 34 
EVOLUTION 
ance as to make the crab hardly distinguishable from 
the rocks and seaweeds. But some crabs do not have 
such armor. A whole group, comprising numerous 
species, called hermit crabs has the hinder part of 
the body unprotected by any horny covering but sub- 
stitutes for this the shell of some mollusk. A dis- 
carded shell is found, or in some cases the rightful 
inhabitant is dragged out and eaten. The crab then 
twists its soft hinder body into the shell with only 
the horny head, pincer claws, and jointed legs pro- 
truding. It has two hooks at the posterior end of its 
body by which it holds itself in the shell. When the 
crab outgrows its first shell it selects a larger one, 
thrusting itself into the new one with extraordinary 
rapidity. Some of these hermit crabs have small 
seaweeds and hydroid polyps growing on their shells. 
In fact, observers have seen them carefully tear off 
from the rocks and "plant" on their shells these 
small polyps, whose stinging tentacles help repel 
any crab enemies. The polyps get the advantage of 
being carried about and of sharing bits of food from 
the finds made by the crab. This is an example of 
a form of interrelation between two animal species 
called commensalism, or messmatism. Other crabs 
showing a special adaptive relation to other animals 
are the little oyster crabs, the females of which live 
a protected life within the gill cavities of oysters. 
These females have a thin, soft skin and weak legs WHAT EVOLUTION MUST EXPLAIN 
and claws, trusting for safety to their unusual situa- 
tion. The males, however, which do not live with 
the oysters but swim about freely, have their bodies 
protected by a horny covering. A similar small crab 
lives in the cavity of the shell of the common mussel 
and the scallop. 
Most abundant of all animals on the tide rocks 
are the various mollusks that form a bewildering 
variety of shells composed of lime, some of a single 
piece and some, like those of the oysters, clams and 
mussels, of two opposed pieces, or valves. All mol- 
lusks are soft bodied and are much sought after by 
many sea-rovers, but their hard shells are their pro- 
tection. A few kinds, the sea slugs, or nudibranchs, 
have no shell, but they appear, from their extraor- 
dinary shape and color pattern, so much like ragged 
bits of seaweed that they are very hard to distin- 
guish, and thus find protection in loss of identity. 
On the other hand, there is no difficulty in perceiv- 
ing the many thick-skinned, varicolored starfishes 
and the incased sea urchins which cling to the rocks 
in all the tide pools. These starfishes and sea 
urchins, by means of hundreds of small, suckerlike 
tube feet, move slowly about or cling very firmly to 
the rocks when disturbed. Some sea urchins bore 
little cavities in the solid rock, in which they remain, 
trusting to the dashing water to bring sufficient food 
to them. They feed chiefly on bits of seaweed, but 
35 36 
EVOLUTION 
the starfishes feed on various mollusks, barnacles 
and sea worms. They fold their arms over a clam 
or oyster; and hundreds of the tube feet fasten them- 
selves to the valves of the shell so that finally the 
mollusk yields to the constant pull of the starfish, 
and the shell opens. Then the starfish protrudes its 
stomach, inside out, through its mouth, and engulfs 
the soft body of the mollusk. It has been found by 
experiment that a large starfish can exert a steady 
pull of over two and a half pounds and that this is 
sufficient in time to open the valves of a clam or 
mussel. 
Other tide-pool and seashore creatures, represent- 
ing still lower animal branches, are the various plant- 
like sea anemones and hydroid polyps, called by the 
older naturalists "plant animals," although they are 
true animals, resembling plants only in general 
appearance when in their fixed, adult condition. 
These animals have a very simple body composed 
of a sort of short, thick-walled tube fastened, at itsi 
base, to a rock and with a single opening at its free 
end. This opening is surrounded by a ring of sensi- 
tive movable contractile and stinging tentacles which 
can grasp and paralyze small aquatic animals, and 
then thrust them down into the tubular cavity to be 
digested. The opening serves both as mouth and 
vent and the tubular cavity serves as stomach. There 
is little or no nervous system, no special sense organs WHAT EVOLUTION MUST EXPLAIN 
37 
except those of touch, and, indeed, none of that 
obvious differentiation of the body into organs and 
parts familiar in the bodies of the higher animals. 
The coral polyps, which produce the extensive coral 
reefs of tropic and subtropic oceans, are sea anem- 
ones of a special kind that secrete a hard skeleton 
of lime salts which is the dead substance we know 
as coral. 
We can find still simpler forms than the sea anem- 
ones, in the sponges, a few kinds of which are to 
be found in every tide pool. They are, in fact, the 
lowest of all the many-celled animals. The common- 
est kinds are in the form of thin reddish incrustations 
on the wave-washed rocks or on the shells of oysters 
and mussels, looking more like lichens than animals. 
Other kinds, more typical of the real sponge shape, 
are like little vases fixed at the base, each with one 
large opening at the upper end and many small 
openings in the side walls. Sponges feed simply by 
constantly drawing in sea water through the numer- 
ous small openings, and throwing it out through the 
large one. Any small organisms suspended in the 
water are taken up by cells which line the small 
openings and the inner wall of the vase. The sponge 
as we know it in the bathroom is the dead tough 
"spongin" skeleton of large sponges which live in 
warm oceans. Some sponges secrete skeletons of 
lime or of a glassy substance; the delicate "glass 38 
EVOLUTION 
sponges," such as the "Venus basket" found in the 
China Sea, being very beautiful. Sponges of the 
same species often vary greatly in form, adapting 
themselves to the situation in which they grow, and 
they possess so little individuality that two sponges 
growing side by side will often fuse into one large 
mass. Live sponges may be cut into pieces and each 
piece will grow into a perfect sponge. 
There are still other kinds of animals to be seen 
in our tide pools, but I have mentioned enough to 
indicate both the wide variety, the range in com- 
plexity and the suggestive adaptations of these lowly 
forms of marine life. They are all living there side 
by side, meeting the same fundamental necessities of 
breathing, food-getting, protecting themselves, pro- 
ducing young and generally doing the same vital 
things with the same general means, but with special 
forms of body and special manners of behavior char- 
acteristic of each and particularly adapted to par- 
ticular environmental conditions and relations to 
other living things. 
This matter of the ecologic interrelations of liv- 
ing things is one of the most fascinating and sug- 
gestive subjects which the biologist can study. In 
recent years much attention has been given to it and 
a great many facts have been revealed about the 
intimate ways in which the lives of different kinds 
of animals and plants are associated with the lives WHAT EVOLUTION MUST EXPLAIN 
39 
of other kinds. Under any particular set of physical 
conditions, climatic, topographic, or other, there 
develop particular "associations" of animal and 
plant kinds which show how intimate and continuous 
is the web of life, and how intimately related it is to 
the environment in which it exists. We can see a 
good example of this by turning again to the Mon- 
terey pines that form our little wood by the sea. 
From the branches, especially the dead branches, 
of many of these trees there hang the gray-green 
streamers of a lichen which finds the pine branches 
convenient for support and free exposure to the air 
and to its frequent, thirst-relieving summer fogs and 
winter rains. The lichen (often mistakenly called 
moss) does not live parasitically on the pine trees, 
as does the mistletoe, which one sees occasionally 
in the tree-tops. Nor is it, indeed, a single kind of 
plant, but an extraordinary combination of two low 
kinds, a fungus and an alga, which live all intermixed 
in a close commensal or symbiotic connection. This 
condition is true of all lichens, of which there are 
hundreds of known species. The fungus, which de- 
rives carbonaceous food substances from the algal 
cells, is the predominant part of the combination and 
cannot live apart from the alga. On the other hand, 
the alga, which derives some benefit from the fungus 
in the way of protection and moisture, and probably  EVOLUTION 
40 
some nitrogenous foodstuffs, can live apart from 
the fungus, and sometimes does. 
If we examine our pine trees carefully for other 
inhabitants, we can readily find an imposing array 
of insects which are entirely at home in the trees 
and make their living at the trees' expense. Differ- 
ent insects take up their abode in different parts of 
the tree. First, there is a tiny midge which lays its 
eggs at the bases of the outgrowing new pine needles. 
From these eggs hatch minute grubs without wings, 
legs, eyes, feelers or even mouth, which, lying bathed 
in the abundant plant sap which the tree provides to 
nourish the growing needles, absorbs this sap or food 
from it through its skin. The needles, thus robbed 
of their food, make only a stunted growth, and since 
the needles are the foliage of the pine tree which 
converts carbon dioxide absorbed from the air, 
under the influence of sunlight, into food for the tree, 
if there are too many pine midges the tree starves. 
On the needles also may be found numerous small 
whitish scale insects, so called because the female 
covers its degenerate, flat, wingless, legless, eyeless 
adult body with a protecting flat scale of white wax 
which it secretes from pores in its skin. It has a 
delicate, flexible, sucking beak which it thrusts into 
the needle to suck sap from it. The adult male of 
this pine-scale insect differs from the motionless 
female by having wings, legs, eyes, but no sucking WHAT EVOLUTION MUST EXPLAIN 
41 
beak or mouth. It takes no food in its adult life, 
having fed sufficiently as a wingless but actively 
crawling larva, provided with sucking beak and 
mouth. 
Leaving the pine needles and examining the upper 
branches and trunk of the tree we shall find two or 
three kinds of bark-boring beetles the adults of 
which burrow in through the outer dead bark to the 
live bark or cambium, and lay their eggs there in 
little niches along a short tunnel. When the beetle 
grubs hatch from the eggs, each burrows a short 
tunnel for itself in the live bark, living on the 
abundant food provided by the cambium. Farther 
down the trunk are the burrows of other species of 
bark borers, and close to the base still others, each 
part of the tree being reserved, as it were, to certain 
beetle species. The tunnel of the adult beetles and 
the tunnels of the grubs which branch off from it 
have a characteristic extent and arrangement for 
each species of bark borer. When the tree is felled 
and the outer bark is stripped off we can read the 
story of how many and what kinds of beetles have 
lived in it. There on the now dead and dry inner 
bark are the curious engravings which, like the hiero- 
glyphs of ancient peoples, can be read by those who 
have the code. 
There are other insect species that live exclusively 
in the Monterey pine trees-to their benefit, and the EVOLUTION 
42 
trees' hurt. There is a moth, called the resin moth, 
which lays its eggs on the outer bark from which 
hatch caterpillars that wound the tree so that resin 
flows out and forms a protecting mass in which the 
caterpillar lives. And there is another moth whose 
caterpillars form a community web of silk stretch- 
ing over a number of small branches, underneath 
which silken web the caterpillars devour the pine 
needles. There are beetles whose strong-jawed 
grubs burrow into the heartwood of the trees. But 
it would be tiresome to catalogue all the insects that 
live in the Monterey pines. It is sufficient to know 
that there are many, and that their ways of living 
are various. Some of them do not rely exclusively 
on the Monterey pine for habitat and food, being 
able to live on other pine trees. But some have be- 
come so habituated to this particular kind of pine, 
and so specialized in their adaptations to it, that they 
have cast in the lot of their species as to success or 
nonsuccess in persistence, with the lot of this one 
species of pine tree. This surrender of general 
possibilities for the sake of the advantage of a very 
precise and successful fitting to specific conditions is 
an oft-repeated story in biology. It is especially 
common in connection with parasitic life. 
Now, the general implications of our cursory 
study of some of the living creatures, and some of 
the ways of life accessible to observation near our WHAT EVOLUTION MUST EXPLAIN 
cottage in the pine woods by the sea, are plain. It 
is the abundance of kinds of animals and plants and 
the variety of form and habit among these kinds and 
the interrelations among them which are the very 
first things to catch the attention of even the most 
casual observer. And hence these are the things 
that evolution, the solver of riddles about life, is 
first called on to explain. 
If we let our study run out from our cottage and 
embrace the whole world, we find in it three quar- 
ters of a million different living species of animals 
and plants. We may be sure, from the rate at which 
new ones are being found-meaning by "new" only 
previously unknown, not newly come into existence- 
that there are quite as many more, probably twice as 
many. And we are faced, as we recognize these 
many different species, with an amazing variety in 
their size, color, form, complexity of structure, habi- 
tat and manner of life. At the same time we see the 
immediate connection of much of this variety with 
the varying conditions under which the different 
forms live. Nor is it difficult to see that many of 
the differences among living kinds are of an obviously 
adaptive sort. To the persistently inquiring student 
of nature these adaptations come almost to obsess 
his attention, they are so many, so ingenious, so 
elaborate, so precise. They affect not only the form 
and structure of the animal or plant but its whole 
43 44 
EVOLUTION 
way of living. These adaptations and all this variety 
of life cry aloud for explanation. Evolution must ex- 
plain them. 
But, perhaps, to most persons, the word "evolu- 
tion" first suggests genetic, or blood, relationships 
among organisms; a genealogical tree of animal life 
with amoeba at the base and man at the tip of the 
highest branch. That is, it suggests, primarily, the 
fundamental identity and continuity of life and the 
similarity and relationships of organisms, the like- 
nesses rather than the unlikenesses. It emphasizes 
the presence of common characters in different or- 
ganisms, as, for instance, the rayed body of all the 
starfishes and sea urchins, the feathers and tooth- 
lessness of all the birds, the milk glands of all the 
mammals. And it is quite true that this is the basic 
conception in organic evolution. It is on this basis 
that we account for fundamental likenesses. But the 
very fact of this recognition of identity in life makes 
the first glimpse of all its variety the more puzzling. 
We demand, straightway, that evolution explain 
both of these conditions. 
Such an attitude toward evolution as I have just 
outlined may seem to reduce it from a great concep- 
tion guiding our manner of thought, a great philoso- 
phy determining our attitude toward all of nature, 
including human nature, to a smaller and more spe- 
cific principle of biology, an idea primarily of use WHAT EVOLUTION MUST EXPLAIN 
45 
to the student of the structure and behavior of ani- 
mals and plants and of their classification. It is 
hard to turn our attention from ourselves; from the 
significance which evolution has for our understand- 
ing of our origin and place in nature; from our en- 
dowment of mind and reason, out of which endow- 
ment comes the very conception of evolution; it is 
hard, I say, to divert our attention from ourselves 
and apply it to insects, starfishes, and the flowering 
plants, and the problem of their variety and ingen- 
ious adaptation to their varying environment. 
We are, entirely understandably, essentially an- 
thropocentric in our interests. What has evolution 
to do with or for us, is our natural first question. 
But we shall best undertake to answer it with some 
confidence in the answer, by trying to find out what 
evolution offers in explanation of the problems of 
the simpler forms of life. Man is so hopelessly 
complex. He is so much more than a starfish. 
Although he is, perhaps, just another form of an 
elaborate chemical and physical phenomenon, or 
group of phenomena, called life he is at least quan- 
titatively so much more than a starfish that he 
presents all the difficulties, to any one who would 
analyze and understand him, of a qualitatively dif- 
ferent object. 
With all my conviction of man's blood relationship 
to the lower animals, and with all my recognition of 46 
EVOLUTION 
the essentially similar chemical and physical phenom- 
ena which go on in starfish life and human life, 
I do not admit at all that I am simply a magnified or 
better starfish. Believing firmly that man is of and 
in nature, and not out of or beyond or above it, I 
nevertheless recognize that included in the great deal 
that naturalists do not yet know about nature there 
is especially much that they do not know about the 
nature of man. Even a very full knowledge of the 
ways and possibilities of starfish life can be but a 
beginning in knowing the ways and possibilities of 
human life. But, and this is important, it may 
really be a sound beginning. CHAPTER IV 
EVIDENCES OF EVOLUTION: COMPARATIVE 
ANATOMY AND EMBRYOLOGY 
We have, so far, rather taken evolution for 
granted. But that is to assume a too easy and gen- 
eral acceptance of something that has excited much 
antagonism and not infrequent indignant repudia- 
tion and denial. The indignant deniers do not really 
care to listen to evidence for evolution; their atti- 
tude is determined more by emotion than reason. 
But the unprejudiced may ask for the evidences of 
evolution. What are the proofs of this grandiose 
conception of the natural production, by transmuta- 
tion, of all the kinds of animals and plants, of the 
blood relationship of all living things, of the identity 
and continuity of all life stuff? If evolution is going 
on all the time we should be able to see it at work. 
If it has been going on for ages we should be able to 
see its results, and as results not explicable by other 
causes, or, at least, more reasonably explicable by 
evolution than by any other cause. These are, in- 
deed, precisely the questions and remarks made to 
me within the day by a young lady of high-school 
age. She has been hearing and reading something 
about science, and that word "evolution" has kept 
47 48 
EVOLUTION 
jumping out at her from all sorts of hiding places. 
She wants to know what it is and how we know that 
there is any such thing. She wants evidences of evo- 
lution. 
Most of these evidences are commonly grouped, in 
the textbooks that are written about evolution and 
by the scholars who present them in lecture rooms 
and laboratories, under four heads: the evidences 
from comparative anatomy, the evidences from em- 
bryology, those from paleontology, and those from 
the geographical distribution of plants and animals. 
But there are some that do not fall readily under 
any of these heads; for example, some, such as blood 
tests, that might be called physiological evidences, 
others, as those of mental reactions and behavior, 
that may be called psychological evidences, and still 
others that come under such general categories as 
adaptations and ecological relations. In fact, there 
are so many of these evidences and they are of such 
a wide variety of character as to make it a puzzling 
matter to select the few that can be set out in such 
a little book as this. A whole book could well be 
devoted to an account of those available in each of 
the groups I have named. 
Comparative Anatomy 
As we are all more interested in human evolution 
than in the evolution of other creatures, perhaps EVIDENCES OF EVOLUTION 
the few cases of evidence from comparative anat- 
omy to which we can give space may advisably be 
chosen from the animals instead of the plants and 
from that single branch of animals, the vertebrates, 
in which humankind finds its zoological place. 
Take, for example, the familiar case of the verte- 
brate skeleton. We all know something about the 
number and character and arrangements of our own 
bones and of the bones of fishes, frogs, snakes, birds 
and mammals. 
If any of us do not, those of us in this sad condi- 
tion should hasten to the nearest museum of natural 
history and stand a few minutes before the case con- 
taining mounted skeletons of representatives of the 
five great vertebrate classes, and then before the one 
showing a group of skeletons of different mammals, 
say a dog or a cat, a horse, a porpoise, a seal, a bat, 
a tailed monkey, an ape, and a man. Then make 
some comparisons among all these skeletons and en- 
joy the pleasure of rediscovering what other observ- 
ers earlier discovered, namely, the fundamental 
identity in character and arrangement of the bones 
which form the framework of the vertebrate bodies, 
even though these different bodies are those of ani- 
mals which vary much in their habits of life. Some 
walk and run, some leap, some crawl, some swim, 
some burrow, some fly. Each kind correspondingly 
shows a modification of skeletal make-up to adapt it 
49 50 
EVOLUTION 
to the special requirements made by each type of 
locomotion. But that modification is clearly only a 
special change rung on a skeletal motive common 
to all. 
Fasten your attention to certain parts. The ver- 
tebrate skeleton consists typically of an axial portion 
comprising the vertebral column and the head, with 
two pairs of appendages or limbs, rising from or 
connected with the axis by a shoulder girdle and a 
pelvic girdle. These limbs are variously developed 
as fins, wings, legs and arms. In a few of the lowest 
fishes there is no trace of limbs, and in various 
amphibians, reptiles, birds and mammals, one or 
both pairs may be quite rudimentary. But precisely 
in these cases of rudimentary limbs, the lack of devel- 
opment obviously corresponding with special man- 
ners of locomotion not requiring functioning fins or 
wings or legs, we get an illuminating illustration of 
the persistence of the basic common type of verte- 
brate skeletal make-up. Where all the limbs are 
present in functional condition but used differently 
as with the bat, the seal, the dog, and an ape or man 
-cases within a single vertebrate class, the mam- 
mals-we see how the fundamentally similar make- 
up, the very same bones, indeed, appropriately modi- 
fied, have been made effectively to serve various 
purposes. We see what the evolution idea calls EVIDENCES OF EVOLUTION 
51 
for, namely, basic identity with gradatory, usually 
adaptive, modification. 
Since our own body especially seizes our attention, 
examine the human skeleton in the museum case be- 
fore you with particular care, and then compare it 
with the skeletons of the chimpanzee or orang-utan 
or gorilla which will be next to it if the museum is 
properly arranged, and also with the skeleton of a 
tailed monkey which will also be close by. In a 
detached and temporarily disinterested attitude, 
carefully go over these skeletons, part by part, bone 
by bone. Draw your own conclusions. Take no- 
body's word for this extraordinary identity. Take 
nobody's explanation for it. Let your own eyes and 
your own reason satisfy themselves. 
If you want more of this kind of evidence for evo- 
lution go over any or all of the other systems of the 
vertebrate and mammal body, including our own; 
the muscular system, the nervous system, the circu- 
latory and respiratory systems. They all repeat the 
story of the comparative anatomy of the skeleton. 
Or, the evidences from comparative anatomy can be 
obtained in another way than by taking up one body 
system at a time. We can run through the story of 
a special category of anatomical facts, as those re- 
lating to the presence in various animals of rudimen- 
tary or, as they are often called, vestigial structures, 
such as the rudimentary limbs in various vertebrates EVOLUTION 
52 
to which we have already referred. Other examples 
of vestigial structures are the reduced eyes in 
various cave-dwelling animals, the "thumb," or 
rather index finger, of birds, the splint bones or 
reduced side toes of the horse's foot, the appendix 
vermiformis and the reduced ear and skin muscles in 
man. Indeed, the anatomist Wiedersheim has re- 
corded 180 vestigial and retrogressive structures in 
man's body alone. They occur in all his systems of 
organs, the skin and hair, skeleton, muscles, nervous 
system, sense organs, digestive, respiratory, circula- 
tory, and urino-genital systems. Some of these rudi- 
mentary structures are to be found completely devel- 
oped in other mammals or other vertebrate groups. 
Eleven of them are fully functional organs in fishes, 
four in amphibians and reptiles. Sometimes they 
appear in more developed condition in particular 
human individuals. Now and then a person can use 
his skin muscles to move his ears slightly, or shake 
the skin of his forehead or scalp. Many of these 
vestigial structures, the tail, for example, are better 
developed in embryonic life but become more and 
more reduced as the body grows and develops. The 
tail is longer than the leg in early stages of the 
human embryo, but gradually becomes more and 
more reduced, until at birth there is no external sign 
of it, although the bony rudiments of it-the coccyx 
-are present all through life. EVIDENCES OF EVOLUTION 
53 
These vestigial structures are evidences of gradual 
evolutionary change. In them one sees evolution 
actually in process. One sees evolution at work. 
Why should a special creator put useless and disap- 
pearing parts into the human body? Why should 
these parts be the remnants of parts useful and used 
by lower vertebrates in their kind of life, but useless 
and sometimes harmful in man, if the explanation is 
not that with his changed manner of locomotion, his 
modified food habits, his new artificial methods of 
defense against enemies, his protection against the 
cold and wet by clothing instead of hair, he has no 
longer use for certain parts with which he has been 
endowed through his relationship to lower verte- 
brates and that they are, consequently, by slow 
evolutionary change, gradually disappearing? What 
other reasonable answers can be given to any of these 
questions except those given by the evolutionist? 
Embryology 
This presence in the embryonic stages of man, 
and of other animals and of plants, of various struc- 
tures which are present in the adult stage only in 
reduced or vestigial condition, or are perhaps not 
present at all, is one of the most striking things re- 
vealed by embryological study. And it is a pecu- 
liarly strong bit of evolutionary evidence. But it is 
only one of the suggestive revelations that come 54 
EVOLUTION 
from the detailed study of the life history of any 
individual organism. In the development of any in- 
dividual plant or animal we can find a swift, much 
condensed and often much modified but, on the 
whole, very enlightening recapitulation of the gen- 
eral evolutionary history of the species to which the 
individual belongs. Embryonic stages occur which 
are essentially similar to stages in the embryology 
of other animals, and also stages are passed through, 
rapidly and incompletely but recognizably, which 
repeat and thus represent, in many characteristics, 
the adult stages of other lower animals or plants. 
This "recapitulation theory" is one of the greatest 
generalizations that has been made in biological 
study. It was first formulated by Karl von Baer 
and later made more specific-too specific, indeed- 
by Haeckel, who, with characteristic optimism, saw 
in it more than the actual facts warranted, and, by 
his overemphasis of its significance and his too de- 
tailed interpretation of the evolutionary history of 
various animal kinds and groups on a basis of it, 
brought it into some disrepute. But it contains, 
without any doubt, a large residuum of truth, and 
is one of the strongest of the evidences of evolution. 
The human body in its growth and development from 
single fertilized egg cell to complex trillion-celled 
adult condition, with its many differentiated tissues EVIDENCES OF EVOLUTION 
55 
and its elaborate systems of organs, tells us much 
of the history of the evolution of man. 
The first steps in the development of an individual 
human being from a fertilized human egg cell, which, 
like all egg cells, has a remarkable power of multi- 
plication and differentiation, are the division of this 
cell into two, then the division of these two into 
four, and of these four into eight, and so on until a 
small solid spherical mass of cells is formed, called 
the morula stage. Some of these cells soon become 
specially massed on one side, and here two cavities, 
or beginning sacs, are formed. One of these becomes 
a sac which gradually incloses the embryo, and the 
other forms the yolk sac, part of which eventually 
becomes the alimentary canal. Some of the cells of 
the two sacs which lie adjacent and are destined to 
form the actual embryo, form two layers, known as 
the ectoderm and endoderm, or the external and in- 
ternal embryonic membranes. Later, through the 
repeated division (multiplication) of the ectoderm 
cells, an elongated area, called the primitive streak, 
is formed, and this indicates the fore-and-aft axis of 
the embryo. 
Along this primitive streak, to follow a recent 
authoritative account by Professor Ferris, professor 
of anatomy in Yale University, a third layer, known 
as the mesoderm, is formed between the outer ecto- 
derm and the inner endoderm layers. There are now 56 
EVOLUTION 
in the region where the embryo is developing in the 
mother's body, three layers of cells, each having its 
own distinctive characteristic. These are known as 
the primary germ layers, and from them all the 
organs and parts of the body are later derived. 
From the outer layer, or ectoderm, are formed the 
outer layer of the skin, or epidermis, including its 
various appendages, such as the hair and sweat 
glands, the cells lining the mouth, the enamel of the 
teeth and the entire nervous system, including the 
sensory portions of the sense organs. From the 
middle layer, or mesoderm, are formed the skeleton 
and other supporting tissues and the muscles, the 
vascular system and the sex cells. From the inner- 
most layer, or endoderm, are developed the cells 
lining the alimentary canal and the essential secreting 
cells of the various organs which develop as out- 
growths from it, such as the thyroid gland, the lungs, 
the liver and the pancreas. In general it may be said 
that the endoderm supplies the alimentary system; 
the mesoderm, the locomotor apparatus and the 
sex cells necessary for the persistence of the race; 
and the ectoderm, those structures which are placed 
in control of the body and put man in touch with his 
environment. This particular course of embryonic 
development, both in its manner and in its relation 
to the origin of different tissues and organs, is EVIDENCES OF EVOLUTION 
essentially similar in all many-celled animals, inver- 
tebrate and vertebrate. 
Up to this point the human embryo appears as a 
rather simple multicellular animal of the inverte- 
brate type. The first indication that it is to become 
a vertebrate is the development of a dorsal, longi- 
tudinal, rodlike axis, called the notochord, which 
eventually extends posteriorly from the base of the 
brain through the length of the body. In the lowest 
forms of aquatic vertebrates (the tunicates and 
lancelets) this is the only longitudinal supporting 
axis the body ever possesses, but in the higher fishes 
and all other vertebrates (amphibia, reptiles, birds 
and mammals) where a more stable axis is necessary, 
the notochord is replaced by a more rigid, segmented, 
bony structure, the vertebral column. 
We have not space to follow, in any detail, all 
the stages in the development of the embryo, but 
may notice simply a few special stages or happen- 
ings which are particularly significant in the evolu- 
tionary history of man. One of these is the segmen- 
tation of the embryo, initiated in its mesodermal 
part, by a series of horizontal clefts which result in 
a linear series of segments extending the whole 
length of the body of the embryo. This primitive 
segmentation, which undoubtedly repeats the adult 
condition of some segmented ancestor of the verte- 
brates, persists in a modified form in adult man in 
57 58 
EVOLUTION 
the serial arrangement of the vertebrae, ribs and the 
spinal nerves. In the lateral mesoderm a cavity de- 
velops which is the beginning of the body cavity, 
called the coelom, which later contains the heart, 
lungs, and abdominal viscera. This cavity splits 
the mesoderm into two layers. The outer layer 
joins with the ectoderm to form the body wall, and 
the inner layer with the endoderm to form the wall 
of the alimentary canal which, in time, becomes en- 
tirely inclosed by the mesoderm and ectoderm of the 
body wall. The human embryo at this stage has 
acquired the characteristics of a typical vertebrate. 
All vertebrate animals are essentially alike in the 
course of their embryology up to this stage. 
The brain in the vertebrate embryo develops at 
the anterior end of a hollow neural tube which ex- 
pands here into three sacs corresponding to the fore, 
mid, and hind brains. From each side of the fore- 
brain sac another sac grows out which expands in 
all directions, but especially backward, spreading 
over the other brain sacs and ultimately forming 
the cerebrum which is so large in man in comparison 
with the lower animals. All of this sac, except the 
lower part, as well as the parts of the adult brain 
formed from it, is known as the mantle or pallium, 
of which that part which forms the major part of 
the cerebrum is known as the neopallium. Now, 
the early condition of the neopallium in the human EVIDENCES OF EVOLUTION 
59 
embryo represents about the whole extent of the 
pallium in the adult fish. As it grows further back- 
ward it represents first the extent of the pallium in 
the next higher class of vertebrates, the amphibians, 
and later the extent in the reptiles. Finally, as it 
begins to cover the cerebellum, the human embryonic 
pallium is like that found in the adult stages of the 
lower mammals, and not until it covers the cere- 
bellum completely do we have the fully developed 
human pallium. 
Thus the forebrain of man passes, in its develop- 
ment, successively through the various stages repre- 
sented in the adult forms of the various major verte- 
brate groups, starting with the fish and terminating 
with the most developed form of the mammalian 
type. Similarly, the structural unit of the nervous 
system, the nerve cell, or neurone, passes, in its de- 
velopment in man, from the very simple neurone of 
the fish through the increasingly complex forms in 
the various vertebrates, to its greatest complexity 
in man. 
The embryonic development of the vertebrate ali- 
mentary canal and the organs that arise from it, 
presents some equally interesting and suggestive con- 
ditions. The canal begins as a closed tube, later 
open, folded off from the yolk sac and lying under 
the notochord of the embryo. At the anterior end 
of the early embryo on each side of the neck, four 60 
EVOLUTION 
crevices appear, which in the fishes open directly 
into the pharyngeal region of the alimentary canal 
and form the gill clefts. In man, however, these 
crevices do not go on to the formation of gill clefts, 
but soon disappear. Their presence, however, is 
indicative of a fish stage in his development. The 
lungs develop from the upper end of the alimentary 
canal by the formation of a single hollow sac, which 
later bifurcates to form the right and left lung sacs, 
which by repeated branchings develop into the highly 
ramified tubular structure of the adult mammalian 
lungs. The early, saclike lung of the human embryo 
is similar in structure to the permanent saccular lung 
of the adult amphibians. 
The heart differentiates from a portion of the 
mesoderm lying underneath the pharynx in the 
head end of the embryo. It consists at first of two 
straight tubes which soon fuse for part of their 
length to form a single tube bifurcated at each end. 
At this stage of development the human heart re- 
sembles that of the adult of the lowest vertebrates. 
Later, this single tube of the developing heart be- 
comes partially subdivided into two successive 
chambers, the auricle and the ventricle, and it now 
resembles the adult heart of the fishes. The auricle 
next divides into two cavities, and now this em- 
bryonic human heart of three chambers resembles 
the fully developed heart of the next highest verte- EVIDENCES OF EVOLUTION 
61 
brate class, the amphibians. Later, the ventricle is 
also divided into two cavities, and thus the four- 
chambered heart characteristic of the highest verte- 
brates and man is reached. The red blood cells of 
the human embryo, are, when first formed, large and 
nucleated. In this stage they resemble the red blood 
cells of adult fishes and amphibians. Later, the em- 
bryonic human blood cells are similar in structure to 
those of adult reptiles. Finally, before birth of the 
human embryo, these blood cells become, as they 
also do in all mammals, nonnucleated and biconcave. 
Thus, it is evident that the human heart and the cells 
of the human blood, in their embryonic development, 
pass through stages representing the different adult 
conditions of the heart and blood in successively 
higher vertebrate classes. 
At the seventh month of prenatal life the chimpan- 
zee and gorilla have well-developed hair on the scalp, 
eyebrows and lips, and the rest of the body is cov- 
ered with fine hair. This is also true of the human 
embryo of the same age, and the hair slopes and 
lines are very similar to those of the apes. But 
before birth the human embryo loses the fine body 
hair. The developing nose of the early human 
embryo goes through a series of stages which repre- 
sent the adult nose of first the gilled fishes, second 
the lung fishes, third the amphibians and finally (in 
the third month) the mammals. In fact, the em- 62 
EVOLUTION 
bryonic history of almost any human part recapitu- 
lates more or less clearly, and more or less nearly 
completely, a series represented by the adult condi- 
tion of this part as possessed by the various verte- 
brate classes beginning with the fishes and ending 
with the mammals. 
This embryonic recapitulation of the evolutionary 
history of the species is necessarily much condensed 
and much modified. Numerous stages of the evolu- 
tionary history are dropped out, and various adap- 
tive stages or characters, which fit the young to carry 
on an independent life while still developing towards 
maturity, may be interpolated. But the ancestral 
stages actually repeated in the embryonic develop- 
ment of the individual are too obvious to be over- 
looked. Equally obvious is the similarity in devel- 
opment, up to very late embryonic stages, of any 
two kinds of animals or plants which are genetically 
closely related, but in adult condition may be super- 
ficially very unlike in appearance, because of adap- 
tive modification of the body to fit the animals for 
life under different conditions. For example, a 
barnacle fixed to a tide-washed rock is a very differ- 
ent-looking creature from a crab crawling actively 
about on the same rock, but barnacle and crab are 
closely related and the barnacle passes through a 
stage in its embryonic development when it is an EVIDENCES OF EVOLUTION 
63 
active, free-swimming larva much like the similar 
larva of the crab. 
An important part of the evidence for evolution 
which embryology adduces, is that implicit in the 
character of the diagram which one could trace to 
illustrate the manner in which different animals (or 
plants) run along together as regards their em- 
bryonic development, or early separate from each 
other in this respect. 
A starfish and a sea urchin, a beetle and a butter- 
fly, a snake and a turtle, a horse, a chimpanzee, and 
a man, all start as single fertilized egg cells. Each 
of these single cells begins its development by a 
series of simple divisions, forming, in each case, a 
little subspherical group of similar cells. Then each 
group of cells begins to become modified in charac- 
ter by foldings and differentiation of continually 
forming new cells. The groups representing the 
starfish and sea urchin, which both belong to one 
branch of animals, change in one way; those repre- 
senting the beetle and butterfly, which both belong 
to another branch, in another way; those represent- 
ing the snake, turtle, horse, chimpanzee and man, 
which are all vertebrates, in still another way. In 
their later development, the starfish and sea urchin 
for some time go through similar changes, as do the 
beetle and butterfly, and as do also the various ver- 
tebrates; but these changes become more and more 64 
EVOLUTION 
unlike in the different groups. At the same time it 
is obvious that the snake and turtle, which are both 
reptiles, follow paths more like each other than 
either is like the path of the horse, chimpanzee or 
man, which are all mammals and which follow paths 
similar among themselves. These two sets of paths 
continue to diverge more and more. Finally, within 
the mammal group the chimpanzee and man go on 
along paths much more like each other than either 
is like the path of the horse. It is, indeed, hard to 
distinguish the embryonic chimpanzee from the em- 
bryonic man until well along in their developmental 
paths. 
To generalize from these instances, we can say 
that animals closely related to each other follow 
similar embryonic paths until late in their develop- 
ment, while animals less closely related diverge 
earlier and more markedly. This divergence is the 
earlier and the more marked, the less closely related 
are the animal kinds. A diagram illustrating graphi- 
cally the facts concerning the embryonic develop- 
ment of all, or many, animals, would, therefore, have 
the form of a repeatedly branching tree. The same 
would be true for plants. Biologists believe that if 
this tree could be correctly worked out it would cor- 
respond with the tree of relationships worked out 
by comparative anatomy. CHAPTER V 
EVIDENCES OF EVOLUTION: PALEONTOL- 
OGY, GEOGRAPHICAL DISTRIBUTION 
The "Emporia Mineral and Fossil Club" was 
composed of a group of scholarly young gentlemen 
of the late grammar and early high school ages who 
had had an earlier organization known as the "Osage 
and Cheyenne Pony Riders"-that "bunch of young 
Indians," the townspeople called them. The trans- 
formation came about as the result of a discovery 
made during the enlargement of the headquarters 
cave of the Pony Riders. This cave was in a sand- 
stone outcropping near the river bank about a mile 
from town, and was a perfect cave for wild Indian 
purposes except that it was too small. In making it 
larger the Pony Riders were astonished to find 
pieces of the sandstone which they broke out stamped 
with the impress of leaves, and these leaves were of 
a kind different from any to be found on the grow- 
ing plants or trees of the neighborhood. The father 
of one of the boys called them "fossil leaves," and 
suggested writing a letter to the state geologist about 
them. This worthy man, being human as well as 
65 66 
EVOLUTION 
scientific, sent back such an answer as immediately 
changed the wild Osages and Cheyennes into an en- 
thusiastic band of geologists and paleontologists as 
devoted to pony riding and roaming as before, but 
bringing back as booty bits of stone and fossil leaves 
and shells, and flat rings from crinoid stems, instead 
of imitation scalps. Especially did the crinoid rings, 
scattered so abundantly on the slopes and crest of a 
low limestone hill near the cave, rivet their attention 
and wonder. For they soon learned that crinoids 
are a kind of marine animal related to starfishes and 
sea urchins, but fixed and plantlike in appearance, 
and hence called sea lilies, which lived in great num- 
bers of kinds and individuals in ancient oceans. This 
meant that the Kansas limestone hill, now nearly two 
thousand miles away from any ocean and a thousand 
feet higher than present sea-level, had been some- 
time part of the bed of an ocean. The state geolo- 
gist guessed this time to be about three or four mil- 
lion years ago I 
In Huxley's words, "fossils are only animals and 
plants which have been dead rather longer than those 
which died yesterday." Each fossil animal or plant 
is a record of prehistoric life, absolutely authentic, 
so far as it goes, admitting of no doubt or question. 
But, as Lyell, the great geological champion of evolu- 
tion in Darwin's time, so expressively stated it, it EVIDENCES OF EVOLUTION 
67 
took one hundred and fifty years of argument and 
dispute to persuade even learned men that shells 
and teeth in the rocks were actual remains of actual 
animals, and another hundred and fifty years to 
demonstrate that the shell-bearing rocks were not 
masses of debris from Noah's flood. Nothing in 
the history of science is more extraordinary than 
the story of the efforts, directed against the first 
students of fossils, to show that these structures 
were mere sports of nature, whimsicalities of crea- 
tion, or freaks developed in the fatty matter 
(materia pinguis) of the earth by the entangling 
influence of the revolving stars! 
Of the four "ancestral documents," anatomy, em- 
bryology, paleontology and distribution, which con- 
tain so much evidence for the reality of evolution 
and so much of the record of plant and animal 
descent, paleontology is at once the most certain and 
the most incomplete. It is the most certain, for 
each fossil is the remains of an actual prehistoric or- 
ganism which has been one of the links in the long 
and ages-old chain of descent from lowest and old- 
est to highest and most recent of organisms. Fos- 
sils help prove evolution, for they help fill the spaces 
in that continuous and branching tree of organic 
genealogy which is called for by the evolution idea. 
But paleontology is the most incomplete of the evo- 
lutionary records because comparatively so few- 68 
EVOLUTION 
although absolutely many-of the myriad plant and 
animal kinds which have lived on this earth have 
left fossil traces for us to examine. No animal or 
plant is preserved as a fossil except as the result of 
an unusual combination of circumstances, of which 
perhaps the most important is that the body of the 
dead organism must have in some way become caught 
in sediment slowly being deposited at the bottom of 
a lake or ocean and which, hardening, formed slowly 
into solid rock. With few exceptions, then, only in 
the sedimentary or stratified rocks of the earth's 
crust do we find fossils. And while these rocks, such 
as limestone, sandstone, and shales, form much of 
this crust, there are in it, also, large masses of other 
rocks, igneous or granitic in nature, in which no 
fossils can occur. In some places, too, the sedimen- 
tary rocks have been so subjected to pressure and 
heat after their deposition that all the fossils in them 
have been destroyed. Finally, recall what a sadly 
small fraction of the unmodified sedimentary rocks 
of the earth has been explored for their contained 
fossils, or can ever be so explored. 
It is obvious that under the conditions necessary 
to the forming of fossils, the plants and animals liv- 
ing normally in the ancient lakes and oceans had a 
very much better chance to leave their remains as 
fossils than the animals living on land. Thus, the 
insects, which at present comprise about three fifths EVIDENCES OF EVOLUTION 
69 
of all known animal species and have undoubtedly 
been abundant through several geologic ages, are 
represented by few fossils compared with the aquatic 
mollusks and crustaceans. It is obvious, also, that 
the hard parts of animals, such as shells, bones, teeth, 
are much more likely to be preserved as fossils than 
the soft parts. We are likely, therefore, to have an 
undue proportion of our discovered fossils repre- 
senting such animals as vertebrates (with bony skele- 
ton) and mollusks (with shells) rather than such 
animals as sea anemones, jelly fishes, spiders, and 
others (with soft bodies). 
We get from the rocks, then, a most incomplete 
picture of the plant and animal life of the earlier 
ages, but what there is of it is indubitably authentic. 
This picture has been in course of painting by the 
master artist, Nature, for millions of years. Here 
and there, and representing different times, it is 
drawn and colored in much detail. In other places, 
or representing other times, it is merely a blank. And 
in many spots it is sadly marred by time and accident. 
We have to face the plain facts that the prehistoric 
plants and animals have had a hard time of it in 
their pleasant intention of presenting to future in- 
quisitive man an easily readable picture or story of 
the succession and kinds of life of the geologic ages. 
Yet, after all, we have been able to read much of 70 
EVOLUTION 
this fascinating tale of who, and when, and where 
were the ancient inhabitants of the earth. 
These inhabitants did not all live at one prehistoric 
time, nor in any one part of the earth. In fact, the 
earth has suffered great changes through the ages; 
where at one time there was ocean, at another there 
was land, and this reversal might be often repeated 
in one and the same part of the earth, with conse- 
quent radical changes in the kinds of organisms which 
lived there. The whole time of the earth's existence, 
in such condition that life might endure on it, is un- 
known in terms of years; we may say millions-for 
it has certainly been millions-and let it go at that. 
But the geologists and paleontologists have divided 
this long stretch of time, for convenience of refer- 
ence, into geological eras or ages, then each of these 
ages into shorter parts called periods, and these 
periods into epochs. Each epoch and period and era 
is more or less sharply distinguished from every 
other by the different kinds of animals and plants 
which lived while its rocks were being deposited. 
Of course some animal and plant kinds persisted 
from one epoch to another, and even from one period 
or era to another. There are various species of 
one-celled plants and animals now living in the 
oceans which can hardly, if at all, be distinguished 
from species which have been found as fossils in the 
earliest (oldest) stratified rocks. But each new EVIDENCES OF EVOLUTION 
71 
geological epoch and period and era is distinguished 
by new kinds and groups of organisms as compared 
with those of the preceding geological time unit. 
The fossils found in the oldest rocks-which, in 
parts of the earth's crust that have not been dis- 
torted by foldings and breaks, are the lowest of the 
stratified series-represent the oldest or earliest 
animals and plants, those in the upper or newest 
rocks chiefly the newest or latest animals and plants. 
Now, an examination of a whole series of rock 
strata shows that the more highly organized and 
specialized kinds of plants and animals did not 
exist in the earliest epochs of the earth's history but 
that the organisms of these epochs were all of the 
simpler or lower kinds. For example, in the older 
(lower) stratified rocks there are no fossil remains 
of the vertebrate animals; there are only inverte- 
brates. When the vertebrates do first appear in the 
less old (higher) rocks, there are none but fishes, the 
lowest vertebrate class, for several epochs. In a 
later period, the amphibians appear; in a still later 
period, the reptiles; and last of all, the birds and the 
mammals. No human fossils have been found below 
the uppermost, that is, most recent, geological strata. 
Of course "recent" used in connection with geologic 
time may mean anything from a few thousand to 
half a million years. As a matter of fact, the oldest 
of human fossils so far known are somewhere be- EVOLUTION 
72 
tween three hundred and five hundred thousand 
years old. We shall pay special attention later to 
these fossil relics of prehistoric man. 
The paleontological history of the plants tells a 
story similar to that of the animals. The oldest fos- 
sil plants are simple types of algae. Ferns appear 
later and are abundant in the coal-bearing rocks. 
The lowest types of seed-bearing plants also ap- 
peared in the later coal measures. Still later, conif- 
erous trees appeared and some species of these 
early conifers have persisted to the present time. 
Thus, the bald cypress of the southern states seems 
to be of the same species as a tree which occurs as a 
common and widespread fossil in a geological era 
before mammals appeared. The famous giant Se- 
quoia and its first cousin, the redwood, which now 
occur only in the mountains of California, were 
common a few million years ago in various places 
scattered over nearly the whole of the northern 
hemisphere. They are the disappearing relics of 
an older geological period. Of the flowering plants, 
only the simpler types are represented by the oldest 
fossils; the more specialized types are represented 
by fossils of only later periods. 
The following table shows the succession of the 
various major geological periods together with their 
characteristic plants and animals. EVIDENCES OF EVOLUTION 
73 
Geologic Chronology 
{Adapted from Pirsson and Schuchert) 
Eras 
Periods and 
Epochs 
Advances in Life 
Dominant 
Life 
Psychozoic 
Recent (Allu- 
vial or Post- 
Glacial) 
Rise of world civilization 
The era of mental life 
Age of Man 
Cenozoic 
Quaternary 
(Glacial or 
Pleistocene) 
Tertiary 
Pliocene 
Miocene 
Oligocene 
Eocene 
Extinction of great mam- 
mals 
Transformation of man 
ape into man 
Culmination of mammals 
Rise of higher mammals 
Vanishing of archaic 
mammals 
Age of 
Mammals 
and Modern 
Floras 
Mezozoic 
Epi-Mezozoic 
Interval 
Cretaceous 
Comanchian 
Jurassic 
Triassic 
Rise of archaic mammals 
Extreme specialization 
and extinction of great 
reptiles 
Rise of flowering plants 
Rise of birds and flying 
reptiles 
Rise of dinosaurs 
Age of 
Reptiles 74 
EVOLUTION 
Eras 
Periods and 
Epochs 
Advances in Life 
Dominant 
Life 
Paleozoic 
Epi-Paleozoic 
Interval 
Carboniferous 
Permian 
Pennsylvan- 
ian 
Missi ssip- 
pian 
Devonian 
Extinction of ancient life 
Rise of ammonites, mod- 
ern insects, and land 
vertebrates 
Rise of insects and primi- 
tive reptiles 
Rise of echinoderms and 
ancient sharks 
Rise of amphibians and 
first known land floras 
Age of 
Fishes 
Silurian 
Ordovician 
Cambrian 
Rise of scorpions and 
lung-fishes 
Rise of land plants, cor- 
als and armored fishes 
First known marine 
faunas and rise of 
shelled animals; domi- 
nance of trilobites 
Age of 
Higher 
(Shelled) 
Invertebrates 
Protoerozoic 
Great E p i - 
P r oterozoic 
Interval 
Algonkian 
Neo - Lauren- 
tian 
Paleo- Lauren- 
tian 
Age of 
Primitive 
Marine In- 
vertebrates 
and Uni- 
cellular Life EVIDENCES OF EVOLUTION 
In some cases the paleontological record is so 
nearly complete that the transformation of certain 
animal kinds can be followed through the ages with 
remarkable detail. The horse is the classic example. 
Its paleontological history is largely revealed by a 
series of American fossils gradually discovered in 
rocks of the Tertiary and early Quaternary ages. 
These fossils are the remains of about thirty differ- 
ent kinds of horselike animals. 
The Eohippus, the earliest of these, found in the 
oldest Tertiary rocks (Lower Eocene epoch) was 
little larger than a fox, and its forefeet had four 
hoofed toes, with the rudiment of a fifth, while the 
hind feet had three hoofed toes. Next, in higher 
strata (Middle Eocene), are the remains of 
Orohippus, also small, but with the rudimentary fifth 
toe of the forefeet gone. Next appear, in the higher 
strata of the Lower Miocene, the fossils of Meso- 
hippus, about the size of a sheep, in which the fourth 
toe of the forefeet is rudimentary and useless, and of 
Miohippus, of similar size, in which the rudiments 
of the fourth toe is almost gone. Also the middle 
toe and hoof of the three usable toes in each foot 
are larger than the other side ones. In the Upper 
Miocene and Lower Pliocene appear the fossils of 
Protohippus, a horse about the size of a donkey, 
with three toes, but with the two side toes on each 
foot reduced in size and probably no longer of use 
75 76 
EVOLUTION 
in walking. In still higher Pliocene rocks comes 
Pliohippus, an "almost complete horse," with hoofed 
toes reduced to one (the middle one) on each foot 
and the side toes reduced to mere splints. Finally, 
in early Quaternary time, comes Equus, type to 
which the present horse belongs, with splint bones 
still smaller and middle toes with rounder hoof. It 
also differs from Pliohippus somewhat in shape of 
skull, length of molar teeth and other details. 
Similar series of fossils representing the gradatory 
development of the elephants and the camel family 
have been beautifully worked out by the paleontolo- 
gists. The earliest animal, called Moeritherium, 
recognized as belonging to the elephant series, lived 
in Egypt in early Tertiary time, and was about three 
feet high. Between this small elephant type and the 
huge mammoths, recently extinct, and the elephants 
of the present day, runs an illuminating series repre- 
sented by fossils of Paleomastodon from Tertiary 
strata in Egypt and India, Trilophodon from still 
higher rocks in Africa, Europe and North America, 
Mastodon from more recent strata in Asia, Europe 
and North America, and Stegodon from the Pliocene 
of Southern Asia and North America. 
The camel family presents another interesting 
series. It now has two main subdivisions, one in- 
cluding the true camels of the Old World, the sec- 
ond, the llamas, guanacos, and others of South EVIDENCES OF EVOLUTION 
77 
America. But for a long geologic time the family 
was entirely confined to North America, where there 
are now no native representatives of it at all. Its 
oldest known members were small animals hardly 
larger than a jack rabbit which lived in North 
America in Upper Eocene time. They had four 
hoofed toes on each foot. 
Another less familiarly known but even more 
striking example of the gradatory appearance of suc- 
cessive animal kinds-which in itself means transmu- 
tation and line of descent-is that which has been 
provided by certain paleontologists who have studied 
intensively the ammonites. This group of curious 
cephalopod mollusks first appeared in Silurian time, 
flourished for several geologic ages, and then became 
extinct, except for one genus of three or four species, 
in the Cretaceous Age. This exception, the only 
living example of this once large and highly devel- 
oped group of mollusks, is the pearly nautilus, the 
many chambered spiral shell of which, with its inner 
surface lined with beautiful nacre, is a familiar curio. 
The other nearest living relatives of the ammonites 
are the squids and octopuses. 
By assiduous study of the abundant fossils of the 
shells of the many kinds of ammonites found in the 
rocks of the geologic ages in which these animals 
flourished, the paleontologists have worked out 
a series of forms, beginning with simple straight 78 
EVOLUTION 
shells, going on to surved ones, then on by grada- 
tions to elaborately spiral ones. These shells, of 
which several hundred fossil species have been found, 
vary from half an inch to a yard in diameter, and 
present, in addition to the gradatory steps in shape, 
a closely continuous series of types of sutures be- 
tween the different chambers running from simple 
straight lines to wavy, and then on by readily dis- 
tinguishable steps to most complexly frilled ones. 
These steps appear successively in different periods 
of geologic time and enable the paleontologists to 
trace the various evolutionary lines within the group. 
But the most striking and informing result of this 
intensive study of the fossils of a group of animals 
which, but for the exception of one representative 
genus, has been extinct for several million years, is 
the discovery that by carefully taking apart the fossil 
shell of one of the more complex, or higher, types 
of the ammonites, and examining closely the sutures 
and the shape of the successive shell chambers, which 
are characters perfectly preserved in the fossils, the 
embryology of this type, at least as regards shell 
formation, can be worked out. For each ammonite, 
beginning as a larva, developed its shell by the for- 
mation and addition of successive chambers, living 
always in the outer or latest chamber. The revela- 
tion of the course of descent of ammonites, derived 
from this embryonic history of a more recent com- EVIDENCES OF EVOLUTION 
plex ammonite, confirms that derived from the 
paleontological history of a series of adult types. 
Altogether the outstanding character of the tale 
of the history of the earth and of the plants and 
animals which have lived on it, which is the tale of 
paleontology, is that it reveals, just as comparative 
anatomy and embryology reveal, the fundamental 
identity of life, and of its steady continuity and 
gradatory progress, and of the genetic relationships 
and the adaptations to changing environment of 
organisms. This is evolution. The great plan of 
life has been slowly and continuously unrolled. The 
great possibilities of life have been steadily unfolded. 
And this unrolling and unfolding is evolution. In 
the paleontological record we see evolution in action 
during the ages. Paleontology is a perfect proof of 
evolution. Evolution is the perfect, and only per- 
fect, explanation of paleontology. 
79 
The Geographical Distribution of Plants 
and Animals 
Paleontology treats of the distribution of organ- 
isms in time; plant and animal geography of their 
distribution in space. These two matters are inti- 
mately connected, for changes in geographical dis- 
tribution occur with the passing of time. A map 
showing the distribution of a certain kind or kinds 
of animals or plants in one geologic age may not be 80 
EVOLUTION 
true for another. For example, the earliest camels, 
those absurd little camel creatures of the size of a 
jack rabbit, which lived in early Eocene time, were 
limited to North America, while the various present- 
day members of the camel tribe live, as natives, 
exclusively in the Old World and South America. 
Even the map of the world itself as the geologist 
would draw it for the Tertiary Age would not be a 
true map for the present age. There have been 
upheavals and subsidences of great land masses 
during the earth's history. Where now is the wide 
dry desert west of Great Salt Lake, there was once 
a great inland ocean. England and Europe were 
once a continuous land mass. The Mediterranean 
Sea has changed shape and position very materially. 
In comparatively recent times America and Asia 
were joined by continuous land where now are the 
Aleutian Islands and Bering Strait. 
With the differences in the configuration of the 
earth's crust in different geologic times there were 
also great differences in temperature and climate of 
specific land regions. In Miocene times Greenland, 
Iceland and Spitzbergen were covered with a luxu- 
rious temperate vegetation, as revealed by the fossil 
plant remains in these countries. In the late Glacial 
period the polar ice sheet extended south to 
40° North latitude in America and $0° in Europe, 
so that the temperate plants of Greenland were EVIDENCES OF EVOLUTION 
81 
pushed south to the shores of the Gulf of Mexico 
and the Arctic plants which line the border of the 
polar ice sheet were pushed to the middle of Europe 
and America. Of course many species of plants and 
animals were killed out by these great world changes, 
so that plant and animal geographic distribution 
to-day is partly the result of great happenings in 
the geologic period just preceding our time. 
But the distribution of the living plant and animal 
kinds of to-day has been determined not alone by 
earth changes in earlier times but by present earth 
conditions. Practically all plant and animal species 
tend to be pressing out in every direction from their 
center of distribution, and are only restrained from 
spreading indefinitely by the existence of various 
barriers. These barriers may be oceans or great 
lakes, mountain ranges, deserts, forests, marked 
differences in climate, or the influences exerted by 
the presence and activities of humankind. Some- 
times animals and plants are helped across these 
barriers by artificial means, most notably by the 
unintentional or intentional aid of man, and thrive 
perfectly well and spread rapidly in their new homes. 
The black rat of Europe was introduced into 
America about the middle of the sixteenth century 
and throve so that it almost crowded out the native 
American wild rats, only to be itself nearly exter- 82 
EVOLUTION 
minated by the Old World brown rat which was 
introduced about 1775. 
These rats were brought across the Atlantic unin- 
tentionally in ships. But the mongoose was intro- 
duced intentionally into Jamaica, the rabbit into 
Australia and the English sparrow into the United 
States, all with similar results of such a rapid and 
enormous increase as to make each a pest. A ma- 
jority of the worst insect pests now in America are 
of foreign origin, brought unintentionally to this 
country on introduced nursery stock, plant cuttings, 
bulbs, etc., all of them finding America an excellent 
breeding ground. Now American entomologists 
roam the world over seeking the natural predaceous 
and parasitic insect enemies of these pests in their 
native lands, and attempting to introduce them here 
to keep them in check. On the other hand, repeated 
attempts to introduce the desirable nightingale, 
starling and skylark from Europe have been failures. 
Thus the present natural distribution of animal 
kinds is determined partly by actual physical bar- 
riers, such as oceans and mountains, and partly by 
unfavorable living conditions outside of the natural 
range of a given kind. This applies also to plants. 
Man alone has special means of crossing barriers 
and special means of adapting himself to all varie- 
ties of world conditions. Hence man has found his 
way to all regions of the earth and can persistently EVIDENCES OF EVOLUTION 
83 
maintain himself in these regions. Yet, as we are 
familiarly aware, the variety of these conditions 
have had their effect in modifying him, so that there 
is on the wrhole a recognized natural distribution of 
the various human races. The races of the tropics 
differ markedly from those of the temperate zones, 
and these from those of the arctic regions. 
The present natural distribution of plants and 
animals presents an interesting and often puzzling 
lot of conditions. But many of these can be bril- 
liantly explained by evolution, and thus become im- 
pressive evidences of evolutionary reality. It was, 
indeed, especially because of their observations on 
the puzzling distribution of various animal kinds 
and groups that both Darwin and Wallace were first 
so insistently driven to an evolutionary explanation 
of this distribution. No other explanation yet of- 
fered, least of all that of specific creation, has such 
a satisfying reasonableness. 
Just as the paleontologists divide earth history 
into a series of geologic and biologic ages and epochs, 
so the students of the geographical distribution of 
organisms divide earth regions and their faunas 
and floras into a number of great realms, with sub- 
sidiary regions within each realm. These realms 
are called the 1 Arctic, North Temperate, South 
1 Other divisions by realms with more or less different realm 
names have been made by various students of distribution. 84 
EVOLUTION 
American, Indo-African, Patagonian, Lemurian (or 
Madagascarian) and Australian. Of these, the 
Australian realm alone is sharply defined. The 
others have outlines rather hazily marked, there 
being much overlapping along their boundaries. The 
general outlines of these boundaries are sufficiently 
indicated by the realm names. 
The distribution of plant and animal kinds among 
these realms, and their subdivisions has obviously 
been determined by several factors; some of these 
are paleontological in character, some strictly geo- 
graphical and topographical, and some adaptive. 
David Starr Jordan, who has given much attention 
to these problems, has formulated certain interest- 
ing generalizations concerning the distribution of 
animal kinds, and these generalizations apply 
equally well to plants. They are as follows: 
"Every species of animal is found in every part 
of the earth having conditions suitable for its main- 
tenance unless: 
"(a) Its individuals have been unable to reach 
this region, through barriers of some sort; or, 
"(Z?) Having reached it, the species is unable 
to maintain itself, through lack of capacity for 
adaptation, through severity of competition with 
other forms, or through destructive condition of 
environment; or, EVIDENCES OF EVOLUTION 
85 
"(c) Having entered and maintained itself, it 
has become so altered in the process of adaptation as 
to become a species distinct from the original type." 
It is the situation referred to under (c) that pre- 
sents especially brilliantly the evidence for evolution 
derived from distributional conditions. Take, for 
example, the conditions presented by the plants and 
animals on oceanic islands, conditions which, as 
already said, were of especially large influence in 
leading both Darwin and Wallace to their belief in 
evolution as the only reasonable explanation of the 
peculiar facts of animal and plant distribution. In 
fact, biographers of Darwin find reason to believe 
that his study of the fauna of the Galapagos Islands 
first fastened his mind on the evolution idea and 
convinced him of the reality of evolution. These 
islands, situated in the Pacific about 500 miles west 
of the South American coast, are of volcanic origin, 
and there are no evidences of their ever having had 
a land connection with the American continent, cer- 
tainly not in recent geologic ages. The depth of the 
ocean around them varies from 2000 to 3000 
fathoms or more. Their animals must have been 
either specially created on them since their upheaval 
from the ocean as volcanoes, or derived in some 
other way. There is such a way which appeals 
strongly to our knowledge of distributional methods 86 
EVOLUTION 
as we know them by actual observation. It is a way 
in perfect line with the assumptions of the evolu- 
tion idea. Let us follow Darwin himself in his 
observations and reasoning in connection with the 
Galapagos fauna. 
"Here," he says, "almost every product of the 
land and of the water bears the unmistakable stamp 
of the American continent. There are twenty-six 
species of land birds; of these twenty-one, or per- 
haps twenty-three, are ranked as distinct species, and 
would commonly be assumed to have been here 
created; yet the close affinity of most of these birds 
to American species is manifest in every character, 
in their habits, gestures and tones of voice. So it is 
with the other animals and with a large proportion 
of the plants, as shown by Dr. Hooker in his admi- 
rable Flora of this archipelago. The naturalist, 
looking at the inhabitants of these volcanic islands 
in the Pacific, distant several hundred miles from 
the continent, feels that he is standing on American 
land. Why should this be so? Why should the 
species which are supposed to be created in the Gala- 
pagos Archipelago, and nowhere else, bear so plainly 
the stamp of affinity to those created in America? 
There is nothing in the conditions of life, in the 
geological nature of the islands, in their height or 
climate, or in the proportions in which the several 
classes are associated together, which closely re- EVIDENCES OF EVOLUTION 
87 
sembles the conditions of the South American coast; 
in fact, there is a considerable dissimilarity in all 
these respects. On the other hand, there is a con- 
siderable degree of resemblance in the volcanic 
nature of the soil, in the climate, height and size of 
the islands, between the Galapagos and Cape de 
Verde archipelagos; but what an entire and absolute 
difference in their inhabitants 1 The inhabitants of 
the Cape de Verde Islands are related to those of 
Africa (these islands lie 300 miles off the west coast 
of Africa) like those of the Galapagos to America. 
Facts such as these admit of no sort of explanation 
on the ordinary view of independent creation; 
whereas in the view here maintained it is obvious 
that the Galapagos Islands would be likely to receive 
colonists from America by flight, on and in floating 
logs, etc., and the Cape de Verde Islands from 
Africa; such colonists would be liable to modification 
t-the principle of inheritance still betraying their 
original birthplace." 
Since Darwin's time, the fauna and flora of these 
islands have been more intensively studied, with re- 
sults expanding, but wholly confirmatory of Darwin's 
observations. Of the birds, more than two thirds 
are species peculiar to the islands and almost all the 
nonpeculiar species are strong-flying and swimming 
aquatic birds capable of crossing wide distances of 
ocean. The true land birds are all, with but few 88 
EVOLUTION 
exceptions, of species peculiar to the islands, while 
more than half of them are of peculiar genera. But 
all these birds are unmistakably allied to South 
American kinds, and they present most beautiful 
series of gradations from perfect identity with con- 
tinental species to genera so distinct that without the 
existing gradatory kinds it would be difficult to de- 
termine to what continental forms they are most 
nearly allied. 
Other volcanic groups of oceanic islands, as the 
Azores, the Bermudas, and the Hawaiian Islands, 
all have an animal and plant life that tells a similar 
story: almost no native terrestrial vertebrates except 
birds, and these of genera and species peculiar to 
each group, but most nearly allied with the bird 
kinds of the nearest continent; the aquatic birds 
mostly of the same kinds as those of the general 
realm in which the islands lie; the familiar animal 
and plant species that travel with man, common to 
all the island groups; the more isolated the island 
group the fewer and the more strictly peculiar the 
animal and plant kinds present; series of gradatory 
forms present in more or less approximate complete- 
ness revealing the changes from original immigrant 
from the continent to latest and most dissimilar 
related species or genus developed from it. 
Australia, which, although continental in area, 
may be looked on as a great island which has been EVIDENCES OF EVOLUTION 
89 
separated for a long geologic time from its nearest 
continent, Asia, presents an interesting case of 
mammalian distribution. The only known living rep- 
resentatives of the lowest order of mammals, the 
curious duckbills and echidnas (Monotremes), are 
found only in Australia and the near-by islands 
of Tasmania and New Guinea. And all of the 
marsupials (kangaroos, wallabies, opossum), con- 
stituting the next lowest order, are, with the ex- 
ception of the opossum, similarly restricted to Aus- 
tralia and neighboring islands. The additional fact 
that these lowly organized mammals are, with the 
exception of various rats, mice and bats, almost the 
only native mammals found in Australia, makes the 
Australian mammalian situation a very interesting 
one. The explanation, reasonable and consistent 
with the known paleontologic facts, seems to be that 
Australia became separated from all other land 
masses not later than in the Jurassic or Cretaceous 
Age, at which time there yet existed in the world no 
other mammals than various small and most lowly 
organized ones. After the separation, the ocean 
barriers prevented the migration into Australia of 
the higher mammalian types, which were later de- 
veloped in the great land masses of the Old World 
and America with their wide and stimulating diver- 
sity of climate, topography and living conditions in 
general, and hence the Australian marsupials have  EVOLUTION 
90 
had no competition, have flourished, and have devel- 
oped a considerable variety of forms. Some of 
these show adaptive, although not fundamental, like- 
nesses to such higher forms as rodents and car- 
nivores. The ancestors of the present rats and mice 
and bats which are now abundant in Australia must 
have arrived from the Asiatic shore in later-prob- 
ably late Tertiary-times, by being carried on float- 
ing logs, or, in the case of the bats, by flight. But 
the large Asiatic mammals have been unable to reach 
Australia. The rapid multiplication of rabbits and 
foxes, introduced recently by man, show how easily 
other mammals might have flourished in Australia 
if they had not been shut out by the ocean barrier. 
With so much space given to the interesting dis- 
tributional conditions presented by ocean islands we 
cannot refer in detail to any of the special features 
presented by continental distribution. Such prob- 
lems, however, are all around us. The meadow 
lark, for example, which we have here in California, 
seems, at first sight, to be just like the meadow lark 
of the East. But closer attention to it shows that it 
has certain slight but positive differences in color 
pattern. Its song, too, is recognizably different. 
The pair of woodpeckers which have a nest in a pine 
tree by my cottage are like a familiar Eastern species 
-but with noticeable slight differences. The flicker 
has reddish instead of yellow wing shafts. Why EVIDENCES OF EVOLUTION 
these differences? Californian life conditions are 
somewhat different from those of the East. The 
birds here respond in one way or another to these 
differences. They are not yet different species of 
birds; but they are on their way to be. They are 
called different varieties or subspecies, which is an 
evolutionary step toward being different species. 
If one traverse a continent in the Northern Hem- 
isphere from south to north, from tropic or subtropic 
regions to north temperate and on to arctic regions 
and then cross it from east to west, he will notice 
greater differences among the plants and animals 
as he moves across the latitudes than are evident 
in moving from east to west. The reason is that 
differences in latitude mean greater differences in 
living conditions than do differences in longitude. 
If we climb a high mountain situated in a south- 
ern region we can mark out by the distribution 
of plant and animal life on it a series of zones cor- 
responding in some degree with differences in lati- 
tude. At the bottom there will be a subtropic zone, 
above it a temperate one, above that a transition 
zone and at the summit an alpine or arctic zone. 
Different kinds of plants and animals fitting the 
different zones form a gradatory series of organ- 
isms in a gradatory series of life conditions. 
But we must make an end of this discussion. Per- 
haps we can most usefully do it by quoting a short 
91 92 
EVOLUTION 
statement recently published by Professor Newman 
of the University of Chicago, summarizing the evi- 
dence for evolution based on geographical distribu- 
tion : 
"On the hypothesis of special creation, or on any 
other hypothesis except evolution that has ever been 
suggested, the extremely intricate patchwork of ani- 
mal and plant distribution remains an unsolvable 
picture puzzle, without rhyme or reason. When this 
puzzle is attacked with the aid of the evolutionary 
idea, the key to the whole maze is furnished, and 
the difficulties clear up with remarkable ease. The 
whole hodgepodge makes sense and we can under- 
stand many previously irreconcilable facts. In no 
field does the working hypothesis of evolution work 
to such advantage as in this field. 
"On the basis that a species arises at one place, 
spreads out over large areas, becoming modified as 
it goes, that new species are formed from old 
through modification after isolation from the parent 
stock, how do the facts of distribution look when 
examined in detail? 
"i. Cosmopolitan groups, those with the widest 
distribution, are those to whom no barriers are suffi- 
cient to check migration, for example, strong fliers, 
man, earthworms carried by man. 
"2. Restricted groups are usually those to which EVIDENCES OF EVOLUTION 
barriers are readily set up and are frequently the 
last remnants of a formerly successful fauna or flora, 
which continue to survive only in some restricted 
area where the conditions are rather more favorable 
than elsewhere. 
"3. The study of the distribution of species be- 
longing to a single genus reveals that the more primi- 
tive or generalized species occupy a central position, 
and the most specialized species are at the outer 
boundaries of the distributional area. 
"4. The faunas and floras of continental islands 
are just what we should expect on the basis that there 
was at one time a land connection with the nearest 
continent; that at this time the faunas and floras 
were the same on both island and continent; that, 
later, the continent and island were separated by an 
impassable barrier of ocean; and that the inhabitants 
of the two bodies evolved separately. 
"5. The faunas and floras of oceanic islands are 
like those of the nearest mainland and are of those 
types, for the most part, that might most readily 
have been blown or carried on floating debris. 
"6. The conclusions arrived at by students of 
geographic distribution, past and present, as to the 
existence of former land connections, now broken, 
are borne out by the independent findings of geolo- 
gists and geographers." 
93 CHAPTER VI 
CAUSAL EXPLANATIONS OF EVOLUTION 
Perhaps it is not a bad thing that Mr. Bryan and 
the Fundamentalists are stirring up matters about 
evolution, and hence stirring up the evolutionists to 
interrupt for a moment their evolutionary research 
in order to take stock of their present knowledge and 
to tell the public, in more or less intelligible language, 
just where evolution now stands. What has been 
learned about evolution since Darwin? What are 
the special things that still need to be learned? 
The principal thing needing now to be known 
about evolution, is to know what causes it. This 
has, indeed, been an outstanding need all along. 
Biologists have, for a long time, had no doubts at all 
about the reality of evolution, but they have always 
had doubts about the validity of the various causes 
that have been suggested to explain it from the times 
of the Greeks to those of the mutationists and the 
Mendelians-which are the times of to-day. Oddly 
enough the antievolutionists have taken little advan- 
tage of this uncertainty among the evolutionists con- 
cerning the causal explanation of evolution. They 
have mostly devoted themselves to affirming dog- 
94 EXPLANATIONS OF EVOLUTION 
95 
matically, or trying to prove, that there is no such 
thing as evolution; at least, and particularly, no such 
thing as the evolution of man. They could have 
made more trouble if they had stressed more the 
differences of opinion among the evolutionists re- 
garding the causes and control of evolution. 
I carefully say causes, not cause, for it is quite 
certain that there is no one thing alone that causes 
evolution. There are certainly several or many 
causal factors, that work together in combination. 
Some of these factors we know, and we understand 
something of the cooperative relation among them. 
But some of the secrets of the combined working of 
the known factors we do not know, and, in addition, 
we almost certainly do not know some of the factors 
themselves. The "unknown factors of evolution" 
are the biologist's great riddle to-day. 
But-let me repeat-because the biologists do not 
know, or only partially know, the causes of evolu- 
tion, to assume from this that they have any doubts 
at all of the reality of evolution, would be to assume 
what is not true. I do not know of a single living 
biologist of high repute-and I do not determine 
repute on a required basis of a belief in evolution!- 
who does not believe in evolution as a proved part 
of scientific knowledge. It is as well-proved a part 
as many other parts of this knowledge that we all 
readily accept. 96 
EVOLUTION 
Since Darwin's day much has been added to our 
knowledge of the facts about the manner and the 
effect of evolution, but only two important new al- 
leged factors have been presented for consideration 
as primary causes of evolution; these are mutations 
and Mendelian inheritance. There is no general 
agreement among naturalists that either is a suffi- 
cient, or is even a chief explanation of either species 
forming or adaptation, which are the coordinate 
fundamental problems of organic evolution. In this 
same post-Darwinian period, also, the two most 
important explanations of evolution current in Dar- 
win's time, namely, Lamarckism, based on the inher- 
itance of acquired characters, and Darwinism, based 
on natural and sexual selection, have been weakened 
rather than strengthened as sufficient causes of evo- 
lution. Hence we are in the curious position of 
knowing now much more about evolution than was 
known a half-century ago, but of feeling much less 
confident that we know the whole story of the causes 
of evolution. If this is ammunition for the antievo- 
lutionists let them make what use of it they can. We 
can afford to be honest. 
The two basic coordinate phenomena which any 
causal explanation of evolution must explain satis- 
fyingly are, as has been pointed out in earlier chap- 
ters, first, the great number and variety of plant 
and animal kinds (species) together with their ge- EXPLANATIONS OF EVOLUTION 
97 
netic relationships, and, second, the adaptation, often 
remarkably precise, of these species, in both struc- 
ture and function, to their special environment 
and ecologic relations. Any satisfactory explanation 
of evolution must explain both of these actually 
existing phenomena. Both Lamarck and Darwin 
faced this necessity squarely. As much cannot be 
said of the mutationists and Mendelians. 
Lamarck's explanation is simple and plausible. 
It bases itself on the familiar fact that plant and ani- 
mal individuals do become adaptively modified dur- 
ing their lifetime in response to environmental condi- 
tions. It assumes that such individually acquired 
changes or characters are passed on, in some degree, 
by heredity to the offspring which in turn, granted a 
similar environment, further change in the same 
direction and similarly pass on their changes by 
heredity. So on through succeeding generations, 
until new types of form and behavior, and increased 
degrees of adaptation or fitness result. 
A plausible explanation, but one wholly depend- 
ent on the "inheritance of acquired characters," 
which, unfortunately, does not seem to happen. Both 
extensive observation and intensive experimentation 
unite in shattering this absolutely essential assump- 
tion in the Lamarckian explanation of evolution. 
The germ plasm from which new individuals arise is 
so distinct from the rest of the body in the parent 98 
EVOLUTION 
individuals, so protected from the influence of ex- 
ternal conditions or of local changes in other body 
parts, that there seems to be no means for causing 
it to produce in its development into new individuals 
a replica of the local changes suffered by the parent 
body in its lifetime. And this conclusion, arrived at 
by modern study of the germ plasm and heredity 
mechanism, is confirmed by the observed results of 
completed development. Acquired characters, in 
the Lamarckian sense, are not inherited. 
Darwin's explanation of species change and 
adaptation is based, like Lamarck's, on both certain 
observed facts and certain assumptions. Small, 
spontaneous, fortuitous variations appear in all new 
individuals born-this is an observed fact-and 
there is an overproduction of young in every spe- 
cies-another fact. Hence, Darwin assumed that 
there will be a severe struggle for existence by these 
young for place and food among themselves and in 
competition with the young of other species. In 
the course of this struggle these small variations 
will play a life-preserving or life-losing role, depend- 
ing on whether in the face of the environment they 
are advantageous or disadvantageous. Those young 
which are better, even very slightly better, equipped 
for this struggle, by virtue of their variations, this 
"better" being, therefore, in the direction of fitness, 
will win in the struggle and leave offspring varying EXPLANATIONS OF EVOLUTION 
99 
as themselves-assuming these variations to be in- 
herited-while the others will be extinguished to- 
gether with their disadvantageous variations. By- 
cumulation through generations this "natural selec- 
tion" through the "survival of the fittest" will result 
in species change and increasing adaptation. 
Also a plausible explanation, but weakened, if 
not shattered, as far as species forming is concerned, 
by the results of modern biological study, which have 
shown that many of these small variations are not 
inherited. They are merely fluctuations around a 
mean, to which mean the offspring tend constantly 
to return. Besides, it is asking too much to ascribe 
a life-or-death-determining value to these minute 
variations, despite any conceivable intensity of the 
struggle for existence. Indeed, most of the species 
differences-let alone the individual differences- 
among such animals as the insects and others repre- 
sented by large numbers of species, are of a kind 
which demand a very lively imagination to be recog- 
nized as of life-and-death-determining value. There 
is a large family of little beetles called ladybird 
beetles, among which some of the species are dis- 
tinguished from each other by very slight differences 
in the number of size or color of minute spots on 
the wing covers. Similarly many little flies are dis- 
tinguished by the number and size of small bristles 
on the back and small differences in wing venation. EVOLUTION 
100 
One often needs a hand lens to discriminate between 
them. Now, are these differences, which we have 
to reinforce our eyes to see, going to decide whether 
a toad or lizard or insect-eating bird sees and de- 
vours, or does not see and devour, individuals of 
one rather than another of these insect kinds-or, 
even more fantastic, one individual rather than an- 
other, both belonging to one species and differing 
from each other by even more microscopic varia- 
tions? 
Mutations are larger variations which are un- 
doubtedly heritable. They were recognized by 
various earlier students of evolution as possible 
factors in the origin of new species, and were not 
unfamiliar to plant and animal breeders, under the 
name of "sports," as the actual beginnings of new 
races or varieties of domesticated plants and ani- 
mals. But it was not until the results of the long 
and painstaking observations and experiments of 
Hugo de Vries, the Dutch botanist, on the large- 
flowered evening primrose (Oenothera lamarck- 
iana) and certain other plants, were published in 
1901, that mutations were seriously considered by 
any considerable number of biologists as possible 
chief elements in species forming. Before De Vries, 
and only ten years after Darwin published the 
Origin of Species, von Kolliker, the great German 
zoologist, in criticizing the assumptions on which EXPLANATIONS OF EVOLUTION 
101 
species forming by natural selection was based in the 
Darwinian explanation, especially the use of the 
small fortuitous fluctuations as handles for selection, 
proposed an alternative theory of species forming 
by leaps (saltations). These saltations, von Kolli- 
ker held, need not be large, but must be changes 
definite and fixed. Later, Korschinsky, a Russian 
botanist, outlined in some detail and with greater 
emphasis a theory of species forming by "hetero- 
genesis" or mutations. Darwin himself referred to 
certain well-known sports which had given rise to 
new races of domestic cattle and sheep (hornless 
Paraguay cattle, short, bent-legged, Ancon sheep; 
extra long, smooth, straight and silky wooled 
Mauchamp-merino sheep). And the well-known and 
now widely spread cattle race called Polled Here- 
ford originated comparatively recently from one or 
more hornless bulls which appeared as sports in the 
horned race. But these were all looked on as excep- 
tional cases, playing little part in the general evolu- 
tion of species. 
Since the work of De Vries, and the growing dis- 
satisfaction with the Darwinian explanation, the 
mutations explanation of evolution has gained a con- 
siderable following. But mutations are, so far as 
much careful observation goes to show, not abun- 
dant, nor can they be assumed to be adaptive in 
character. They may be so pathologic or abnormal EVOLUTION 
as to insure early death to the individual showing 
them, and to that extent are "selected out." That 
is, the very bad ones get extinguished, but if not too 
bad they may persist and really establish a new 
species. That they actually do this is a proof that 
it is not merely the fittest which survive: it is just 
the sufficiently fit that survive. But to explain the 
extraordinary precise adaptations of orchids and 
other insect-pollinated flowers to their insect visi- 
tors, and the equally extraordinary adaptations of 
these visitors to their plant hosts, or the remarkable 
adaptations of parasites, or of protectively colored 
and patterned butterflies and moths, mutations are 
simply hopeless. 
Then, there is the offered explanation of the 
origin of new species through hybridization in 
nature, and the juggling of characters and character 
combinations in these hybridizations through the 
Mendelian formula of heredity. This Mendelian 
inheritance will be discussed in the next chapter, 
but here again it is only the origin of new kinds 
of plants or animals, and not adaptations, which 
are explained-if anything at all in the way of 
species forming is explained. Thus only one half 
of the evolution problem is even approached by the 
Mendelian explainers. 
An explanation, more auxiliary than replacing in 
its character, was especially urged by Moritz 
102 EXPLANATIONS OF EVOLUTION 
103 
Wagner, an explorer and German naturalist, and by 
Romanes, the brilliant student and upholder of Dar- 
win, and has been strongly championed by David 
Starr Jordan, the eminent American zoologist. This 
explanation has received much support from field 
naturalists and students of systematic botany and 
zoology and of geographic and topographic distri- 
bution. It is that based largely on the element or 
factor of isolation, both geographical and physio- 
logical. Its special strength lies in its great useful- 
ness to the natural selection explanation. Indeed it 
seems self-evident to many naturalists that natural 
selection is impotent as an actual cause of species 
forming without some effective sort of isolation fac- 
tor to assist it. 
Whenever the individuals of a species move evenly 
over an area, its members freely interbreeding, the 
character of the species remains substantially uni- 
form. Whenever freedom of movement and conse- 
quent freedom of interbreeding is checked by some 
barrier or other means, the character of the species 
is rapidly altered. It is changed even though ex- 
ternal conditions seem to be practically identical on 
both sides of the barrier, and if there is no visible 
distinction in the original stock on the two sides. 
Presumably there are differences in the variations 
which become perpetuated in either group. 
If, therefore, individuals of a plant or animal EVOLUTION 
104 
species are able in some way to cross a barrier and 
are then isolated from the rest of the species, or 
are segregated in any way so that free interbreed- 
ing with the bulk of the species is prevented, a grad- 
ual change occurs in the isolated group. 
One of the most striking examples of such species 
and varietal change, accompanying sharp localiza- 
tion of groups of individuals, is afforded by certain 
land snails of the Hawaiian island of Oahu. The 
naturalist Gulick, in a classical study of the distribu- 
tion and changes of these snails which inhabit a series 
of adjoining but rather sharply separated valleys in 
the wooded part of the island, has shown that they 
have become split up into about 175 species, includ- 
ing between 700 and 800 varieties or subspecies. In 
all cases the valleys that are nearest each other 
furnish the most nearly allied forms of the snails, 
while a full set of the varieties of each species pre- 
sents a minute gradation between the more divergent 
types found in the more widely separated localities. 
Romanes pointed out that such a segregation of 
a group or groups of individuals of a species can 
come about by other means than geographic or topo- 
graphic isolation. Anything that could lead to 
exclusive or discriminate breeding among certain 
individuals of a species would result in the isolation 
of these individuals as effectively as their actual 
separation from others by a geographic or topo- EXPLANATIONS OF EVOLUTION 
105 
graphic barrier. Now, there are various influences 
or conditions that might conceivably bring about 
such a state of affairs, and some of these have been 
actually observed to exist. In the case of plants, 
for example, slight differences in the time of flower- 
ing and hence pollination among groups of individ- 
uals in the same region might determine a certain 
physiological segregation of groups within the 
species. 
The evolution explanation by isolation is not with- 
out importance, but, as already said, is more of an 
auxiliary than an independent causal explanation. 
Without question, isolation is an important factor 
in helping to effect species modification. But it is a 
condition, rather than an active force, making for 
species change. 
Many naturalists have called attention to the 
existence of long series of related plant or animal 
kinds which show a succession of small gradatory 
steps along some particular line of modification, 
these steps being of a character which it is not 
reasonable to interpret as of selective value. Some 
of these naturalists have become convinced that the 
explanation for this condition must be found in some 
subtle controlling environmental influence or some 
inherent capacity in the organisms to change in a 
given direction-or, at least, they say when such 
change is once begun by reason of some extrinsic 106 
EVOLUTION 
influence, there must be some force to keep it up 
even to a point where it may prove disadvantageous 
to the more recent and largely modified species and 
hence be arrested by selection. Many paleontolo- 
gists, especially, are convinced that this is a common 
phenomenon, and explain the extinction of various 
lines of plant and animal evolution on this basis. 
This phenomenon is called orthogenesis, or develop- 
ment in straight lines, and is explained by assuming 
the possibility of determinate variation within a 
given line of related organisms. 
Paleontologists see in this orthogenesis and its 
results the explanation for the extraordinary devel- 
opment in size and character, and final extinction of 
the great Jurassic and Cretaceous reptiles, dinosaurs, 
ichthyosaurs, mosasaurs, etc., which were the largest 
land animals that have ever lived, but nearly all of 
which became extinct before the next geological 
epoch (Tertiary). The famous horse series (de- 
scribed in an earlier chapter) with its persistent 
evolutionary increase in size and functional and 
structural reduction of toes is another example. So, 
also, is the classic series of species of the fresh-water 
snails (Paludina) of Slavonia. They extended 
from Tertiary times to the present, and show a per- 
sistent increase in size, addition and roughness of 
whorls and modification of shape of the aperture. 
But all these differences follow by such small grada- EXPLANATIONS OF EVOLUTION 
107 
tory steps that one cannot reasonably attribute a 
selective value to them, and hence cannot accept an 
explanation of the evolution of these snails on the 
basis of natural selection. 
But this orthogenetic or determinate variation 
itself calls for a causal explanation. This has led 
to the rise of two schools of orthogenesists, one of 
which assumes some as yet unexplained environ- 
mental influence capable of setting up and continuing 
such variation, while the other assumes internal in- 
fluences which determine this particular kind of per- 
sistent variation. It is easy to see, however, that 
these explanations are confessions of ignorance of 
evolutionary cause or causes, and readily lead to a 
form of mysticism which is not conducive to the 
advancement of a scientific explanation of evolution. 
In fact, the assumption of determinate variation and 
orthogenesis has already led to the development and 
too wide uncritical acceptance of such an evolution 
explanation as Bergson's evolution creatrice, pre- 
supposing an internal elan in life stuff which compels 
it to move ever on toward complication and special- 
ization along particular lines. But it must be ad- 
mitted that the proposal and more or less general 
acceptance of such semimystic types of evolution 
explanation is an evidence that the other types such 
as Lamarckism, Darwinism, and mutations, are not 
generally satisfactory. The plain truth is, as pointed 108 
EVOLUTION 
out at the beginning of this chapter, that a satis- 
factory causal explanation, or a sufficient number 
and combination of causes of evolution, has yet to be 
worked out. Evolution is a fact, obvious to any 
one who will see it, and accepted by all scientific men; 
but its explanation is incomplete. CHAPTER VII 
FUNDAMENTAL FACTORS IN EVOLUTION 
(Reproduction and Development, Variation, Heredity) 
Even though the full explanation of evolution is 
incomplete, certain inevitable elements in it, however 
it may be formulated, are plainly recognizable. In 
all the various causal explanations of evolution 
which have been offered by different men at differ- 
ent times, there figure certain fundamental phe- 
nomena and conditions common to all life, such as 
variation, heredity, segregation, selection, plasticity 
and adaptive response to environment. Antecedent 
to any evolution, and even to these fundamental fac- 
tors in it, there must be life itself, and the capacity 
of living creatures to reproduce themselves and de- 
velop from egg to egg-producing stage. 
Students of organic evolution are not necessarily 
concerned with the actual origin of life. They are 
concerned with the origin of the great variety of 
form and manner which life assumes. Yet they can- 
not help asking themselves, just as all of us ask, the 
ultimate question: If evolution explains the unfold- 
ing and outrolling of life with its myriad variety, 
what explains the beginning of life? Who, or what, 
109 100 
EVOLUTION 
breathed life into nonlife, and when and how was it 
done? Did life come to this earth from elsewhere 
in the solar system, as the meteors come? Did the 
Creator of matter and energy create life as a special 
act at a special time? Or were the created inorganic 
matter and energy able to create life some time in 
their own evolution? 
As we trace life down from its manifestation in 
the form of the highest, most complex living crea- 
tures to its character in the lowest, simplest organ- 
isms, and at the same time trace inorganic matter 
up from its simplest or elemental forms to its most 
complex combined forms, we find a very suggestive 
approach. Much of the form and behavior, the 
detail of make-up and activity of the simplest organ- 
isms, the microscopic one-celled plants and animals, 
are determined by familiar physical and chemical 
laws. Small masses of oil foam made in the labora- 
tory with a viscosity and colloidal structure similar 
to that of protoplasm, imitate in surprising manner 
the physical appearance and simple movements of 
the simplest organisms. The students of biophysics 
and biochemistry are daily taking some of the mystic 
"vitalism" out of life. 
But nobody has yet made an amoeba in a test tube, 
nor infusoria in a sterilized hay infusion. Pasteur 
and Tyndall long ago exploded the naive claims of 
the believers in spontaneous generation. Omne FUNDAMENTAL FACTORS 
111 
vivum ex •vivo. It is only life that produces life. 
The amoebalike bit of oil foam, with all of its realis- 
tic imitation of amoeba's movements, the most com- 
plex molecules created by the organic chemist, with 
all their identity of chemical elements with proto- 
plasm, are all of that long way from amoeba and 
protoplasm which is measured and defined by the 
phrase nonlife and life. There is a great gulf 
between what is living and what is not. And that 
gulf creates the great question for evolutionists and 
nonevolutionists alike; the question of the origin of 
life. 
The thoroughly logical evolutionist, or trans- 
formationist, who sees the whole world, inorganic as 
well as organic, with all its present variety of matter 
and form, as the result of slow continuous transmu- 
tation and evolution-a view greatly strengthened 
by the recent revelations in radioactivity and atomic 
structure and behavior-simply says, sometime, 
somewhere, some way, living matter, in its simplest 
form, arose from nonliving matter, probably in its 
most complex form. But he has not seen that hap- 
pening, nor does he attempt to say when, where, 
or really how, it happened. He does occasionally 
amuse himself by guessing at possible "hows," but 
that is chiefly because of the pressure of his con- 
sistency. EVOLUTION 
112 
Variation 
But in whatever way living matter, with its ca- 
pacity for self-reproduction, was, or, possibly, is, 
produced, it exists. Evolution has it ready at hand. 
But there could be no evolution of organisms unless 
this life stuff itself varied in its reproduction. 
Whether this variation, slight or large, is imposed 
on reproducing life stuff by the inevitable variability 
of the conditions under which reproduction takes 
place-there can certainly be no identity of such 
conditions in two succeeding instants of time or two 
different points of space-or whether this variation 
is something inherent and spontaneous in the complex 
physico-chemical phenomenon we call life, the ob- 
served fact is that such variation does always occur. 
As I have already said, there are no two animal or 
plant individuals, whether of different or the same 
species, whether the offspring of different or the same 
parents, whether usual twins, or even so-called iden- 
tical twins, which are alike. This unlikeness begins 
with their beginning as separate organisms. Varia- 
tion is a fact. It is the basic factor of evolution. 
It is a basic element in any causal explanation of 
evolution. 
In Darwin's explanation of evolution, variations, 
small, spontaneous, fortuitous variations, as well as 
larger ones called sports, were the building stones FUNDAMENTAL FACTORS 
113 
with which he began the erection of his explanation 
of species forming and adaptive change by natural 
selection. He took these variations as given by na- 
ture, and assumed their heritability. He then incor- 
porated into his explanation the further observed 
fact of overproduction of young by all parents. All 
parents normally produce either at one time or at 
scattered intervals more than enough offspring to 
replace themselves. The female codfish has been 
known to produce 9,000,000 eggs in one year; the 
Columbia salmon produces 4,000, and then dies. 
Some seabirds lay but one egg a year, but they con- 
tinue to produce eggs for several years. The ele- 
phant, reckoned the slowest breeder of all animals, 
does not begin to produce young until thirty years 
old, but it continues breeding until ninety, producing 
an average of six young in the interval. 
On the basis of this overproduction, which would 
lead, should all produced young survive to maturity, 
to the filling of the sea by codfish and the covering 
of the land by elephants, Darwin predicated a sharp 
struggle for existence by the young of any species. 
This he saw as a struggle among themselves for 
place and food, a struggle between them and the too 
many young of other species needing the same place 
and food, a struggle between them and their pre- 
daceous and parasitic enemies, and a struggle with 
cold, and heat, and wet, and dryness and inclement 114 
EVOLUTION 
nature generally. He then assumed that those indi- 
viduals with variations, however small, that were of 
advantage to them in this struggle would live to pro- 
duce more young, while those with disadvantageous, 
or not so advantageous, variations would be snuffed 
out. He then further assumed that those individuals 
that lived would hand on their favorable variations 
by heredity to their young. Thus he used in his 
structure of explanation another fundamental phe- 
nomenon of life and basic factor of evolution, 
heredity, to which we shall recur in a moment. 
Among the young there would be further variation, 
in both right and wrong directions, with further 
success in living and reproduction on the part of 
those with advantageous variations, and extinction 
on the part of those with disadvantageous varia- 
tions. So, by cumulation, there would come about 
a gradual modification of species in an adaptive or 
fit direction; the "survival of the fittest." 
Now variations, which do actually occur and are 
undoubtedly the basis of evolution, have been the 
subject of much study among biologists since Dar- 
win's time. They have found out that there is much 
variety among variations with special regard to their 
heritability. This is an all-important matter in con- 
nection with their relation to evolution. Some are 
heritable, and some are not. Especially are those 
spontaneous, fortuitous, small variations on which FUNDAMENTAL FACTORS 
115 
Darwin placed so much reliance in building his evo- 
lution explanation, not directly inherited in full 
measure. They turn out to be simply an ephemeral 
fluctuation of size, of color, of any character at all, 
around a mean according to the law of probabilities, 
and are not handed on by inheritance in their own 
particular degree of variance from this mean. 
Further, as already suggested, they are so slight 
that they seem unable to act as handles for natural 
selection, even were they directly heritable and thus 
capable of cumulation. I have studied the variation 
in color pattern in a species of small ladybird beetle 
which has typically twelve little black spots on its 
two red-brown wing covers. Now, among a thou- 
sand of these beetles, collected in the spring just 
after their emergence from pupae and not yet ex- 
posed as full-fledged beetles to all the struggle for 
existence, there were individuals with no spots, indi- 
viduals with one spot on each wing cover, individuals 
with two spots on each wing cover, and so on up to 
beetles with nine spots on each wing cover, or eigh- 
teen altogether. A large majority, however, had 
the typical twelve spots. Among another thousand 
collected in the autumn, after a full season's exposure 
to the struggle for existence, in which struggle the 
character of color pattern should cut an important 
figure according to the natural selection explanation 
of evolution, there were also individuals with no 116 
EVOLUTION 
spots, with two, four, and so on up to eighteen spots, 
and in almost exactly the same proportions as among 
the spring collection. In other words, although 
twelve black spots on the red-brown wing covers are 
the normal color pattern of this ladybird beetle, its 
individuals may vary materially as regards this con- 
spicuous character and yet come through a season's 
struggle for existence none the worse for it. 
Lamarck, too, depended on variations as the basis 
of his causal explanation of evolution, but they were 
not the so-called Darwinian variations. They were 
rather the differences between offspring and parents 
which were produced during the development of the 
offspring by special use or disuse of its parts; by 
modifications directly induced in the developing 
individuals by the influence of varying environment, 
by adaptive reactions to the external conditions of 
life. These are the so-called "acquired characters" 
of Lamarck. They were assumed by him to be 
handed on to the next generation by inheritance, and 
to be cumulated by further modifications and their 
inheritance through successive generations until an 
effective and often most extraordinarily precise 
adaptation to environmental conditions was reached. 
It is these extraordinary adaptations which so im- 
press us in any examination of plant and animal life. 
But these "acquired characters" do not seem to be 
heritable; they certainly are not in the manner of FUNDAMENTAL FACTORS 
117 
Lamarck's conception. Mutilations produced ex- 
perimentally, like the cutting off of the tails of mice 
during many succeeding generations, or produced by 
conformity with some custom, as the binding and 
deforming of Chinese women's feet, leave no trace 
in heredity. Valley plants carried up to Alpine 
gardens and grown there for several generations be- 
come dwarfed in size and otherwise modified as the 
result of the high mountain conditions, but their 
seeds sown again in the valley produce plants of the 
usual valley type. I have reared silkworms on a 
starvation ration and produced moths but half of 
the usual size, but the descendants of these moths, 
fed normally, produced full-sized moths. 
These nutritional, or climatic, or otherwise en- 
vironmentally produced changes, these individually 
adaptive modifications, or so-called acquired charac- 
ters are not directly inherited. From the days of 
Weismann, who began the destructive criticism of 
the reputed instances of such inheritance, up to the 
very present time, there has been a steady attack on 
the fundamental assumption in Lamarck's most 
plausible and attractive evolution explanation-and 
the Lamarckians have had the losing side. Yet they 
do not give up. They cling to some of their in- 
stances. New experimentalists offer new alleged 
cases of the inheritance of acquired characters. 
Even as I write, the Austrian experimentalist Kam- 118 
EVOLUTION 
merer is exciting anew the hopes of the Lamarckians 
by accounts of his experiments in inducing changes 
by environmental influence in the mode of repro- 
duction of various salamanders and in the color of 
various amphibians and reptiles, with a claimed 
definite hereditary transmission of these changes in 
later untreated generations. Also, recently, two 
American zoologists of repute, Guyer and Smith, 
have reported the positive inheritance of certain 
eye defects induced in rabbits by a toxic serum. The 
unusually carefully conducted experiments of these 
men and their elimination of alternative explana- 
tions give their claims a very serious importance. 
Perhaps even more arresting are the claims of Pav- 
lov, the great Russian physiologist, for the direct 
inheritance of certain conditioned reflexes as the 
result of his experiments with white mice. He was 
able to train some mice after 300 lessons to run to 
their feeding place on the ringing of a bell. But it 
required only 100 lessons to train similarly the off- 
spring of these mice, only 30 lessons to train their 
offspring, only 10 lessons to train their young and 
only 5 lessons to train the next generation. He 
believes it "very probable that after some time a new 
generation of mice will run to the feeding place on 
hearing the bell, with no previous lesson." No 
matter how many carelessly claimed instances of a 
modification of a species character by an inheritance FUNDAMENTAL FACTORS 
119 
of acquired characters can be proved to be uncertain, 
and thus to be useless as evidence for the Lamarckian 
explanation of evolution, any single one that cannot 
be otherwise explained will have a grave consequence 
in the search for the actual causes of evolution. 
There is a great reasonableness and a strong at- 
traction about the Lamarckian explanation of species 
change and adaptation. If its assumptions could be 
substantiated it would offer a direct method of 
adaptive change and it would satisfactorily explain 
a fact that seems to me to be the strongest logical 
argument which can still be made for species adapta- 
tion growing out of individual adaptation. This is 
that so many of the species adaptations are precisely 
identical with those adaptations that are produced 
in individuals during their lifetime in response to 
environmental conditions. This situation presents 
an important problem not yet explained away by the 
anti-Lamarckians. 
But they assume the offensive. They ask how, in 
the light of all we know about the mechanism of 
heredity, and about the sharp and early setting off 
and isolation of the germ cells from the body cells 
of the parents, the Lamarckians are going to find 
the means by which their acquired characters can so 
modify the germ cells that they will produce in the 
new generation a replica of these changes in the 
body cells. There is no doubt that anything inside 120 
EVOLUTION 
the body or extrinsic to it which can directly affect 
and modify the germ cells will modify the new indi- 
viduals. Experiments of various kinds prove this, 
and it is, besides, something quite in harmony with 
modern knowledge of the mechanism of heredity. 
But how the enlargement of the right biceps of a 
blacksmith, or the callousing of the heel of a bare- 
foot negro, or the storing away of an unusual mass 
of information in the brain, is going to affect the 
germ cells in such a way as to cause them to develop 
into children especially muscled or calloused or edu- 
cated does not readily appear. Certainly all the new 
knowledge of the material basis and mechanism of 
heredity, which has been acquired in the last sixty 
years, and which is more than we had acquired in 
all time before, does not reveal how. 
Heredity 
But there are heritable variations, small ones as 
well as larger ones, mutations, sports, etc. There is 
heredity, or the passing on of characters and traits 
from generation to generation, new characters and 
traits as well as old. There would be no evolution 
without heredity any more than without variation. 
Heredity is a phenomenon as fundamental to life, 
and as important an element in any explanation of 
evolution, as variation. If variation is the all- 
important centrifugal element in evolution, heredity FUNDAMENTAL FACTORS 
is the all-important centripetal element. Like 
produces like, although always, always, with a dif- 
ference. Sometimes this difference is small-the 
minute variation; sometimes large-sports, salta- 
tions, mutations. But larger than the unlikeness is 
always the likeness. Heredity is more obvious than 
variation; but both are in evidence in every birth, 
and both are fundamental factors in evolution. 
The modern knowledge of heredity began with the 
work of Francis Galton in England and Gregor Men- 
del in Austria in the eighteen-sixties. Galton studied 
heredity statistically and paid a special attention to 
the inheritance of human mental capacity and traits. 
His work was published in well-known scientific 
journals and in books which had an immediate hear- 
ing and influence. Mendel studied heredity experi- 
mentally, using garden peas and other plants for 
subjects, and published his results in the obscure 
journal of a small natural history society where they 
lay unregarded until 1900. In that year three 
famous European botanists, all working independ- 
ently of Mendel and of each other on experimental 
problems in inheritance, discovered-each for him- 
self and all three practically simultaneously-Men- 
del's papers, and made Mendel's work known to the 
world. Now Mendel, "Mendelism" and "Men- 
delian inheritance" are names nearly as familiar to 
121 122 
EVOLUTION 
biologists as Darwin, Darwinism and Darwinian 
selection. 
On the basis of his statistical studies of heredity, 
Galton formulated two major generalizations now 
commonly known as Galton's laws of heredity. They 
express in summary form average results when a 
large number of cases is taken into account. The 
first of these generalizations, which may be called a 
general law of ancestral inheritance, is to the effect 
that an individual derives on the average one half 
of his inheritance from his two parents, one fourth 
coming from each; one fourth of his inheritance 
from his four grandparents; one eighth from his 
eight great-grandparents; and so on, by diminishing 
fractions, until the sum of this infinite series reaches 
i, or the total inheritance of the individual. 
He also formulated a second generalization, 
which he called the law of filial regression. This 
may be expressed by saying that the children of par- 
ents who vary from the mean of the population 
vary similarly, but to less extent than the parents. 
"The stature of adult offspring must, on the whole," 
he says, "be more mediocre than the stature of their 
parents; that is to say, more near to the mean or 
mid-type of the general population." 
These generalizations or laws of Galton, based 
on the examinations and statistical treatment of many 
data, mark a distinct step forward in the study of FUNDAMENTAL FACTORS 
123 
heredity. But they give us little information about 
the probabilities of the inheritance of specific char- 
acters, and the hereditary make-up of specific indi- 
viduals. They do not indicate just what special 
traits we may expect to derive, or may not expect to 
derive, from the parents, or the grandparents, or 
great-grandparents; nor do they tell us what will 
be the hereditary fate of a given individual with a 
given ancestry. It is precisely that kind of informa- 
tion that we most desire. 
Mendelism makes no such broad generalizations 
as Galton's, but it makes much more precise ones. 
It does not treat of halves or quarters or eighths of 
one's whole inheritance, but of the inheritance of 
specific characters. And it treats of the inheritance 
expectancies of the offspring of a single pair of 
parents instead of those of the members of a large 
mixed population. Mendel's own experiments were 
made on various varieties of garden peas (and some 
other plants) which can be freely crossed, these 
crossings resulting in fertile offspring. Mendel chose 
for his crossings races of peas characterized by such 
readily contrasted characters as tall and dwarf 
stem, wrinkled and smooth seed coats and so on, and 
fastened his attention to the results of crossing as 
regards these specific or unit characters. He de- 
termined and recorded the outcome for every one 
of the offspring. He then mated these recorded off- 124 
EVOLUTION 
spring among themselves and with typical repre- 
sentatives of the races to which the parents belonged, 
and determined and recorded again the results, as 
regards the same special characters, for all of the 
offspring produced by each mating. And so on, for 
still other generations. 
Out of all this experimentation and detailed re- 
cording of results came a surprising and important 
revelation of an orderly and definite inheritance be- 
havior. On the basis of this, Mendel was in a 
position to state in advance just what could be relied 
on to happen when the same experiments were re- 
peated. And when Mendel's work was repeated by 
others the same things did happen. Since then Men- 
del's results have been strikingly confirmed by other 
men studying inheritance in other plants, and also 
in animals and human beings. 
Among the outstanding features of the Mendelian 
conception of inheritance, is the recognition of the 
body as being made up of a great number of more 
or less independent unit characters grouped together 
as a mosaic to form the whole. These unit charac- 
ters can be rearranged and recombined by crossings, 
but not destroyed, or essentially modified. One 
character may be temporarily extinguished as regards 
bodily manifestation by the presence of a dominat- 
ing contrasting character, but it persists as a ger- 
minal possession, to reappear again as a body char- FUNDAMENTAL FACTORS 
125 
acter under favoring circumstances. That is, the 
germinal and bodily possessions of an individual may 
differ, and it is his germinal rather than his bodily 
character and history that is of prime importance 
in understanding and prophesying his hereditary 
possibilities and those of his offspring. His own 
mind may be entirely sound but his germ plasm may 
carry the possibility of feeble-mindedness. Let me 
illustrate this by recounting some of the details of 
an experiment in hybridization that I have repeat- 
edly made. 
If we make a cross-mating between two silkworm 
moths of certain different artificially developed races, 
one of these races producing exclusively golden silk 
(cocoons) and the other white silk, we shall get a 
family of about three hundred brother and sister 
silkworms. When cocooning time comes these will 
spin, not, as one might expect, pale yellow (color 
blend) cocoons, nor yellow-and-white blotched (color 
mosaic) cocoons, nor some golden cocoons and 
some white cocoons, but all of them will spin golden 
cocoons like the cocoons of the golden-silk-spinning 
race to which one of the parents belonged. And it 
makes no difference whether this parent was the male 
or the female parent. It is the hereditary trait, 
golden-silk-spinning, that dominates over the heredi- 
tary trait, white-silk-spinning, not one parent over 
the other. The dominance seems complete, and, as 126 
EVOLUTION 
regards physical or bodily manifestation, it is. But 
let us carry the experiment a step further. 
If we mate two of these golden-cocooning off- 
spring of the golden-white cross we shall get a family 
of silkworms which will not all spin golden cocoons, 
as both their parents did, but three fourths of the 
young will spin golden cocoons and one fourth white 
cocoons, and this proportion will be nearly exact. 
If now, two of these white spinners, which are the 
offspring of two golden-spinning parents, are mated 
together, all the offspring produced by them will spin 
white cocoons, while the offspring of two of the 
golden-spinning children of the golden-spinning 
parents will again divide in the proportion of three 
golden spinners to one white spinner. 
That is, although the golden-spinning trait is 
dominant, in bodily manifestation, over the white- 
spinning trait, when a pure golden race is crossed 
with a pure white race the germ cells of the offspring 
produced by this crossing will still carry the white- 
spinning trait, which is able again to manifest itself 
under certain conditions. 
Mendel's results in crossing his races of garden 
peas differing in various contrasted traits were just 
like these silkworm results. On their basis he 
offered a theoretical explanation of this behavior 
which indicates what the conditions are which make 
the recessive trait appear again after its apparent FUNDAMENTAL FACTORS 
127 
extinguishing by the dominant trait. And this ex- 
planation so well accounts for the happenings that, 
with some modifications made necessary by post- 
Mendelian studies, it may be accepted as the true one. 
It assumes that hereditary traits are represented 
in the germ cells by specific physico-chemical 1 de- 
terminers, or combinations of them. These are 
brought together in the fertilized egg cell produced 
by any mating, pure or cross, and handed on in the 
male and female sex cells produced by the offspring 
of the cross, without destroying or materially influ- 
encing each other. Although, when two kinds or 
groups of determiners representing contrasting 
traits, such as yellow-and-white-silk-spinning or 
high-and-dwarf stem of pea plant, are in the egg 
cells, one of these contrasting characters is domi- 
nant over the other as regards actual bodily mani- 
festation. 
Now, applying this explanation to the pea and silk- 
worm experiments, let us see how it accounts for the 
results. 
When a moth of the pure white-silk race is 
1 The modern study of plant and animal cells, particularly of 
the germ cells, shows definitely that these determiners, called 
genes, are situated in small bodies called chromosomes which lie 
in the cell nuclei. An elaborate study of the character and 
behavior of these chromosomes has been made by cytologists, with 
the result of revealing their enormous importance in the mechanism 
of heredity. 128 
EVOLUTION 
crossed with a moth of the pure golden-silk race, 
the offspring will all spin golden cocoons, because 
golden is dominant over white in the struggle for 
manifestation. But half of the germ cells of these 
hybrid golden-silk spinners will carry the determiner 
for golden, and half the determiner for white. When 
these golden-spinning hybrids are mated together, the 
differing sex cells should meet, by the law of proba- 
bilities, in the following proportions: male cell 
carrying golden with female cell carrying golden in 
one fourth of the cases; male carrying golden with 
female carrying white, or female carrying golden 
with male carrying white, in one half of the cases; 
and male carrying white with female carrying white 
in one fourth of the cases. Now, the results of these 
junctures in the fertilized egg cells from which the 
young develop should be that, in all the cases where 
golden meets golden, the developing young should 
spin only golden cocoons and produce sex cells con- 
taining only golden determiners. In all the cases, 
where white meets white, the young should spin only 
white cocoons and produce sex cells containing only 
white determiners. But in all the cases where golden 
meets white, the young should spin only golden 
cocoons (because golden dominates white in bodily 
manifestation where the two traits meet), but these 
young should produce sex cells, one half carrying 
golden and one half carrying white determiners. FUNDAMENTAL FACTORS 
That is, although all of the young produced by 
mating a moth of the pure golden race with a moth 
of the pure white race should spin golden cocoons, 
only three fourths of the young produced by a mat- 
ing of these hybrids should spin golden cocoons. 
One fourth should spin white, and these whites 
mated together should produce young spinning only 
white. But the goldens mated together should pro- 
duce again a certain proportion of whites, because 
only one third of these goldens are germinally pure, 
the other two thirds possessing both germ cells rep- 
resenting white and germ cells representing golden. 
Which is just what happens. 
This is only the beginning of the new-heredity 
story. In some cases, the first hybridization pro- 
duces a blend between the crossed characters, be- 
cause neither character is actually dominant over the 
other. But crossings of the blend generation result 
in a breaking-up among the offspring into some (ac- 
tually one fourth) showing one of the original traits, 
some (another one fourth) showing the other, and 
the rest (one half) showing the blend again. The 
one fourth showing one of the original traits are ger- 
minally pure for that trait, and, mated together, 
produce offspring showing only that trait; and simi- 
larly with the one fourth showing the other original 
trait. But the blends are germinally impure, that is, 
they produce in equal numbers sex cells carrying one 
129 130 
EVOLUTION 
trait and sex cells carrying the other, and, when 
mated together, they produce offspring, one fourth 
manifesting only one trait and germinally pure for 
that trait, one fourth manifesting only the other 
trait and also germinally pure for it, and one half 
showing blends and germinally impure. 
But it would take too long, and lead us into too 
much detail, to go on with the story. It is sufficient 
to affirm that the facts of Mendelian inheritance 
and their explanation have carried us a long way in 
our attempts to reach the goal of being able to 
prophesy, with a high degree of confidence, what will 
be the specific hereditary outcomes of matings of 
plants and animals and men in which contrasting 
specific traits are involved. The principles and the 
mechanism of Mendelian inheritance are well de- 
termined. But the behavior of each trait has to be 
worked out for each species of plant or animal, or 
for man. Golden color may be dominant over white 
in the silk of silkworms; but because we know this, 
we cannot say that golden is dominant over white 
in flower petals. It may be in one kind of flower, and 
the reverse may be the case in another. 
The actual determinations can be fairly easily 
worked out in plants and in those animals suscepti- 
ble to experiment. In the case of man, however, 
planned and controlled experimentation is impos- 
sible. Here advantage must be taken of unplanned FUNDAMENTAL FACTORS 
131 
experiment (miscellaneous matings), and of family 
(genealogical) records which have paid attention to 
physical and mental characteristics. 
Much has already been done in this way. The 
hereditary behavior of a number of human patho- 
logical conditions, like six-fingeredness, web-fingered- 
ness, dwarfism, color-blindness, night-blindness, and 
the like; and a number of diseases, and especially 
disease diatheses, as diabetes and Huntington's 
chorea; and some less important but interesting 
physical characteristics, as eye color and hair form; 
and finally, and very importantly, several mental 
traits, as certain types of feeble-mindedness, have 
been pretty clearly worked out and found to be 
typically Mendelian. But only a beginning has been 
made. And, despite the sweeping claims of the 
Mendelians, there is undoubtedly much heredity that 
is not Mendelian in character. CHAPTER VIII 
FUNDAMENTAL FACTORS IN EVOLUTION 
(Selection, Segregation, Response to Environment) 
Selection 
Another fundamental factor in evolution is se- 
lection. Various forms of selection have been 
either actually observed or assumed, such as natural 
selection, artificial selection, and sexual selection. 
The first and third of these will always be closely 
associated with the name of Darwin, for their de- 
tailed elaboration as explanations of evolution was 
Darwin's peculiar contribution to the evolution 
idea. In fact, the word Darwinism when used by 
biologists means the natural and sexual selection 
theories of evolution explanation, and not evolution 
itself, for which Darwinism is popularly much used 
as a synonym. Darwin also called wide attention to 
artificial selection and was undoubtedly much 
strengthened in his belief in natural selection by the 
obvious processes and results of the plant and animal 
breeders. 
The essential elements of observed fact and logical 
induction that go to make up Darwin's conception 
132 FUNDAMENTAL FACTORS 
133 
of natural selection as a causal agent in species form- 
ing have been noted in previous chapters (Chapter 
VI and VII). And I have referred, too, to the fact 
that most biologists now refuse to believe in its 
capacity to serve as such agent. But this lack of 
acceptance of natural selection as a competent cause 
of the origin of new species must not be confused 
with a disbelief in the reality of natural selection as 
a general evolutionary factor, exercising a definite 
influence on the determination of major lines of 
evolutionary movement. There is a general-al- 
though not unanimous-acceptance of natural selec- 
tion as that factor in life which accounts for the 
nonpersistence of too unfit lines of plant and animal 
evolution. Perhaps it would be sufficient simply to 
say that only such organisms can exist as are suf- 
ficiently fitted to exist under the actual conditions 
on the earth. The nonpersistence, then, of any 
others-and it is certain that such others do come 
into being-may need no further explanation, and 
the phrase "natural selection" might have, from this 
point of view, no sufficient meaning to keep itself 
alive. 
However, there is a real place for the idea of 
natural selection in our understanding of evolution, 
for it indicates a group of special conditions, inci- 
dental to the life of plants and animals and to their 
arrangement in evolutionary lines, which gives us 134 
EVOLUTION 
some understanding of how it is that some lines 
persist and some do not. A line of gradual develop- 
ment and modification may go on for some time and 
distance and yet come to a forced ending. The con- 
ditions which bring about this forced ending may be 
summed up under the name natural selection. 
Sexual selection as conceived and expounded by 
Darwin may be roughly defined as a selection in mat- 
ing brought about by the exercise of a choice by the 
males or females of an animal species among the 
varying individuals of the other sex which offer 
themselves, or are available as consorts. This 
choice can be made on grounds of superior size, or 
virility, or striking character, or beauty of color 
pattern, or loudness or melodiousness of song, or 
what not else. As a result of this choice in mating, 
young would be produced inheriting the special char- 
acters of the parents. Darwin would thus explain 
the existence and high development of the striking 
color pattern and plumes and the curious dancing 
and mating-time antics of many male birds, and the 
songs and decorations of many male insects, the fe- 
males of the same species being plain and quiet. 
Such a theory was necessary to account for the 
numerous cases in which these striking possessions 
of the males would seem to expose them dangerously 
to their enemies, a condition not at all compatible 
with Darwin's assumption in his natural selection FUNDAMENTAL FACTORS 
135 
theory of a rigorous struggle for existence. Indeed, 
it was the incapacity of the natural selection hypoth- 
esis to account for the various highly developed 
secondary sexual characters that exist in so many 
animal species that led Darwin to the formulation 
of the sexual selection theory. 
But this theory faced difficulties in its necessary 
assumption of a highly developed aesthetic sense on 
the part of such animals as the insects and birds. 
Also, in the fact that in most species of animals 
about equal numbers of males and females occur, 
so that after all the more beautiful males or the 
louder singers had been chosen there would still be 
mates left for all the others, and hence a reproduc- 
tion of the less adorned as well as the more adorned. 
But the major difficulty is that much close observa- 
tion and some ingenious experimentation have failed 
to substantiate the sexual selection hypothesis. The 
late Dr. A. G. Mayor, the brilliant biologist in 
charge of the Carnegie Institution's laboratory in the 
Dry Tortugas Islands, cut off the strikingly patterned 
wings from the males of the large Promethea moth, 
and fastened the plainer wings from females on 
them. He found no hesitation on the part of the 
females to accept these males, robbed of their spe- 
cial attractions as consorts, even in the presence of 
unmutilated males. Other similarly crucial experi- 
ments have had similar results. 136 
EVOLUTION 
Darwin's sexual selection theory may be said, 
therefore, to be largely discredited. It ought, how- 
ever, to be said at the same time, that no other more 
satisfactory theory to explain these strange and 
striking secondary sexual differences which occur in 
many animal species has yet been presented. 
The phrase artificial selection may be used to sum 
up the methods, or it may also be used to denominate 
the results of man's work in domesticating and modi- 
fying various plants and animals for his advantage. 
From the standpoint of self-dependence in nature, 
these changes, especially in the case of animals, 
constitute usually a sort of retrogression. 
It is certain from the records of history, and from 
ancient pictures and carvings, and still more ancient 
bones and relics, that man has had domesticated ani- 
mals for the last ten thousand years. How long be- 
fore that he made a practice of taming and using 
and perhaps breeding his animal companions of pre- 
historic times, we may never know. In the caves 
where are found the bones and rude implements of 
early man, that primitive man of the Glacial epoch, 
there are also found the bones of various animals, 
but these seem to be the remains of kinds that were 
either his victims or his conquerors in the raw 
struggle for existence of those ancient times. How- 
ever, when the prehistoric Egyptians and Cretans 
emerged from the Stone Age into the earliest light FUNDAMENTAL FACTORS 
137 
of history, they appear with cattle, sheep, donkeys 
and dogs, already fully domesticated. 
The domestication of animals is the result of 
several different factors-and similar factors enter 
into the domestication of plants. First, there may 
be the simple capture and taming and using of indi- 
viduals of a wild species. Then comes the rearing 
in captivity of young of this species, and the easier 
taming of these home-reared individuals because 
of their earlier acquaintanceship with man. 
But in this rearing in captivity a new element 
enters almost at once. That is the choosing, or se- 
lection, of certain of these young to be allowed to 
grow up, and again the choosing among these when 
grown up of those to be the parents of more young. 
This selection may be almost unconscious, or it may 
be made intentionally and carefully, so as to preserve 
the most desirable individuals and have them give 
birth to others like themselves. Then there comes 
the crossing of special individuals, or the hybridizing 
with other races in the hope of adding or combining 
in the offspring the desirable qualities of both kinds 
of parents. 
It is easy to see, as Darwin saw, the striking 
analogy between artificial selection and an assumed 
natural selection in which man's place is taken by an 
overproduction of young, and a struggle for exist- 
ence. But the weakness of the argument for nat- 138 
EVOLUTION 
ural selection on the basis of the reality of artificial 
selection lies in the assumption that the slight varia- 
tions among the too many young will determine their 
life or death. In the case of artificial selection, this 
life or death determination among the young is 
made by man, using all his powers of discriminating 
among the slightest differences in individuals. I have 
seen Luther Burbank, one of whose special qualifi- 
cations for plant breeding is an unusual capacity for 
such discrimination, crawling about on his hands 
and knees among the thousand little seedlings in a 
plant bed all derived from a single crossing, and 
selecting the few that he wishes to live and repro- 
duce themselves, while the others go to the bonfire. 
It would be interesting to trace, if space permitted, 
the origin and lines of modification of some of our 
more familiar domesticated plants and animals. 
The dogs, for example, undoubtedly the oldest do- 
mesticated animals, as they are also the closest and 
the most nearly universal animal companions of man, 
and represented now by about fifty breeds ranging 
in character and size from the tiny toy dogs of Paris 
that a lady can carry in her muff to the great Danes 
and St. Bernards that stand three feet high and 
weigh one hundred and fifty pounds, are believed by 
most zoologists to have descended from several 
different wild species of wolves and jackals of vari- 
ous lands. The house cats, on the contrary, as FUNDAMENTAL FACTORS 
139 
various and widely distributed as are their present 
thirty or more breeds, all seem to be descended from 
a single wildcat (Felis maniculata). It is a native 
of northeast Africa and was domesticated in Egypt 
at least 1,300 years before Christ. 
The horses of modern times can be traced back to 
two wild ancestor species, Equus przewalski, of 
northern Asia, from which all the Oriental, Mon- 
golian, Arabian, North African and East European 
races have sprung, and Equus caballus fossilis, or the 
diluvial horse of Europe, from which the German, 
Norman, English and West European horses gen- 
erally have arisen. The existing horses in America 
have been derived from Europe, although a remark- 
able series of fossil horses dating back to the begin- 
nings of the Tertiary Age have been found in this 
country. Remains of horses are associated in Europe 
with human relics of the Bronze Age, and figures of 
the wild horse are abundant among the drawings 
made by prehistoric man on cave walls in Spain and 
France. 
The many races of domesticated cattle also seem 
to trace back to two sources, the wild banteng (Bos 
sondaicus), of Java and South Asia, from which are 
derived the zebus, the old Egyptian longhorns, and 
many of the races of Europe, such as the Spanish, 
Albanian, Sardinian, Polish and brown Alpine 
cattle; and the primitive wild ox of Europe (Bos EVOLUTION 
140 
primigenius'), from which have descended most of 
the English, North German, and Holland races. 
This wild species persisted in Germany until the 
twelfth century, and in Poland up to the eighteenth 
century. The races of domesticated hogs are also 
descended from two wild races, the European wild 
boar (Sus scrofa), and another species (Sus vit- 
tatus) from eastern Asia. From this latter the 
swine of China and those of the Romans, and indeed 
most of the European races have descended. The 
lake dwellers of Switzerland had domesticated hogs, 
and pig remains have been found with prehistoric 
relics in Denmark. China has had domesticated 
swine for thousands of years. The domesticated 
races of sheep seem to have had three original wild 
sources, Ovis musimon of South Europe, Ovis arkal 
of Western Asia, and Ovis trageiaphus of North 
Africa. Most of our present European and 
American races come from the second named of 
these wild kinds. The earliest certain remains of 
tame sheep appear in the Stone Age. In the Bronze 
Age, sheep domestication was well developed. The 
oldest Assyrian drawings picture domesticated sheep, 
among which the still persisting fat-tailed race ap- 
pears. The Egyptians had domesticated sheep in 
the times before the Pharaohs. 
Of birds there are domesticated races of doves, 
chickens, turkeys, ducks, geese, swans, peafowls, FUNDAMENTAL FACTORS 
141 
pheasants, canary birds, ostriches, cormorants and 
others. Of these, the doves and chickens are repre- 
sented by the most varieties. Brown, an English 
authority on domestic birds, lists more than seventy 
races of chickens now living, thirteen races of ducks, 
ten of geese, and eight of turkeys. Of pigeons there 
must be nearly as many domestic races as there are 
of chickens. Yet all of them, with their extraor- 
dinary variety of crests, and ruffs, and tails and 
plumage pattern, and all their various special man- 
ners, such as tumbling, dancing, and the like, are 
descended from a single wild species, the common 
rock dove {Columba livia), of Europe, Asia and 
North Africa. 
The domestic races of chickens are by some natu- 
ralists also held to be descended from a single wild 
species, the jungle fowl {Gallus bankiva), which 
ranges from Hindukoosh to the Chinese island of 
Hainau and through most of the Indonesian Islands. 
But other naturalists believe that one or two other 
wild species of fowl are concerned in the ancestry 
of our barnyard hen. The domestic ducks are de- 
rived from the wild duck {Anas boschas), and have 
evidently originated from this ancestor independ- 
ently both in China and in Europe. The domestic 
geese seem to have an older origin than the ducks; 
in fact, geese are probably the oldest of domesti- 
cated birds. The ancestor of our races is the wild 142 
EVOLUTION 
species Anas cinereus. The Chinese races, however, 
are descended from Anas cygmoides, and the early 
Egyptians seem to have tamed and used the Nile 
goose (Chenalopex egyptiaca). 
There are even two species of insects that have a 
right to be called domesticated animals, namely, the 
honeybee and the mulberry silkworm. Man has 
long used the honeybee (Apis mellifica) to obtain 
honey, but only in modern times has the species been 
the subject of true "breeding." However, already 
several distinct races have been produced. The bee 
is native to Europe and Asia, and "wild" honeybees 
in America are only communities established by wan- 
dering swarms from hives, or from other "wild" 
communities which have descended from such es- 
caped swarms. 
The silkworm (Bombyx mori) has, on the con- 
trary, been an artificially bred animal for five thou- 
sand years, and scores of races, with differently 
colored and shaped cocoons, exist. The actual wild 
species from which the domesticated races are 
descended is not known, but it is most likely some 
one of the several wild species of northern India. 
The cocoons of certain of these wild Indian species 
are to-day still collected for the silk and sold under 
the commercial name of "Tussoor" silk. The an- 
cient breeding and care of silkworms was mostly FUNDAMENTAL FACTORS 
done in China and Japan. To-day it is also carried 
on extensively in France, Italy, Syria and the Levant. 
143 
Segregation 
Finally, there remain to be considered two other 
fundamental factors in the life of plants and animals 
which have a positive influence in helping to deter- 
mine the lines, or directions, of evolutionary move- 
ment. These are the factors of segregation, or 
isolation, and that of the response of living matter 
to environment. But both of these factors have been 
briefly discussed in a preceding chapter, and need for 
the purposes of this elementary treatise to be only 
a little further elaborated here. 
Under the natural conditions of life, as it exists 
to-day and has existed for many geologic ages, it is 
inevitable that a certain geographical segregation 
of plant and animal kinds should occur. For ex- 
ample, the present flora and fauna of Australia are 
largely isolated from the floras and faunas of other 
parts of the earth, and this has a directly determin- 
ing effect on the possible lines of plant and animal 
evolution that can exist in Australia. Similarly, 
although not by ocean barriers, the plants and ani- 
mals of a desert are segregated more or less nearly 
completely from other groups of organisms. And 
this is also true of those of a great forest, or of a 
great marsh, or of the upper altitudes of a mountain 144 
EVOLUTION 
range. The important matter of "associations" of 
plants or animals, or plants and animals, produced 
by such geographical, or topographical, or other 
environmental conditions, is attracting much atten- 
tion at present; and the evolutionary importance of 
segregation of groups of plant and animal species 
is becoming more and more apparent. 
But there can be, and often is, a segregation not 
of species but of individuals of species. A number 
of individuals of a given species may, by one means 
or another, escape from their habitual home and 
thus be forced to mate with each other, without 
opportunity of mixing with the bulk of the species. 
This results in a special perpetuation and cumula- 
tion of the particular heritable variations possessed 
by the members of this segregated group of individ- 
uals. Such a segregation of individuals can also be 
produced even within the usual range of a species 
by special physiological or environmental conditions 
which bring about a restricted instead of a general 
interbreeding on the part of certain individuals. 
It is this segregation and forced interbreeding of 
a few individuals in a different environmental setting 
from that of the bulk of the species which has seemed 
to some biologists to give "isolation" a considerable 
importance as a causal factor in the modification of 
species, or, in effect, in the origin of new species 
(see Chapter VI). FUNDAMENTAL FACTORS 
145 
Response to Environment 
The remaining important factor which we should 
not overlook, is that of the response of life stuff 
and of individual organisms to their environment. 
The plasticity of life is notorious. But we are 
too much given to seeing fixity in individuals and 
kinds of plants and animals, especially animals. We 
see more of this plasticity in plants. Differences in 
nutrition and other environmental conditions so 
readily affect plants that the botanists even recog- 
nize "nutritional varieties" in their classifications 
of plant kinds. But animals are also extremely plas- 
tic, and individuals of a given species can reveal very 
considerable differences directly traceable to differ- 
ences in environmental conditions during their de- 
velopment from egg to adult. Dr. Stockard is able to 
cause certain kinds of fishes to develop with only one 
eye, or with no eyes, by giving them, during their 
growth, an environment somewhat different from 
their usual one. In fact, it is very important for us to 
hold always in mind the fact that even what may be 
termed normal development depends not alone on 
influence of heredity, but also on environmental in- 
fluences. For the normal development of any indi- 
vidual organism, certain definite environmental 
conditions, constituting what may be called normal 
environment, are as necessary as normal heredity. 146 
EVOLUTION 
Without any doubt at all, the plasticity and indi- 
vidual response to environment is a very important 
factor in evolution. Here we are using the word 
"environment" in a very generous sense to include 
the active or limited exercise of body function, use 
and disuse of parts, as well as external or internal in- 
fluences or conditions affecting either directly or indi- 
rectly the whole individual or special parts of it. It 
must be confessed, however, that in the present state 
of our knowledge of heredity, and especially of the 
inheritance of acquired characters, we cannot under- 
stand just how this individual response to environ- 
ment plays the role in modifying species that many of 
us believe it simply must play. There is undoubtedly 
a growing belief in the importance, as a fundamental 
evolutionary factor, of the adaptive response to 
environmental conditions on the part of individual 
organisms. And one of the reasons for this is the 
conviction of biologists generally that the natural 
selection, mutations and Mendelian explanations of 
the origin and adaptation of species are insufficient 
explanations. 
Osborn, the paleontologist, and others, have made 
familiar to us the phrase "the unknown factors of 
evolution." It is a suggestive and useful phrase. 
We have yet much to learn before we shall have a 
full understanding of the fundamental factors and 
the effective causes of evolution. CHAPTER IX 
THE EVOLUTION OF THE PLANTS 
The plants which grow about my little green 
cabin in the Monterey pine wood near the Pacific 
shore are many and various, and they represent a 
considerable number of the major groups into which 
botanists conveniently divide all plants in order to 
indicate the likenesses and unlikenesses among them. 
But this grouping does more than simply that; it 
gives a general picture of their relationships, their 
genealogy, their evolution. 
First, starting with the higher types, there are 
the flowering, or seed plants. They are commonly 
divided by botanists into two principal classes: those 
with inclosed seeds and usually well developed, bril- 
liantly colored and variously shaped flowers, like the 
yellow monkey flowers and dandelions, the blue 
asters, orange poppies and delicate creamy white 
fairy lanterns that are all blooming so abundantly 
now about me; and those with exposed or "naked" 
seeds, which include our Monterey pines and the 
various other pines, as well as all the firs and cedars 
and cypresses and larches and redwoods and other 
cone-bearing trees. Among the naked-seeded plants, 
147 148 
EVOLUTION 
too, are a few strange trees called cycads, of which 
the "sago palm" of the florists is the most familiar 
example, as well as the curious maidenhair tree or 
ginkgo, the sole survivor of an ancient race which 
was represented by many species in older geologic 
times. Indeed, the whole class of naked-seeded 
plants is an older and much more primitive plant 
group than the flowering plants. It is also much 
more limited in number of kinds, being represented 
by perhaps not more than 500 living species to the 
100,000 or more of the flowering plants known to 
exist now. However, some of the conifers have 
such myriads of individuals, and these individuals 
are, as great trees, so imposing and important and 
so useful to us as plant kinds that we are likely to 
think of them as the crown of plant evolution. In- 
deed, the greatest plants we know are the cone-bear- 
ing ancient Sequoias of the Sierra Nevada in Cali- 
fornia, represented now by comparatively few 
individuals, some of which are 300 feet high and 
more than thirty feet in diameter at the base, and 
are of an age of 2,000 years and more. In earlier 
geologic times the Sequoias grew in abundance over 
most of America and even in the Old World. 
But it is really the closed-seeded flowering plants 
with their manifold kinds, from the tiny and tender 
white violets in the near-by forest, to the aspiring 
and hardy alders and sycamores of the stream side, EVOLUTION OF PLANTS 
and sturdy oaks of the open hills, which are the 
crown of plant evolution. 
It is among these flowering plants that those 
marvelous adaptations of stem and leaf and flower 
and seed, and those extraordinary relations with 
other plants and animals, reach their culmination. 
To them belong the strange array of desert plants: 
the water-storing, leafless cactuses, with their animal- 
repelling spines and prickles and their minimum of 
evaporating surface to bulk of body; the small 
leaved, resin-covered creosote bushes, and the 
slender-stemmed ocatillas, which swiftly put out tiny 
leaves in the short rainy season, and let them as 
promptly wither away when the scanty rains cease. 
Among the flowering plants, also, belong various 
pond kinds with special adaptation to aquatic life, 
like the floating, small-flowered duckweed, the an- 
chored, long-stemmed, flat-leaved, large-flowered 
pond lilies, and the completely immersed pond weeds 
with their hidden flowers lacking all petals. 
To the more developed of the flowering plants 
belong those many kinds with strangely shaped and 
strikingly colored and patterned flowers that are 
cross-pollinated by insects. The insects, attracted by 
the scent and colors of the flowers, visit them to 
gather nectar and pollen, and are compelled by the 
intricate pattern and shape of the flower cup and in- 
genious mechanical devices of stamens and pistils to 
149 EVOLUTION 
150 
carry a load of pollen from one flower and deposit 
it in the next similar one visited. The orchids attain 
the extremest cases of these cross-pollinating devices, 
accompanied by such specialized conditions that the 
flowers of some orchids are absolutely sterile unless 
visited by particular kinds of insects. This means 
that not only are the plants specially adapted to 
their insect visitors, but the insects are specially 
adapted in structure and habit to their flower hosts. 
When Wallace discovered a great sphinx moth in 
Madagascar with a sucking proboscis twelve inches 
long, he prophesied that there would be found in the 
same region a kind of plant with flower cups twelve 
inches deep, and his prophecy was realized. In the 
warmer parts of the United States there grow sev- 
eral species of the plant genus Yucca, showy lilylike 
plants of which some are grown in gardens. These 
plants depend for pollination on the visits of a small 
moth (Pronuba) whose larvae live on the seeds of 
Yucca. The parent moth lays her eggs in the ovary 
of a Yucca flower, and then collects a small mass of 
Yucca pollen and forces it down the central part of 
the pistil, thus insuring fertilization of the Yucca 
ovules and consequently a supply of developed seeds 
to provide food for the moth larvae. The larvae 
do not eat all the seeds, some remaining to produce 
new Yucca plants. The Pronuba thus generously 
pays for the seeds eaten by its young! EVOLUTION OF PLANTS 
151 
There are various flowering plants which have 
developed a parasitic life at the expense of others. 
A well-known example is the dodder, which twines 
its leafless stems about other plants, sending suckers 
into its host, from which it derives all its nourish- 
ment. The various species of mistletoe are partly 
parasitic, but get some of their needed carbon from 
the carbonic dioxide of the air. Of another type 
are the carnivorous insect-catching plants, such as 
the sundew, and Venus's flytrap, whose leaves are 
modified into traps which catch small insects and 
afterward digest them by means of digestive fer- 
ments not unlike those secreted by the alimentary 
canal of animals. In the pitcher plants and bladder 
weed the leaves also trap insects, but there is little 
digestive effect, the products of the decomposing 
bodies of the dead insects being simply absorbed 
by the leaves. Certain tropical trees, especially of 
the genera Cecropia and Acacia, have developed 
extraordinary symbiotic relations with certain kinds 
of ants, harboring colonies of these ants in their 
hollow stems or thorns, and putting out certain 
peculiar growths much relished by the ants. When 
the marauding leaf-cutting Aztec ants come to these 
trees they are repelled by the friendly ants living 
in them, and thus their foliage is saved. 
But the adaptations of the flowering plants are 
altogether too numerous and various to be cata- 152 
EVOLUTION 
logued here. These plants range from minute, 
almost microscopic, herbs living but a few weeks, to 
stately trees which outlive man; from plants with 
soft, succulent stems to those with rigid trunks of 
the hardest wood; from plants living in salt and 
fresh water, to those living in deserts or on moun- 
tain summits and soil-covered arctic glaciers. In 
form and habit plastically fitting themselves to all 
sorts of conditions they yet show a remarkable 
homogeneity in their essential structure and funda- 
mental manner of life and self-reproduction. They 
reveal readily their mutual relationship to each 
other; they show a fundamental identity of form 
and function through all the changes rung on their 
basic plan. They are a beautiful example of evolu- 
tion making variety out of identity. 
But we must pass to other kinds of plants, to 
simpler and older types, for the flowering plants are, 
as geological time goes, very modern. They did not 
appear on the earth until the Cretaceous epoch, mil- 
lions of years after that period known as the Car- 
boniferous, when lower types of vegetation ran riot 
over the earth and laid down those inconceivable 
masses of plant remains which we delve now from 
the earth's crust as coal. In those Carboniferous 
forests and marshes and land regions there were no 
flowering plants, even no true conifers. Ferns were 
the most abundant plants of the Carboniferous Age EVOLUTION OF PLANTS 
153 
both as to variety of species and numbers of indi- 
viduals. Ferns and some strange trees called Lepi- 
dodendrids, Sigillarids, and Calamites, whose re- 
mains form most of the coal, were the characteristic 
plants that gave the great swamps of the Carbonifer- 
ous Age, the age dominated by plants, their peculiar 
character. There were fishes and amphibians and 
a few reptiles, but as yet no birds or mammals-and 
of man, no whisper. 
As one moves down the evolutionary ladder of 
the plants from the true flowering plants at the top, 
and the conifers forming the next rung below, one 
comes next to the ferns. They differ from the flow- 
ering plants in general structure and characteristic 
appearance, but especially and most importantly, 
from the evolutionist's point of view, in the repro- 
ductive parts and methods. Although the ferns are 
land plants-they, with the mosses and liverworts, 
were the first land plants-they do not produce seeds 
but spores, and the male germ cells are not pollen 
grains but motile sperm cells, which must find their 
way to the female germ cells, or spores, in water. 
There is an existing genus of fernlike plants called 
club mosses (Selaginella), which shows a remark- 
ably close approach to the seed-bearing condition 
and reveals undoubtedly the intermediate evolution- 
ary stage between the typical ferns and the lowest 
seed-bearing plants. And one of the most important  EVOLUTION 
154 
botanical discoveries of recent years is that of the 
fact that in a number of the lowest seed plants ferti- 
lization of the ovules is still effected by large motile 
sperm cells very much like those of the ferns. The 
plants in which this condition exists are the fernlike 
cycads and the curious ginkgo, both of which had 
long been recognized by botanists as being very an- 
cient types of seed plants-they date back to Car- 
boniferous times-and as showing in various ways 
close resemblances to the ferns. In both of them the 
ovary and ovules are much like those of pines, and 
the pollen grain develops a pollen tube as it does in 
the pine. But this pollen tube carries an accumulation 
of water and two large motile sperm cells to the 
female organ. The fertilization and germination of 
the seed-bearing plants has great advantages over 
the aquatic method of the ferns. In the first place, 
a special supply of water is not needed for fertiliza- 
tion, and, secondly, the seed has food stored in it by 
means of which the young plant is able to make a 
good start despite the possible absence of an im- 
mediate food supply in the ground. 
The beautiful bracken and maidenhair ferns which 
are so familiar to all of us, and which grow abun- 
dantly in my near-by canon, are modern types adapted 
to the earth conditions of present geologic time. 
And although there is undoubtedly no such abun- 
dance of fern kinds now as existed in the Carbonif- EVOLUTION OF PLANTS 
155 
erous Age, yet the ferns are by no means, a dying 
race. "In some especially favorable regions, such as 
the higher mountains of Jamaica and in New Zea- 
land," says Douglas Campbell, the Stanford botanist, 
"the number and variety of the ferns is extraor- 
dinary, and they are perhaps the most numerous and 
conspicuous plants that one encounters. From the 
tiny filmy ferns, sometimes less than an inch in height, 
to the majestic tree ferns, raising their magnificent 
crowns of fronds thirty or forty feet above the 
ground, every available spot is occupied by a be- 
wildering variety of these beautiful plants. Mois- 
ture loving as they are, one finds that they become 
scarcer in the drier parts of the world, but many 
species have become adapted to dry regions. For 
instance, there are a number of ferns found in the 
coast regions of California, where for months during 
the long rainless summer they become completely 
dried up and apparently lifeless, but promptly revive 
with the advent of the first autumn rains. In the 
moisture and warmer regions many ferns become 
epiphytes and grow upon the trunks and branches of 
trees. These epiphytic ferns are among the most 
beautiful growths that one encounters in the tropics. 
A few species of ferns are also aquatic in habit, but 
the number of these water ferns is small." 
Below the ferns, and lowest of the land plants, 
are the mosses and liverworts. Even more clearly 156 
EVOLUTION 
than the ferns, these plants show their origin from 
aquatic ancestors (algae) by the character of their 
reproductive organs. They also lack any consider- 
able development of firm or woody tissues and thus 
do not assume the upright position common to most 
of the higher plants, but have a prostrate habit. 
They send out simple hairlike roots upon which, 
however, they depend to only a limited extent for 
their supply of water, as they readily absorb water 
through their leaves. Most of the mosses and liver- 
worts are small plants growing flat on the ground, 
although a few liverworts normally float on water. 
If the water dries up, however, the liverwort settles 
upon the mud and grows very luxuriantly, the con- 
tact with the earth acting apparently as a stimulus. 
Roots are developed, penetrating the mud, and the 
plant assumes quite a different form from that of the 
floating condition. The behavior of these liverworts 
may perhaps illustrate the first step in the develop- 
ment of the higher plants from algalike aquatic an- 
cestors, and the botanists are undoubtedly justified in 
looking at them as representing the most primitive 
of existing green land plants and those which have 
given rise to all the higher types of plant life. 
A curious aberrant group of the lower plants, 
widespread, showing much variety of form and 
habit, and second in numbers only to the flowering 
plants, is that of the fungi, including the mushrooms, EVOLUTION OF PLANTS 
157 
puffballs, molds, mildews, rusts, etc. We know 
nearly 50,000 living species of fungi, all of which 
agree in their lack of that green coloring matter 
called chlorophyll, which is so characteristic of 
almost all other plants and upon the presence of 
which depends the plant's power of breaking up 
carbon dioxide and rebuilding organic compounds 
containing carbon and water in the presence of light 
(photosynthesis). The chlorophyll-less fungi must, 
therefore, derive their needed carbon from already 
formed organic carbon compounds. They are com- 
monly found growing on dead and decaying plant or 
animal matter or living as parasites on and in ani- 
mals and other plants. Some of them, notably the 
wheat rust, corn smut, rye ergot, potato rot, and 
others, cause great injury to various crops. All 
fungi, whether living on dead or live organic matter, 
and whatever their variety of external form, have a 
fundamental identity of make-up. They consist of 
few or many ramifying rootlike filaments which pene- 
trate the organic matter and take up food from it, 
and a number of special filaments which bear spores. 
These spores may be produced in uncountable num- 
bers, as in the toadstools and puffballs, where they 
form a mass of whitish powder. 
Various types of fungi are familiar to all of us. 
Bread exposed to warm moist air soon becomes 
covered with "mold" which is composed of one or 158 
EVOLUTION 
more species of fungi, whose floating spores falling 
on the moist bread have germinated and quickly pro- 
duced a tangled, webby mass of fine threads. These 
ramify through the bread near the surface, breaking 
down the starch by means of the ferments or enzymes 
secreted by the fungus filaments, and using this 
broken-down starch as food. Later, special short 
erect filaments are sent up and from them are given 
off many minute spores. A mushroom or toadstool 
as we see it above ground is merely the spore- 
producing part of the fungus rising quickly at fruit- 
ing time from a mass of underground filaments which 
take up food from the decaying organic matter in the 
soil. A familiar parasitic fungus is that which often 
kills house flies in autumn. It causes the dead flies 
to stick to the windowpane, where each is surrounded 
by a whitish ring composed of tiny spores. These 
are shot off from the ends of filaments protruding 
from the body of the fly, within which the feeding 
filaments have abundantly ramified and broken down 
the tissues of the fly for food. 
The fungi evidently do not lie in the direct line 
of evolution from the simplest green plants (the 
algse), through the generalized liverworts and 
mosses and ferns, to the modern, highly developed 
and specialized flowering plants. They are a side- 
wise line with a kind of specialization of their own. 
It is a successful line as evidenced by the host of EVOLUTION OF PLANTS 
159 
species and their great variety and wide distribution. 
And it is probably a fairly recent line, for some of 
the parasitic fungi occur exclusively on flowering 
plants and hence must have developed since their 
hosts came into existence. Very few fossil fungi 
have been discovered-the habits are against ready 
fossilization-and these few throw no particular 
light on the evolution of the group. It is, however, 
generally assumed by botanists that the fungi are 
descended from plants containing chlorophyll, 
although it is conceivable that they represent a series 
of plant forms which never developed chlorophyll. 
There are a few fungi, like the water molds, whose 
resemblance to certain algae, both in general struc- 
ture and in method of reproduction, is sufficiently 
close to suggest a real relationship. The lichens, 
those curious plant combinations of fungi and algae 
living together in a symbiotic or unusual parasitic 
relationship, constitute a highly specialized group 
of plants, which are quite outside the general line of 
plant evolution. It is a successful special kind 
of adaptation, which involves both fungi and algae of 
several different groups. 
In our further descent of the evolutionary plant 
ladder we find ourselves now facing a series of plant 
groups, all of which are aquatic. We take leave of 
land plants when we get below the ferns and mosses 
and liverworts and the aberrant fungi. All of the 160 
EVOLUTION 
lowest and simplest plants live in the water or under 
such conditions of wetness as are nearly equivalent 
to strictly aquatic conditions. Even in the ferns and 
mosses we found strong indications, especially in the 
structure and habits of their fertilizing cells, of their 
evolution from aquatic ancestors; and our next step 
down the plant ladder gets us definitely into water. 
There is no doubt, indeed, that land plants have de- 
veloped from water plants-just as we shall find in 
tracing the evolution of animals that they, too, origi- 
nated as aquatic organisms. In fact, we shall find 
that both plants and animals trace their ancestry 
back to a few groups of very simple, one-celled 
acquatic organisms that are difficult, if not impossible, 
to differentiate clearly either as animals or plants. 
They are just simplest organisms from which both 
great branches of life, plant and animal, developed. 
Many years ago, the German evolutionist Haeckel 
proposed founding, for the sake of a discriminating 
classification of living creatures, a third great branch 
of organisms called Protista, to include the simplest 
and oldest creatures representing the evolution of 
life before it had become definitely differentiated 
into plant and animal life. 
The plants below the mosses and liverworts are 
called algae, and are divided into three main groups, 
known as the red algae, the brown algae and the 
green algae. The red and brown algae are almost EVOLUTION OF PLANTS 
161 
all marine plants and are familiar to us as the kelp 
and seaweeds abundant on any ocean shore. I see 
dozens of kinds of them each time I go down to the 
near-by tide pools. They mostly live attached by 
rootlike holdfasts to rocks or shells, although a few, 
like the famous Sargassum or gulfweed of the 
Caribbean Sea and South Atlantic, can live as float- 
ing masses far out from land and far above the 
bottom of the ocean. The green algae, examples of 
which are the familiar green pond scums, are mostly 
fresh-water forms, but a few live in the ocean. 
The red algae compose the majority of the sea- 
weeds, but, because of their usually small size and 
their habit of living in deeper water or under the 
shelter of the larger brown seaweeds or rocks, are 
not so well known to casual observers as the brown 
algae. A few forms, however, like the Irish moss, 
which grows where it is exposed to the action of the 
waves and becomes large, and the smaller, curious 
corallines, which deposit carbonate of lime in their 
softer tissue and thus come to look like delicate 
corals, are familiar to any seaside visitors. The 
red seaweeds contain chlorophyll, but it is hidden by 
the red pigment which characterizes most of these 
rather specialized algae. A few of the fresh-water 
forms are, however, greenish or olive or even black- 
ish rather than red. 
The brown algae, owing their color also to brown- 162 
EVOLUTION 
ish and yellowish pigments which hide their chloro- 
phyll, include a number of very large and con- 
spicuous species. Largest and most remarkable of 
all are the giant bladder kelps of the Pacific coast. 
These are characterized by a long, hollow axis with 
fleshy walls, tapering from the smaller end, which is 
attached to the ocean bed by strong holdfasts, to the 
swollen bulbous floating end, which is furnished with 
long, flat, leaflike projections. Some specimens grow 
to a length of 300 feet or more, thus rivaling in 
linear dimensions those greatest land plants, the 
giant Sequoias and redwoods. Sometimes these 
giant kelps grow in such thick beds as to form 
veritable offshore breakwaters, as they do at Santa 
Barbara and near Monterey. Various other brown 
algse, like the "devil's aprons" of the New Zealand 
shore, are broad and flat with tough leathery skin, 
while yet others, as the curious sea palm of the 
Pacific coast, growing on rocks exposed to the full 
attacks of the heaviest surf, resemble small palms 
in appearance. The bodies of all these brown algae 
have a gelatinous tough covering, and are capable 
of resisting not only the beating of waves but ex- 
posure to the dry periods of low tide. They have 
their own peculiar and necessary adaptations, and 
though simple in make-up compared with the land 
plants, are a considerably specialized group. 
The green algae bring us almost to the very EVOLUTION OF PLANTS 
163 
bottom and beginning of the plant series. Most of 
them live in fresh water, but a few live in the ocean, 
especially on coral reefs in the tropics. Some of 
these marine forms have fan-shaped, flattened bod- 
ies; others are jointed and much branched with the 
ends of the branches tipped with bright green tufts 
of hairs looking like the tentacles of the coral polyps. 
Like the corals, too, they secrete a calcareous skele- 
ton and help to build the coral reefs. Some of them 
have been found as fossils in rocks as old as those of 
the Silurian Age. But these marine green algae are 
specialized forms and very unlike the simpler fresh- 
water forms, which abound in ponds and slow 
streams. Many of the green algae are unicellular 
in structure, but some, like the green pond scums, 
have bodies consisting of chains of similar simple 
cells floating free in the water. Closely related to 
the pond scums are the beautiful little unicellular 
desmids, familiar to microscopists who study the 
microcosm of a drop of stagnant water. 
Usually classed with the algae, as their lowest 
types, but by some biologists treated as a separate 
group of lower organisms probably related to the 
bacteria, are the so-called unicellular "blue-green 
algae." These, together with the bacteria, also one- 
celled in structure, are known as the fission plants 
because their only form of reproduction is by simple 
division of the body into two (or more) parts, each EVOLUTION 
164 
of which becomes a new individual. The blue-green 
algas occur in stagnant water or on damp earth and 
possess chlorophyll and can thus live independently, 
but some apparently live as parasites or messmates 
in the bodies of higher plants and animals. Some 
species have been found in hot springs like those in 
Yellowstone Park and, like many of the bacteria, 
are able to endure temperatures fatal to most plants 
and animals. The bacteria, which include many 
species, some of which are the cause of putrefaction 
and of numerous animal and plant diseases, seem to 
lack a definite nucleus in their minute one-celled 
bodies, and may be closely related to the very oldest 
and simplest of living creatures. 
But there are other simple organisms that claim 
this distinction. The one-celled nucleated monera, 
claimed also by zoologists as the simplest animals, 
and the curious slime molds, also claimed by both 
botanists and zoologists, are mere naked masses of 
soft slimy protoplasm, which can move slowly by a 
sort of flowing of their viscous bodies, and reproduce 
themselves by simply breaking up into small nucle- 
ated bits. The slime molds are terrestrial, but are 
active only under conditions of dampness, contract- 
ing and secreting a protective covering when dryness 
comes on. The best known of these slime molds 
live on old tanbark, where they may be found in 
damp weather on cloudy days or in the shade as EVOLUTION OF PLANTS 
165 
slowly moving, light-yellowish soft masses of naked 
protoplasm. 
But of all these simplest living creatures an aquatic 
group of microscopic, one-celled organisms, called 
the Flagellata, are the most interesting to the evo- 
lutionist, as they seem to be the living representatives 
of the immediate ancestors of both plant and animal 
branches alike. They get their name from the deli- 
cate whiplashlike flagellae that project from their 
minute one-celled bodies and by the lashing of which 
the microscopic creatures can swim freely about in 
the water. To them are closely related another 
group of organisms called Volvocales, which are 
small colonies of similar cells living in fresh-water 
ponds or puddles. They are especially significant 
because they seem to reveal the manner in which 
many-celled plants and animals have evolved from 
the one-celled types. These two groups are com- 
posed of active, free-swimming organisms, some of 
which possess chlorophyll, while others lack it. The 
chlorophyll-bearing flagellates closely resemble many 
of the simplest green algae, while the chlorophyll-less 
species show a close resemblance to the cells of such 
low animal types as the sponges. It may, indeed, 
very well be that, as already suggested, from the 
flagellates have developed, and branched off from 
each other, both the plant and animal lines of life. 
Related, perhaps, to the flagellates, are two 166 
EVOLUTION 
groups of microscopic simple unicellular organisms, 
the diatoms and the Peridineae, which occur in 
enormous numbers in the surface waters of the sea 
-the diatoms include many thousand species. They 
form the greatest part of the so-called "plankton," 
or floating life of the ocean, on which all marine 
animals ultimately depend for food. For all of the 
smaller animals and even some of the larger ones 
feed directly on these minute organisms, while the 
other marine animals feed in turn on these plankton- 
fed kinds. The diatoms are especially abundant in 
the colder oceans, and the Peridineae in the tropical 
seas. The diatoms secrete delicate and beautifully 
marked siliceous shells, which, constantly falling to 
the ocean bottom as their minute makers die, build 
up great beds. Upheavals of ocean bottoms in 
various geologic periods have lifted some of these 
beds to be part of the present land, and in various 
parts of the world thick strata of this diatomaceous 
earth or rock have been found. 
This swift survey of the major plant groups, ar- 
ranged in a descending series from highest, most 
specialized and most recent groups to lowest, sim- 
plest and oldest, has, I hope, revealed a glimpse, at 
least, of the general course of plant evolution and 
some of the major kinds of adaptations character- 
istic of it. Various as plant kinds are, and amazing 
as are some of their adaptations, they show, on the EVOLUTION OF PLANTS 
167 
whole, no such extremes of specialization as do the 
animals. Nor are there as many diverging lines of 
evolution, nor perhaps more than half as many 
species, as among the animals. But for this very 
reason one can see in the plants with perhaps more 
clearness than in the animals, the outstanding gen- 
eral course of their evolutionary development and 
the limits of their adaptations. 
Plants are different from animals in structure and 
in physiology; but they are less different than the 
definitions in the old textbooks would indicate. Most 
of them are fixed, a general characteristic of plant 
life; but so are many animals; while some plants, 
mostly lower aquatic ones, swim freely about. All 
of them have certain powers of movement, shown 
by the moving of the protoplasm in the cells, and 
the "nutation," or twisting movement, of the grow- 
ing apex. Some of them have twining tendrils; some 
keep their flower faces to the sun. While in their 
photosynthetic method of obtaining carbonaceous 
food from the air, plants do "take in carbonic acid 
gas and give off oxygen" as stressed by the textbooks, 
yet they also have a true respiration like that of 
animals, that is, they take in oxygen and give off 
carbonic acid gas. And although all of them which 
contain chlorophyll can and do get food directly 
from inorganic materials, which animals cannot do, 
all of the fungi as well as various other higher and 168 
EVOLUTION 
lower forms live on already formed organic sub- 
stances as animals do. Also, if the flagellates and 
volvocales are animals, as the zoologists strenuously 
claim, then there are some chlorophyll-bearing 
animals which can obtain food directly from inor- 
ganic substances. 
But most plants do differ markedly from most 
animals, both physiologically and in structure. The 
power of photosynthesis possessed by all green 
plants is, perhaps, the most important and funda- 
mental distinction of plants, as contrasted with ani- 
mals. Plants are less specialized in their parts and 
in the interdependence of these parts. Plant cells 
are usually well set-off from each other by walls; 
animal cells are not. Although the usual method of 
reproduction is by means of fertilized egg cells, as in 
animals, most plants, even the higher ones, can re- 
produce themselves from other parts of the body. 
They can regenerate parts, and make new wholes 
from small pieces. This is true of only a few ani- 
mals, mostly low in the evolutionary scale. Plants 
are less individualized; indeed they may almost be 
looked on as colonial organisms. They are very 
plastic, and respond individually in very marked de- 
gree to environmental influences; a beech tree in the 
temperate zone may grow to be fifty feet high and 
have widely spreading branches; in arctic regions or 
near the top of a high mountain it may never reach EVOLUTION OF PLANTS 
169 
more than a few inches in height. The power of 
individual adaptation in plants is, in general, much 
greater than that of animals. 
Like the animals, many plants have been domes- 
ticated by man and greatly modified by changed 
environment, selection and hybridization, and the 
distribution of many has been much affected by man. 
The United States Department of Agriculture main- 
tains a bureau entirely devoted to the search for, and 
importation and domestication of, foreign plants. 
Man has played a great modifying role in relation to 
the forests, grasses, grains, edible vegetables, fruits 
and nuts, and ornamental flowering shrubs and trees. 
Quite as true is it that these plants have had a large 
influence on man. Much of the pattern of man's 
present spread over the earth has been determined 
by forests and by agriculture; so, although plant evo- 
lution has no such intimate relation to human evolu- 
tion as has animal evolution, nevertheless it has had 
its influence in determining human evolution, espe- 
cially that part of it which we call social or societal 
evolution-a very important part indeed. CHAPTER X 
THE EVOLUTION OF THE ANIMALS: 
THE INVERTEBRATES 
When we speak or hear of evolution, our first 
thought is likely to be of the evolution of man- 
our view of the world is naturally anthropocentric. 
The next thought is of the evolution of the animals 
-we recognize their kinship to us. And only the 
last thought is of the evolution of the plants-they 
stand farthest from us. In the fleeting survey, 
however, of the evolution of the various groups of 
organisms, begun with the last chapter, I have, for 
various reasons, arranged the consideration of the 
evolution of man, the animals, and the plants in 
inverse order to that of our interest. One reason is 
precisely that of this relative difference in our inter- 
est in human, animal and plant evolution. By taking 
up the plants first we avoid too much self-interest, 
too much prejudice, perhaps. Another reason is that 
the plants have gone less far, less widely, and less 
variously in their evolution than the animals, and we 
are, therefore, rather easily able to comprehend the 
general course of their evolution. Thus, we gain a 
certain amount of confidence in our capacity to 
170 EVOLUTION OF INVERTEBRATES 
171 
trace evolutionary lines, a confidence that we need 
more and more as we approach the almost baffling 
evolutionary complexes and extremes met with in the 
higher animals and man. 
Just as the plants are divisible into two great 
groups, of different evolutionary rank, namely, the 
higher, or flowering plants, and the lower, or flower- 
less plants, with some uncertain and perhaps linking 
forms on the border line between them; so the ani- 
mals are divided into the two major groups of ver- 
tebrates, including the higher forms, and inverte- 
brates, which include a great host of lower kinds, 
some, however, of extreme and very successful spe- 
cialization. It is of interest to note that the higher 
group of plants, the flowering kinds, contains nearly 
as many living species as all the various groups of 
flowerless plants together, while among the animals 
the higher vertebrates are greatly outnumbered by 
the lower invertebrate kinds. Indeed, the class of 
insects alone includes nearly three fourths of all the 
known living animal species. 
Although recognizing the plants as a group in- 
ferior in specialization and general evolutionary rank 
to the animals, we must not think of the plants and 
animals as constituting a linear evolutionary series, 
the lower animals rising from the higher plants. 
The plants and animals constitute two separate, or, 
better, divergent lines of evolution, which arose from EVOLUTION 
172 
simple, generalized, common ancestors in the early 
history of the earth. For just as the lowest plants 
are aquatic, motile one-celled kinds of very simple 
structure and behavior, so it is, also, among the ani- 
mals. Indeed, as we have already noticed, there are 
at the bottom of the plant and animal ladders a con- 
siderable number of living kinds of organisms which 
are claimed by the botanists as the simplest plants, 
and by the zoologists as the simplest animals, or are 
looked on by less self-interested biologists as con- 
stituting a separate ill-defined group of primitive 
organisms in which the two great living kingdoms of 
plants and animals find a common origin. 
The simplest animals comprise a large and various 
group called the Protozoa, and are almost all micro- 
scopic in size. The bodies of most of them are com- 
posed, for their whole lifetime, of but a single cell. 
A few, however, sometimes called colonial Protozoa, 
have the body made up, for a part of their life, at 
least, of from a few to many similar cells, some of 
which may become modified to form special germ 
cells. But even in these colonial Protozoa there is 
no organization of cells into tissues or organs. By 
cell is meant simply a small unit bit of protoplasm, 
with a nucleus that is also protoplasm, but of some- 
what different character from that of the rest of the 
cell body. The single cell of some protozoan bodies, 
as those of Amoeba and similar forms, is quite naked, EVOLUTION OF INVERTEBRATES 
173 
and has no more definite shape than has a droplet 
of any thick viscous substance. In others, the pro- 
toplasm forms a thin but firm outer covering, or 
even secretes a tiny calcareous or siliceous shell, 
which give the little creatures a definite fixed shape. 
These shell-secreting Protozoa remind one, by this 
character, of the diatoms (one-celled plants with 
siliceous shells). Like the diatoms, also, they occur 
in immense numbers in certain ocean regions and 
their shells accumulate on the ocean bottom in thick 
beds of great extent, some of which have been up- 
heaved in past geologic times to form part of the 
earth's crust. The great chalk beds and cliffs of 
England, France, Greece, Spain and America are 
composed of countless numbers of tiny shells of 
lime-secreting Protozoa, while the rock called 
Tripoli found in Sicily, and the Barbadoes earth 
from the island of Barbadoes, are composed of the 
minute siliceous shells of other ancient Protozoa. 
Although the one-celled Protozoa have a body of 
great simplicity compared with that of any of the 
many-celled animals-our own bodies have been esti- 
mated to contain about three trillions of cells-the 
single cell body of some of these Protozoan bodies 
shows a good deal of differentiation and specializa- 
tion. In amoeba and other similar generalized 
forms any part of the tiny naked protoplasmic 
body can do what any other can, that is, help in 174 
EVOLUTION 
locomotion by slowly flowing out into temporary 
fingerlike projections (pseudopodia), take in food 
by flowing around it, digest this food, secrete wastes, 
breathe, and take part in reproduction, which con- 
sists simply in the dividing of the body into two 
parts (as in the fission plants). But in the more spe- 
cialized Protozoa the body has a fixed shape, with 
a definite number of projecting flagellae, or cilia, 
for swimming organs, a definite mouth opening and 
an opening for discharging waste, special contractile 
parts, and an accessory nucleus besides the regular 
larger one. Also, reproduction may involve the per- 
manent or temporary fusing of two individuals be- 
fore division takes place. 
The first step in the evolution of the many-celled 
from the one-celled animals is illustrated by the 
make-up of those interesting small creatures called 
volvocales, which are common in fresh-water ponds, 
and are claimed by botanists as the simplest multi- 
cellular plants, and by the zoologists, under the name 
of colonial Protozoa, as the simplest multicellular 
animals. There are several kinds of these interest- 
ing links between strictly one-celled and true many- 
celled organisms, but all agree in having a body 
composed of a few (sixteen or thirty-two) to many 
similar cells which hold together (usually in a ge- 
latinous envelope) in a flat or spherical group, until 
the time for reproduction arrives. Then each cell EVOLUTION OF INVERTEBRATES 
175 
divides into a small group of daughter cells (new 
colony), or a few of the cells become modified to act 
as quiescent egg and active sperm cells while the rest 
of the cells die, thus ending the old colony. Each 
of the cells of the old colony is like every other cell 
and can do what any other cell can do, at least until 
the time for reproduction comes. Each one of these 
colonies is thus an independent organism, with its 
make-up resembling, in its essential character, that 
early embryonic stage in the life of all many-celled 
organisms which is produced by the repeated division 
of the fertilized egg cell. 
From the one-celled and colonial Protozoa the 
first step up the existing evolutionary animal ladder 
brings us to the sponges, the least complex of all 
the strictly multi-cellular animals. They possess so 
little differentiation of cell character and such a lack 
of individualization that they are hardly more than 
larger, and somewhat more complex, colonial Pro- 
tozoa. The body contains no such systems of organs 
as characterize the higher animals; there are no 
heart, no lungs, no alimentary canal, no nervous sys- 
tem, or eyes or ears or other organs of special sense, 
no organs of locomotion. It is simply an aggre- 
gate of cells, arranged in two layers, with a gelat- 
inous substance between them in which protoplasm 
ramifies and a number of separate cells lie. The 
whole is supported, usually, by a skeleton of horny 176 
EVOLUTION 
fibers or siliceous spicules. It is of a simple vase 
shape, or of hardly any definite shape at all, without 
front or back, right or left, growing attached to a 
rock or shell in the ocean, or, in the case of the few 
fresh-water kinds, to a stone or piece of wood in 
pond, river or canal. There are numerous small 
openings scattered all over the body surface, lined by 
cells with waving flagellae, and a single larger open- 
ing at the free end of the body. By the waving of 
the flagellas, currents of water are drawn into the 
small openings, bringing oxygen to be breathed and 
tiny organisms to the cells to be captured and 
digested. These cells also give up carbon dioxide 
and food wastes to the water, which passes from the 
lateral openings into the central cavity or cavities 
of the sponge and on out through the large opening 
at its free end. The one necessary condition for 
the life of a sponge is the streaming of water 
through its body. 
The sponges reproduce themselves, both sexually 
by fertilized egg cells, and asexually by small budded- 
off groups of body cells which either swim away, 
become attached to a firm object and grow into a new 
sponge, or remain attached to the mother body and 
develop there, thus producing an irregular colony 
with the general appearance of a branching plant. 
In the sexual mode of reproduction, male and female 
germ cells are developed in the same individual, and EVOLUTION OF INVERTEBRATES 
177 
the motile male cells swim about in the canals and 
cavities of the sponge body until they find the egg 
cells, which they fertilize. The fertilized egg cells 
then begin to develop and pass through their first 
stages in the sponge body. Finally the embryo 
sponge, which is usually a tiny oval mass of similar 
cells, with cilia on the outer cells for swimming, 
escapes from the body of the parent into the outer 
water where it swims about for some time, then 
comes to rest on the ocean floor and attaches itself 
to some rock or shell, and begins to take on the 
form and character of the parent, leading, from this 
time on, a fixed sedentary life. 
"Sponges," as most of us know them, are merely 
the horny sponge skeletons of a few species. But 
other sponges are found in almost all oceans, from 
the tide lines to the greatest depths, and vary in size 
from a small fraction of an inch to a yard in height. 
They may be reddish, purple, orange, gray or even 
bluish in color. The horny sponge skeletons, which 
are the sponges we use, all belong to a few species 
which grow in the Mediterranean and Red seas and 
along the coasts of Greece, Asia Minor, Africa and 
the Bahama Islands. 
The next step is to a large branch of animals 
which includes the; polyps, corals, sea anemones 
and jellyfishes. Most of them live in the ocean (a 
few in fresh water) and, like the sponges, are fixed EVOLUTION 
178 
in the adult stage, and somewhat resemble plants 
in general appearance, hence the name "zoophytes" 
(plant animals), given them by earlier naturalists. 
The jellyfishes, however, and the young polyps, 
which are jellyfishlike in make-up, swim about 
freely. Along any seashore we can see members 
of this great animal group. In some of the tide 
pools near my cottage there are hundreds of sea 
anemones of various kinds, some of them very beau- 
tiful. And after every storm at sea the cast-up, 
soft, gelatinous bodies of jellyfishes, from tiny 
"umbrellas" of half an inch to great ones two feet 
or more in diameter, strew the beach. 
The sea anemones and polyps have a body shape 
and structure something like that of the sponges, 
but more definite, more differentiated and special- 
ized, and more individualized. The body is a short 
thick tube, composed of two distinct cellular layers, 
separated by a thin noncellular membrane. It is at- 
tached by its base to some rock or firm object, and 
has a large opening at its free end surrounded by a 
circlet of sensitive contractile tentacles which seize 
objects of food and thrust them into the interior of 
the hollow body. The numerous small lateral open- 
ings of the sponge body are missing, water and food 
entering the body, and water and wastes leaving it, 
by the one hole at the upper end. 
The exquisite jellyfishes, or medusae, have the EVOLUTION OF INVERTEBRATES 
179 
body in the general form of an umbrella or shallow 
bell. Around the edge of this bell are set numerous 
threads or tentacles with stinging cells (correspond- 
ing to the circlet of tentacles in the polyps and sea 
anemones). The mouth opening is at the middle of 
a longer or shorter projection which hangs down 
from the middle of the underside of the umbrella. 
The body cavity extends out into the umbrella- 
shaped part of the jellyfish, usually as a series of 
canals radiating from the center, with a connecting 
canal running around the margin of the umbrella, 
all forming a sort of special digestive cavity. 
In both the polyps and jellyfishes there are the 
beginnings of cell specialization into muscle and 
nerve and stinging cells, and the beginnings of spe- 
cial organs, such as simple sense organs and diges- 
tive canals. Reproduction is carried on much as in 
the sponges, that is, both asexually by budding, 
and sexually by the fusion of quiescent egg cells and 
motile sperm cells produced by the same adult indi- 
vidual. But the process of reproduction and de- 
velopment of new individuals is carried on in more 
specialized manner than it is in the sponges. Some 
of the polyp buds develop into jellyfishlike bodies 
(medusae) which swim away from the parent body 
and later produce both egg and sperm cells. After 
fertilization, the egg cell develops into a free-swim- 
ming larva called a planula, which resembles neither 180 
EVOLUTION 
a jellyfish nor a polyp, but later becomes attached 
and develops into a polyp. With many polyp kinds 
the young polyps, and even also the young medusae, 
remain attached to the parent body and, like the 
sponges, produce a colony. But this colony may 
show a considerable variety among its adhering in- 
dividuals. The most specialized of these polyp- 
medusae colonies are the extraordinary floating 
colonial jellyfishes, like the marine "Portuguese man- 
of-war" which appears as a delicate bladderlike float, 
usually about six inches long and brilliant blue or 
orange in color, bearing on its upper surface, which 
projects above the water, a raised particolored crest, 
and on its under surface a tangle of various thread- 
like appendages bearing grapelike clusters of little 
bell- or pear-shaped bodies. Each of these parts is 
a modified polyp- or medusa-zooid produced by bud- 
ding from an original central zooid. 
In these colonies we find an extraordinary condi- 
tion. They are made up of many polyp and medusa 
individuals, each of which sacrifices all its functions 
except one which it performs for the whole colony. 
(Thus, some individuals serve as swimming organs, 
some as feeding organs, some as sense organs, some 
as stinging organs, and some as reproductive organs. 
But each one originates as a distinct individual and 
not as a single part or organ of an individual. 
Some of these colonial polyp-jellyfishes have for EVOLUTION OF INVERTEBRATES 
181 
a central zooid a long upright tube with hundreds 
of variously shaped parts attached around it. The 
upper end of the tube is enlarged to form an air- 
filled chamber, a saclike boat, by means of which the 
whole colony is kept afloat. Around the central 
stem are many delicate bells, the opening and clos- 
ing of which make the whole colony swim through 
the water. Each swimming bell is a modified 
medusa-zooid, without tentacles or digestive or re- 
producing organs, but retaining simply the function 
of swimming. Below the swimming bells, at the 
lower end of the central stem, are grouped many 
structures presenting at first sight a confusion of 
variety and complexity, but on careful examination 
revealing themselves to be polyp- and medusa-zooids 
modified to form at least five kinds of functioning 
structures. There are many flattened scalelike parts 
whose function is simply that of affording a passive 
protection, in times of danger, to the other struc- 
tures. These protecting scales are greatly modified 
medusa-zooids, each consisting of a simple cartilage 
like gelatinous mass penetrated by a food-carry- 
ing canal. Under these broad leaves are a number 
of pear-shaped bodies which have a wide octagonal 
mouth opening at their free end and possess in 
their interior certain digestive glands. Each one is 
provided with a very long flexible tentacle which 
bears many fine stinging threads. The tentacle 182 
EVOLUTION 
waves back and forth in the water, and on coming 
in contact with an enemy or with prey its poisonous 
stinging threads shoot out and paralyze or wound 
the unfortunate animal. These pear-shaped bodies 
are the feeding organs, each being a modified polyp- 
zooid. Scattered among these dangerous structures 
are many somewhat similarly shaped but wholly 
harmless structures, the sense organs. Each of 
these, too, has a pear-shaped body, but without 
mouth opening, and also a long, very sensitive, ten- 
taclelike process. The sense of feeling is very highly 
developed in the tentacles, and they discover for 
the colony the presence of any strange body. These 
sense structures are modified polyp-zooids. Finally, 
there are two other groups of structures, usually 
arranged in grouplike bunches of grapes, which are 
the reproductive structures, male and female. They 
are modified medusa-zooids grown together, and 
without tentacles. 
This whole colony, or this compound animal, 
floats or swims about at the surface of the ocean, 
and performs all of the necessary functions of life 
as a single complex animal does. Yet it is, in 
truth, a community in which the hundreds of parts 
are different individuals all of one species. It is, 
in effect, the same kind of communal life, with a 
differentiation of labor and specialization of struc- 
ture among the community individuals, as is re- EVOLUTION OF INVERTEBRATES 
183 
vealed by those familiar communal animals, the ter- 
mites, social wasps, bumblebees and honeybees. In 
these insect communities, however, the individuals 
are all separate, while in the colonial polyp jelly- 
fishes they are all fastened together. They are 
fastened together so closely, and so in the manner 
of the various organs of a single complex individual, 
that it is only by a study of the origin of each part 
that we find that it is, in reality, a single, much 
modified polyp or medusa individual. 
Thus, in this great branch of lower animals, made 
up of the polyps, corals, sea anemones and jelly- 
fishes, we find the plain beginnings of a specializa- 
tion of cells and of simple organs and also an ex- 
ample of colonial, or communal life. And we find 
these animals, although possessing their own pecu- 
liar specializations and adaptations, clearly stand- 
ing in the general evolutionary line toward the higher 
animals which we have seen to begin with the one- 
celled and colonial Protozoa, and advance through 
the generalized sponges to the more specialized sea 
anemone and polyp type. But from here on we 
find no such clear single line of evolution. The 
other great branches of animals, although showing 
in the character of their most generalized members 
enlightening indications of such a line, are so diver- 
gent and reach such extremes in their own peculiar 
development that they can hardly be arranged in 184 
EVOLUTION 
any straight linear series. The arrangement must 
be of a pronounced tree-branching character. 
The branch of animals which is usually treated 
in the zoological textbooks, next after the polyps and 
jellyfishes, is that of the echinoderms, which include 
the starfishes, sea urchins, sand plates, sea cucumbers 
and feather stars. They are all marine, the star- 
fishes and sea urchins being among the most familiar 
animals of the tide pools, and are all, except the 
feather stars, not fixed but able to move freely, 
although only slowly, about. These feather stars 
are the living representatives of the crinoids, those 
widespread and abundant echinoderms of earlier 
geological ages. They differ from all other mem- 
bers of the branch in being fixed either permanently 
or for a part of their life, being attached to rocks 
on the sea bottom by a longer or shorter stalk, which 
is composed of a series of rings or segments. 
The body shape of the echinoderms varies from 
the flat, rayed body of the starfish and the thin disc 
of the sand plate, to the thick, flattened egg-shape 
of the sea urchin, the melonlike sac of the sea 
cucumber, and the delicate, many-branched head of 
the feather star, with its supporting slender stalk 
attached to some firm object. But in all these 
shapes we can see, more or less plainly, a symmetri- 
cal radiate arrangement of the body. For there is 
always a central portion from which radiate sepa- EVOLUTION OF INVERTEBRATES 
185 
rate arm- or branchlike parts, or about which are 
arranged radiately the internal body parts; al- 
though the external appearance may, at first sight, 
give no plain indication of the radiate arrangement. 
The radiating parts of the body are usually five. 
There is a certain degree of radiation in the body 
structure of the sea anemones, polyps and jellyfishes, 
and it is possible that the echinoderms have derived 
and further developed this radiate condition, so char- 
acteristic of them all now, from an ancestral polyp- 
like type of generalized radiate character. But there 
is little else in the echinoderm body to suggest the 
polyp, or jellyfish type of structure. 
All the echinoderms have specialized cells and 
tissues and well-developed systems of organs. They 
are far above the sponges and polyps in degree of 
specialization of these tissues, organs and organ 
systems. They have, for instance, a well-developed 
digestive system, with mouth, alimentary canal com- 
posed of esophagus, stomach, intestine, caeca and 
special glands secreting digestive fluids and an anal 
opening. This alimentary canal is not, as in the 
polyps, simply the body cavity, but it is an inclosed 
tubular cavity lying within the general body cavity. 
There is a well-developed nervous system consisting 
of a central nerve ring around the esophagus, and 
branches radiating from it into the various radially 
arranged arms or regions of the body. There is no EVOLUTION 
186 
brain as in the higher animals, but the central nervous 
ring contains nerve cells as well as nerve fibers. The 
only sense organs are special tactile or touch organs 
in all the members of the branch and very simply 
composed eyes or eyelike organs at the tips of the 
rays of starfishes. Some of the echinoderms breathe 
simply through the outer body wall, taking up, by 
osmosis, the air mixed with the water in which their 
bodies lie, but some have very simple, special gill- 
like respiratory organs. There is also a distinct cir- 
culatory system, but the "blood" which is carried by 
this system and which fills the body cavity consists 
mainly of sea water containing a number of amoeboid 
blood cells. There is no organ really corresponding 
to the heart of the higher animals. There are dis- 
tinct organs for the production of the germ or re- 
productive cells, and the sexes are distinct (except in 
a few species). All the echinoderms (except some 
of the feather stars) have organs of locomotion and 
well-defined muscles to move these organs, which are 
short flexible processes called tube feet. The spines 
of the sea urchins also help in their locomotion. 
Differing from that of the sponges and the polyps 
and jellyfishes, the reproduction of the echinoderms 
is always sexual. Young or new individuals are 
never produced by budding, or in any other asexual 
way, although most echinoderms have the power of 
regenerating lost parts, even to such a degree that EVOLUTION OF INVERTEBRATES 
187 
some starfishes can regenerate all the rest of a body 
from a single ray with a bit of the central disc. The 
new individual is always developed from an egg pro- 
duced by a female individual and fertilized by the 
sperm of a male individual. The eggs are very 
small (about i-50th inch in diameter in certain star- 
fishes) and are fertilized by the sperm cells after 
leaving the body of the female. That is, both sperm 
cells and egg cells are poured out into the water by 
the adults, and the motile sperm cells in some way 
find the egg cells and fertilize them. From the 
eggs hatch tiny larvse which do not at all resemble 
the parent starfish or sea urchin. They are active, 
free-swimming creatures more or less ellipsoidal in 
shape, and provided with cilia for swimming. Soon 
the body changes form and takes on a curious shape 
with prominent projections. From these larval 
stages the adults develop by changes, or metamor- 
phoses, as striking as those of the butterflies and 
moths. 
Of a markedly different general plan of body are 
all those mostly small and extremely various lower 
animals which the older naturalists lumped together 
in one great heterogeneous branch called the articu- 
lates; that is, animals with jointed bodies, such as 
the earthworms, leeches and other worm kinds, and 
the crabs, crayfish, centipedes, insects and spiders. 
Although modern naturalists break up this miscella- 188 
EVOLUTION 
neous group into several branches, the older classi- 
fication stresses their common possession of a 
segmented body. There is, however, a great va- 
riety of appearance among them. In the more 
specialized forms the segments may be so fused and 
modified that they are recognizable only in the 
younger embryological stages, but in such forms as 
the earthworms and tapeworms, the crayfish and 
lobsters, the centipedes, scorpions and simpler in- 
sects, the successive body segments are plainly vis- 
ible all through life. There may be many of these 
segments and, except for the head and perhaps the 
hindmost segments, they may be much alike, as with 
the earthworms and centipedes. Or, they may be 
very few and mostly different from each other in size 
and shape, as with the more specialized insects and 
the spiders. 
Corresponding to the general segmentation of the 
body is the segmental arrangement of external 
mouth parts, legs, wings and organs of special sense 
(feelers, eyes, etc.), which are attached, usually in 
pairs, to various segments. The legs may be many, 
or few, or none. The wings, which occur only in 
the insects, are never more than two pairs and, like 
the three pairs of legs possessed by most insects, are 
attached to the segments of the middle (thoracic) 
part of the body, these locomotory organs thus being 
concentrated about the body's center of gravity. The EVOLUTION OF INVERTEBRATES 
189 
internal organs and organ systems are (except in the 
case of various degenerate parasitic worms) well 
developed and also show a distinct segmental and 
symmetrically bilateral condition. The more com- 
pact the body, and fused its segments, the more con- 
centrated and less obviously segmented are the vari- 
ous organ systems. The general evolutionary line 
of specialization among the segmented animals is 
from an elongate, slender body, composed of many 
similar segments with many pairs of similar external 
segmental appendages and distinct repetitive seg- 
mental arrangement of the internal organ systems, 
to a short, compact body, composed of a few much 
modified and closely fused segments with few pairs 
of external appendages and strongly concentrated in- 
ternal organ systems, with little repetition of parts. 
From centipede to house fly illustrates this evolu- 
tionary line. 
These segmental animals show different special 
lines of evolution. Present-day knowledge of these 
diverging lines is indicated by the modern classifica- 
tion. This breaks the articulates up into five 
branches of worms and wormlike animals, and one 
very large branch including the crustaceans, myria- 
pods, insects, and spiders, mites and ticks. Some of 
the wormlike groups are perhaps not fundamentally 
articulate in structure and hence should not be in- 
cluded in the general evolutionary line of segmental 190 
EVOLUTION 
development. In the branch with the crustaceans, 
myriapods, insects, and spiders, and forming an in- 
teresting sort of link between them and the seg- 
mented worms, is a small group of wormlike animals 
called slime slugs, which show in their make-up a 
combination of characters of the segmented worms 
and the myriapods, which in turn connect closely 
with the simpler insects. 
In the branches of flat worms and round worms 
there are many kinds which live as external or in- 
ternal parasites on other animals, and which have 
developed extraordinary adaptive specializations 
both of structure and habit, to fit themselves spe- 
cially for parasitic life. Much of the structural 
specialization of these parasites is along the line of 
degeneration, loss of parts (locomotor and special 
sense organs, etc.), and a retrogression toward sim- 
plicity of body make-up. This is a simplicity by 
specialization, however, and not by generalization; a 
lack of complexity, not original but acquired by evo- 
lution. Not all evolutionary movement is advance 
toward higher forms and higher powers; it may be 
toward acquired simplicity and degenerative condi- 
tions. But it is still evolution toward fitness. The 
parasite is often among the most fit of creatures; it 
is fit for its particular mode of life. But it is such 
a specialized and canalized kind of life that in the 
event of a catastrophe to host species the parasite EVOLUTION OF INVERTEBRATES 
191 
species also faces catastrophe. It cannot change 
back or away from its too complete adaptation to 
just one set of living conditions. 
It seems absurd to pass by the great insect class 
without even a reference to the amazingly many 
and various examples of evolutionary development 
and adaptation it presents. Greater in number of 
kinds than all the other animals put together, almost 
all the changes conceivable are rung by the insects 
on the basic motif of a single fundamental type 
of form and physiology. Adapted for life in the 
air, the water, the soil, the trunks of trees, the 
leaves of plants, the bodies of other insects and ani- 
mals, feeding on leaves and fruits and seeds or hard 
wood, burrowing into animal flesh, eating feathers 
and hair, sucking plant sap or blood, taking food 
through a mouth, or simply through the skin, hav- 
ing wings and legs or no wings and no legs, clad in 
all the colors and patterns of a kaleidoscope, and 
showing such examples of protective coloration and 
mimicry as are to be found nowhere else in the ani- 
mal kingdom, the insects, abundant in species and 
individuals, with long series of continuous small 
gradations from species to species, and yet attain- 
ing large differences and great evolutionary dis- 
tances, offer a most fertile field to the student of 
evolution. But we cannot enter it here. In a later 
chapter, however, we shall have opportunity for, EVOLUTION 
192 
at least, a brief account of insect instinct. It is, 
above all, among insects that instinct reaches its 
greatest diversity and highest development. 
Still of another body plan are the mollusks, con- 
stituting the last great branch of invertebrates, and 
including the mussels, clams, oysters, snails, slugs, 
cuttlefishes and octopuses, and all that host of ani- 
mals we call "shells" or shellfish. We know them 
familiarly only by the houses which they make, live 
in, and leave at death to tell the tale of their ex- 
istence. The variety in form, colors and markings 
of these shells indicates the great diversity among 
their makers. They live on land, in fresh water 
and the ocean. No depths of the ocean abysses are 
too great for the octopuses, no coast is without its 
many shells, hardly a pond or stream but has its 
mussels and pond snails, and in all regions the land 
snails and slugs abound. 
The mollusks are not to be mistaken for any 
other of the lower animals; they have a structure 
peculiarly their own. In them the body is not ar- 
ticulated or segmented as with the worms and crabs 
and insects, nor radiate as in the starfishes and sea 
urchins, nor plantlike as with the sponges and polyps. 
Where the typical molluskan body is well developed, 
it is composed of four principal parts: a head, with 
the mouth, feelers, eyes and other organs of spe- 
cial sense; a trunk containing the internal organs; EVOLUTION OF INVERTEBRATES 
a foot, which is a thick muscular mass not at all 
foot- or leglike in shape, but which is the organ of 
locomotion by means of which the creature crawls; 
and a mantle, which is a fold of the skin inclosing 
most of the body and which produces the shell. Such 
a typical molluskan body is possessed by most of 
the snails. But in most of the other mollusks one 
or more of these four body regions are so fused with 
some other region as to be indistinguishable. In the 
mussels and clams the head is not at all set off from 
the rest of the body, the cuttlefishes and octopuses 
have no foot, and the slugs have no shell. 
The internal organs and organ systems are well 
developed. The nervous system includes a brain, 
and the circulatory system has a pulsating sac com- 
posed of two or three chambers which can fairly 
be called a heart, and there is a well-defined closed 
system of arteries and veins. Especially in the de- 
velopment of their circulatory system do the mol- 
lusks stand above all other invertebrates. 
Reproduction is always sexual. In most species 
the young mollusk on hatching from the fertilized 
egg does not resemble the parent, but is a free- 
swimming larva which must undergo a considerable 
metamorphosis before reaching the adult stage. 
In this respect the mollusks are like the echino- 
derms and many of the articulate animals, notably 
the crustaceans and most insects. It seems prob- 
193 194 
EVOLUTION 
able that this marked metamorphosis in the course 
of development often accompanies, and is an indi- 
cation of, a high degree of specialization or diver- 
gence from the ancestral generalized type of the 
group. In such cases the larvae, unless too highly 
modified adaptively to meet immediate needs, should 
give some idea of the group ancestor. For example, 
the caterpillar of a moth or butterfly, although itself 
much modified so as to meet the immediate condi- 
tions of life it faces, undoubtedly represents in some 
degree the segmented wormlike ancestors of the in- 
sects. The winged butterfly has come such a long 
distance from its wormlike ancestor that we ordi- 
narily would never connect the two. But if we wish 
to visualize the far ancestors of the butterflies we 
have but to look at their caterpillars. What an in- 
teresting revelation of evolution at work I CHAPTER XI 
THE EVOLUTION OF THE ANIMALS: 
THE VERTEBRATES 
All the thousands of animals of the great branches 
we have so far referred to agree in being inverte- 
brates, creatures without a backbone and the rest 
of that internal bony skeleton characteristic of the 
vertebrates. These latter, which include the fa- 
miliar classes of fishes, amphibians, reptiles, birds 
and mammals, we call the "higher" animals, to dis- 
tinguish them from that long and confusing array 
of smaller creatures of sea and land which are con- 
veniently lumped together as "lower" animals. That 
the higher animals have arisen by evolution from the 
lower ones is proved by such a mass of evidence that 
all biologists accept such origin as a fact. But from 
what particular branch or group of invertebrate ani- 
mals the vertebrates have arisen is still a matter of 
question. 
But before we reach the true vertebrates we find 
a few odd kinds of marine creatures, simpler in struc- 
ture than the vertebrates, but yet associated with 
them by modern students of classification. They 
group them with the vertebrates to form a branch 
195 196 
EVOLUTION 
called Chordata, so called because of the notochord 
characteristic of all of them. This notochord is 
an internal, slender, gristly rod which extends the 
length of the body along the back and serves to sup- 
port the nervous system. It is present in the young 
of all vertebrates, being replaced in older stages by 
the more highly developed cartilaginous or bony 
jointed backbone, but in the lowest Chordates it 
persists throughout life; no vertebral column is 
formed. In a small, delicate, almost transparent, 
fishlike animal called the lancelet, or amphioxus, the 
notochord persists throughout life, and is the only 
internal skeleton possessed by it. In the lampreys, 
a group of larger eel-like animals, the notochord also 
persists through life, but these creatures have a car- 
tilaginous skull at its anterior end. Both the lance- 
lets and the lampreys are classed by some biologists 
with the fishes, as the lowest members of the group, 
but by others they are classed as independent pro- 
fish forms. 
The lowest Chordates are certain curious marine 
creatures, called sea squirts, or ascidians. They 
have leathery, saclike bodies, and live singly or in 
colonies, or even so closely associated as to form a 
sort of compound animal. We might expect them 
to show such traces of their invertebrate ancestry as 
would indicate from which of the lower inverte- 
brate branches they have arisen; but they are so EVOLUTION OF VERTEBRATES 
197 
degenerate in structure that they give no satisfactory 
clues to their evolutionary origin. This degenera- 
tion is due probably to-the fact that although they 
are active tadpolelike creatures when hatched, they 
mostly soon become attached to rocks or shells, 
and take on the simple saclike form characteristic 
of their adult condition. 
The lancelets, of which only about ten living, 
rather widely scattered, species are known, are only 
from half an inch to four inches in length and live 
chiefly in sand, in warm seas. Heretofore, they 
have been looked on as rather rare animals, but 
recently one species has been discovered to exist in 
great numbers in a limited region of the China 
coast. Although they have a well-developed car- 
tilaginous notochord running from head to tail, with 
a nervous cord above it, inclosed in a special mem- 
branous sheath, they have no skull or brain. The 
mouth is a mere vertical slit without jaws. The 
circulatory system is fishlike, with closed blood ves- 
sels, but there is no heart, the blood being driven 
about by the contraction of the walls of the vessels. 
Along the edge of the back and tail is a rudimentary 
fin, but there are no paired lateral fins which, in the 
true fishes, correspond to the arms and legs of 
other vertebrates. In the character and arrange- 
ment of its parts, the lancelet is certainly a fish; but 
in degree of development it differs more from the 198 
EVOLUTION 
lowest true fish than such a fish does from a mammal. 
Lancelets may be regarded as vertebrates expressed 
in the lowest terms. 
Let us begin now with the true vertebrates. They 
are characterized by the possession of a bony in- 
ternal skeleton composed of a longitudinal axis, the 
backbone, terminating anteriorly in a skull and pos- 
teriorly in a tail, with a smaller or larger number 
of ribs inclosing incompletely the main body cavity, 
and with two pairs of limbs connected with the axis 
by a shoulder and a pelvic girdle. In them we have a 
remarkable series of animals showing, despite much 
plasticity of adaptation and marked lines of lateral 
development, a close adherence to a general struc- 
tural plan and a steady advance along a major evo- 
lutionary line. The fishes, the amphibians, the rep- 
tiles, the birds and the mammals grade with more or 
less clearness into each other, even the living link- 
ing forms being sufficient to establish these genetic 
gradations, let alone the impressive confirmatory evi- 
dence derived from past forms preserved as fossils. 
Comparative anatomy, embryology, paleontology 
and geographical distribution all offer their strong 
and mutually supporting evidence of the general line 
of vertebrate evolution, with its triumphant termina- 
tion in that highest mammal, man. 
Of these vertebrate classes the largest is that of 
the fishes, of which about 15,000 living species are EVOLUTION OF VERTEBRATES 
199 
known, 3,000 of them living in North America. The 
typical fish body is one well formed for progres- 
sion in the water, being pointed at each end (the 
shorter point in front) and with the sides flattened, 
the back and belly rather narrow, and the motive 
power located in the tail. But from this typical 
form diverge manifold variations, adaptations to a 
wide variety of habit and specific mode of life. 
These adaptations affect the size and shape of the 
body, the character of the fins and tail, the colors 
and pattern of the skin and scales. In the flounders, 
which are flattened and lie on one side on sandy 
bottoms, the eye that would normally be on the 
under side, moves during development, around to 
the upper side of the twisted head. When the 
flounder is first hatched, the eyes are on the two 
sides of the head and the creature swims upright in 
the water like other fishes. 
But whatever and however radical the adaptive 
modification of the body, whether flattened as in the 
flounders, slender, cylindrical and snakelike as in the 
eels, long and narrowed from side to side as in the 
ribbon fishes, or almost spherical as in the globefish, 
a fundamental plan of body make-up is always 
present and readily recognizable. And this body 
plan of the fishes is also the fundamental structural 
plan of all the vertebrates from the lowest fishes 
through to the highest mammals, always persisting EVOLUTION 
200 
and plainly to be made out. There is the internal 
bony skeleton with its longitudinal axis, skull, ribs, 
shoulder and pelvic girdles and attached limbs; the 
closed circulatory system with pumping heart and 
elaborate system of ramifying arteries and veins; a 
respiratory system with lungs in the land forms and 
gills in the aquatic forms; a digestive system with 
stomach, intestine and attached liver and pancreas; 
a system of reproduction which is exclusively sexual; 
and a nervous system composed of brain, dorsal 
spinal cord and nerves reaching all parts of the 
body. 
The amphibians, including the coecilians, sirens, 
mud puppies, salamanders, frogs and toads, stand 
in a fairly intermediate position between the fishes 
and the reptiles. Despite their difference in appear- 
ance and habits, they are really much like fishes, 
resembling them in all but a few essential charac- 
ters, such as absence of fins, the presence usually 
of well-developed legs for walking and leaping, 
and the absence or reduction of certain bones of 
the head connected with the gills and lower jaw 
which are well developed in fishes. In their adult 
condition some of the amphibians are terrestrial 
and some aquatic (fresh water), but all have an 
aquatic larval life. The young, called tadpoles, 
are extremely fishlike in their earlier larval stages, 
being long-bodied, tailed, swimming freely about by EVOLUTION OF VERTEBRATES 
201 
means of the finlike flattened tail, and breathing by 
means of external gills. As the tadpoles grow and 
develop the legs begin to appear, the hind legs 
first in the frogs and toads, the forelegs first in the 
salamanders; lungs develop as two simple sacs with 
more or less folded walls, and the gills disappear 
(except in the cases of the few forms which in addi- 
tion to developing lungs retain gills through life). 
The tail shortens and finally disappears in the frogs 
and toads; with the salamanders the tail fin only is 
lost. At the same time the change from water to 
land is made. 
The body varies from a long and slender, truly 
snakelike form, as in the tropical coecilians, through 
the familiar salamander shape, where it is more 
robust but still elongate and tailed, to the heavy 
squat, tailless condition of the toads. Legs, with five 
digits, are usually present, but in the few species of 
coecilians they are wholly wanting. These coecilians 
may have as many as 250 vertebrae and about as 
many pairs of ribs. The salamanders may have as 
many as 100, but the short, squat frogs and toads 
have but 10 vertebrae, and no ribs at all. The 
heart is always three-chambered (two auricles 
and a ventricle), while in fishes there are but two 
chambers (one auricle and a ventricle). The cir- 
culation of the simpler salamanders is essentially 
like that of a fish, but in the frogs and toads there 202 
EVOLUTION 
is a distinct advance beyond this condition. The 
nervous system is well developed, although the hind 
brain (cerebellum) is very small. There are tac- 
tile nerve endings in the skin over the whole body, 
and taste organs on the tongue and lining of the 
mouth. The eyes have no lids in some of the lower 
forms, but most of the frogs and toads have an 
upper lid although no under one. The ears have 
no external parts other than the thin tympanic mem- 
branes. 
The reptiles, including the lizards, snakes, tor- 
toises, turtles, crocodiles and alligators, resemble the 
amphibians in general shape, but in internal struc- 
ture and the more essential characters are more like 
the birds. They all breathe exclusively by lungs, 
although some kinds live in water, both salt and 
fresh. As among the amphibians, the body shape 
varies from very long and slender-some snakes 
have as many as 400 vertebras-to short and squat, 
some turtles having only 34 vertebras. The rep- 
tilian skull, in the number and disposition of its parts 
and in the manner of its attachment to the spinal 
column, resembles that of birds, although the cranial 
bones remain separate, not fusing as in the birds. 
Four legs, each terminating in a five-toed foot, are 
present in the turtles; the lizards, also, usually have 
four, but some have only two and some none at all; 
while the snakes are legless or at least without more EVOLUTION OF VERTEBRATES 
203 
than mere rudiments of hind legs. The lungs of 
reptiles are simple and saclike, but in the turtles and 
crocodiles they are divided by septa into a number 
of chambers. The reptilian heart consists of two 
auricles and two ventricles which are, however, usu- 
ally only incompletely divided, the division into right 
and left ventricles being complete only in the croco- 
diles and alligators, the most highly organized of 
living reptiles. The nervous system reaches a con- 
siderable degree of development. The brain, in size 
and complexity, is plainly superior to the amphibian 
brain and resembles quite closely that of the birds. 
Of the organs of special sense, taste seems to be little 
developed, but smelling organs of considerable com- 
plexity are present in most forms and consist of a 
pair of nostrils with olfactory papillae on their inner 
surfaces. Ears are present, but crocodiles and alli- 
gators are the only reptiles with a well-defined outer 
ear. Eyes are always present and are well de- 
veloped, resembling the eyes of birds in many re- 
spects. All reptiles have movable eyelids, including 
a nictitating membrane like that of the birds. In 
addition to the usual eyes there is in many lizards a 
remarkable eyelike organ, the so-called pineal eye, 
which is situated in the roof of the cranium, and 
seems to be the vestige of a true third eye which in 
ancient reptiles was probably well developed, but has 
been lost by degenerative evolution. 204 
EVOLUTION 
Most reptiles lay eggs from which the young 
hatch after a longer or shorter period of incuba- 
tion. Usually the eggs are simply dropped on the 
ground in suitable places (although certain turtles 
dig holes in which to deposit them), where they are 
incubated by the general warmth of the air and 
ground. However, some of the giant snakes, the 
pythons, for instance, hold the eggs in folds of the 
body, and in some snakes and lizards the eggs are 
retained in the body of the mother until the young 
hatch, but in all these cases, the young, although born 
alive, are in reality inclosed in an egg shell until 
the moment of birth. The newly hatched young 
resemble the parents in most respects except in size. 
With the birds, readily distinguished from all 
other animals by the covering of feathers, we come 
to a distinct advance in vertebrate evolution. Yet 
they have many important points in common with the 
reptiles, and the paleontological record shows a 
number of striking linking forms uniting the two 
classes. The birds, unlike the fishes, amphibians 
and reptiles, have warm blood and a complete double 
circulation, and they have more complex lungs to 
provide for the increased aeration of the blood made 
necessary by the more active blood movement. The 
lungs are divided into small spaces by numerous 
membranous partitions, but they are not lobed as in 
the mammals. Connected with the lungs are a EVOLUTION OF VERTEBRATES 
205 
series of scattered air sacs which in turn connect 
with bones that are hollow and contain air. Thus, 
a bird's body contains a large amount of air, an 
adaptation connected with flight. 
The power of flight is made possible by the modi- 
fication of the fore limbs to be wings and the special 
development of large muscles attached to the breast- 
bone which has, except in the ostriches and a few 
other birds which do not fly and have only rudi- 
mentary wings, a marked ridge or keel to provide 
space for this attachment. The fore limbs, or 
wings, have only three digits, while the legs usually 
have four, although a few birds have only three toes 
and the ostriches but two. This is a condition 
brought about by a reduction from the typical five 
digits. The hind limbs or legs are present and func- 
tional in all birds, adaptively varying, as pointed out 
in an earlier chapter, in relative length, shape of 
feet, etc., to suit the special perching, running, wad- 
ing or swimming habits of various bird kinds. Liv- 
ing birds are toothless, but certain ancient forms 
now extinct and known through fossils had large 
teeth set in sockets on both jaws. 
The heart of birds is composed of four distinct 
chambers, the septum between the two ventricles be- 
ing complete. The birds have an active and intense 
circulation, the pulse being even quicker and the 
blood hotter than in the mammals. The brain is 206 
EVOLUTION 
compact and relatively large and more highly devel- 
oped than in the amphibians and reptiles, but the 
forebrain (cerebrum) has not the convolutions of 
the mammalian cerebrum. Of the special senses, the 
organs of touch and taste are apparently not keen, 
but those of smell, hearing and sight are especially 
well developed. The optic lobes of the brain are of 
great size relatively, compared with those of other 
vertebrate brains, and there is no doubt that the 
sight of birds is keen and effective. There is no 
external ear, other than a simple opening, but the 
organs of the inner ear are well developed and birds 
have excellent hearing. 
All birds are hatched from eggs, which undergo a 
longer or shorter period of incubation outside the 
body of the mother, and are, in most cases, laid in 
a nest and incubated by the parents. The time for 
this incubation varies from ten to thirty days among 
the more familiar birds, to nearly fifty among the 
ostriches. When the young are ready to hatch, they 
break the egg shell and emerge. Either their eyes 
are open and the body is covered with down and they 
are able in a few hours to feed themselves (precocial 
young), as with the grouse, quail, and others; or they 
are blind and almost naked, and dependent upon the 
parents for food until able to fly (altricial young), 
as in the case of the perching and song birds, and 
others. The preparation of a nest, sometimes of EVOLUTION OF VERTEBRATES 
207 
much elaborateness, and the faithful care of the 
young for longer or shorter periods after hatching, 
mark a distinct psychological advance on the part of 
the birds over the lower cold-blooded vertebrates. 
And now the mammals-and we are nearing 
home! Let us recall the story of the evolution of 
the vertebrates as we have so far tried to picture 
it. First, we find them in the water, as fishes, 
breathing by means of gills, cold-blooded, with a 
heart of but two chambers not separating the arterial 
and venous blood, small-brained, and with sense 
organs of dull perception, trusting for the persistence 
of the kind to many eggs carelessly strewn, rather 
than care of a few young. Then, as amphibians, 
half-aquatic and half-land inhabiting, with lungs 
as simple sacs with folded walls richly supplied with 
blood vessels (though this blood is still cold and 
mixed and pumped by a heart of three chambers), 
with the first legs, and a better brain and sharper 
senses. Next come the reptiles, mostly typical land 
animals, all breathing by lungs which begin to have 
their surface increased by membranous partitions, 
with blood still cold and mixed, but driven by a four- 
chambered heart, with nervous system better de- 
veloped, the brain larger; and practicing the begin- 
nings of nest-making for the eggs. Then, the birds, 
warm-blooded, active, intense, possessing a definitely 
double circulation with a heart of four chambers 208 
EVOLUTION 
in which the two ventricles are completely sepa- 
rated, a brain large but compact, keen special senses, 
especially of sight and hearing, and capable of elab- 
orate nest-building, and care of the eggs and young. 
They are animals of the free air, thanks to fore- 
limbs become wings; animals of highly perfected 
instincts and a dawning intelligence. 
And now, as mammals, the vertebrates reach their 
evolutionary height. They reach man. But not at 
one leap. The mammals are of many sorts; there 
are 2,500 living species of them grouped into eleven 
different orders. They all agree in certain distinc- 
tive characters of structure and physiology: above 
all, they are distinguished from the other vertebrates 
by their mammae, or milk glands, from which they 
feed, for a while, their few carefully tended young. 
They all, except one very small group (three genera) 
representing the very lowest of mammal kinds, which 
produce young from eggs hatched outside the body, 
give birth to free young. These young, as em- 
bryos, have developed in the uterus of the mother 
body, to which they are intimately connected by a 
membrane called the placenta. (In the kangaroos 
and opossums, composing the next lowest mamma- 
lian group, there is no placenta.) Mammals dif- 
fer from fishes and amphibians and agree with 
reptiles and birds in never having external gills. 
They differ from reptiles and agree with birds in EVOLUTION OF VERTEBRATES 
209 
being warm-blooded and in having a heart with two 
distinct ventricles and a complete double circulation. 
Finally, they differ from both birds and reptiles in 
having the skin more or less clothed with hair, the 
lungs freely suspended in a thoracic cavity sepa- 
rated from the abdominal cavity by a muscular 
partition, the diaphragm, and in the possession by 
the females of milk glands. 
In size, mammals range from the pygmy shrew 
and harvest mouse, which can climb a stem of 
wheat, to the great sulphur-bottom whale of the 
Pacific Ocean, which attains a length of a hundred 
feet and a weight of many tons. There is a great 
range of variety in external form and in habits of 
life. Though most species live on the surface of 
the earth, some, like the moles and gophers, are 
burrowers in the ground; some, like the bats, have 
the forelimbs modified to be wings; and some, like 
the seals and walruses, the porpoises and the whales, 
have taken to the water and have the limbs modi- 
fied to flippers. In the dolphins, porpoises and 
whales, the hind limbs have been lost, and the tail 
ends in a broad horizontal fin, or paddle. While 
most mammals have the typical five toes on each 
foot, the hoofed mammals have from but one to 
four. 
The bones of mammals are firmer than those of 
other vertebrates, containing a larger proportion 210 
EVOLUTION 
of salts of lime. The spinal column varies in the 
number of vertebras, this difference being chiefly due 
to the varying length of tail. Apart from the 
caudal vertebrae the usual number is about thirty. 
The skull is very firm and rigid, all the bones com- 
posing it, excepting the lower jaw, the tiny auditory 
ossicles and the slender bones of the hyoid arch, 
being immovably articulated together. The teeth 
vary in number and character, for they are adapted 
to varying habits of feeding and of offense and de- 
fense. The alimentary canal differs greatly in 
length, being very long in vegetable feeders-in the 
cow it is twenty times the length of the body-and 
short in the carnivores-in the tiger, for example, 
it is but two or three times the body length. The 
nervous system and organs of special sense reach 
their highest development among the mammals. In 
all of them the brain is distinguished by its large 
size and by the special preponderance of the fore- 
brain, or cerebrum, over the mid- and hind-brain. 
In man and the higher mammals the surface of the 
forebrain is thrown into many convolutions; among 
the lowest the surface is smooth, as it is in the bird's 
brain. Man's brain is many times larger than that 
of any other known mammal of equal bulk of body. 
As already pointed out, all mammals except a 
very few give birth to free young. There are three 
or four peculiar creatures, undoubtedly the lowest EVOLUTION OF VERTEBRATES 
211 
of all mammal forms, which produce their young 
from eggs hatched outside the body. One of these 
kinds, which lives in Australian rivers and is called 
the duckmole, or duckbill (Ornithorhynchus) be- 
cause of the flat sheathed snout, lays two eggs in a 
carefully constructed burrow nest. The other kinds, 
which are land animals, deposit a single egg in an 
external pouch on the body, and here it hatches. In 
various structural details these egg-laying mammals 
show remarkable resemblance to birds and reptiles. 
They are all found in those "lands of living fossils," 
Australia, Tasmania, and New Guinea. So different 
are they from all other mammals that some biolo- 
gists prefer to call them promammals, and establish 
them in a separate class. But they agree with 
other mammals in that distinctive character of feed- 
ing their young a secretion from milk glands, al- 
though the glands in these low or near mammal 
forms are much less compact and well developed 
than they are in other mammals. 
Another low and ancient order of mammals is 
that of the marsupials (kangaroos and opossums). 
They give birth to their young in a very early and 
helpless stage (the young of the American opossum 
is only about half an inch long at birth) and carry 
them about in an external pouch. This pouch is on 
the underside of the body, and in it are the teats to 
which the young cling constantly. All of the mar- 212 
EVOLUTION 
supials except the opossums live, as do the duck- 
moles, in Australia and neighboring islands. 
Most of the other orders of mammals-the 
rodents, the shrews and moles, the bats, the ceta- 
ceans, the herbivorous hoofed mammals, and the 
carnivores-are more or less familiar to all nature 
students, hunters and frequenters of zoological gar- 
dens. They are all, except perhaps the seals and 
walruses and the dolphins, porpoises and whales, 
easily recognizable as belonging to the mammalian 
class. They are the "quadrupeds" of the older text- 
books, the "beasts" of common parlance. 
But there is one order, the highest in the whole 
class and by far and away the most fascinating to us, 
to which a few special words must be given before 
we pass on to our next chapter, that on the evolution 
of man. This order is that of the Primates, or man- 
like mammals, and includes the lemurs, tailed mon- 
keys, baboons and apes. It is in this order that the 
classifying zoologists place man. He is put here on 
the basis of the known facts concerning his anatomy, 
physiology, embryology and paleontological history. 
He has been studied in a detached and unemotional 
way by the same methods and in the same manner as 
other animals have been studied, and the results of 
this study compel the zoologists to classify him as 
an unmistakable member of the great branch Chor- EVOLUTION OF VERTEBRATES 
213 
data (which includes all vertebrates), belonging in 
it to the class of Mammals, and in this class to the 
order of Primates, within which finally he finds his 
closest genetic relationships with the anthropoid 
apes, the living representatives of which are the 
gibbons, orang-utan, gorilla and chimpanzee. The 
structural resemblances of all the Primates, as well 
as their physiological and embryological character- 
istics, set them off clearly and unmistakably from all 
other mammals, as a group of closely related forms. 
First and lowest among them are the curious, 
small, superficially squirrel-like lemurs of Mada- 
gascar and neighboring regions. They live chiefly 
in trees and feed on insects. Then come the tailed 
monkeys and the apes (not anthropoid) which 
are divided into two general groups. One of them 
lives in Central and South America and is character- 
ized by having a flat nose with the nostrils far apart 
and directed laterally. All of the members of this 
group are arboreal, and many have long prehensile 
tails. In the other, or Old World group, the nos- 
trils are close together and directed downward, the 
tail is never prehensile, and in some cases is rudimen- 
tary or even absent. These Old World apes include 
the baboons, mandrills, the tailless Macacus and the 
Barbary ape, which extends across from Northern 
Africa into Spain. EVOLUTION 
214 
Finally, at the head of the order (excluding man, 
for the present) come the tailless anthropoid apes, 
whose structure is very close in almost all details to 
that of man. In fact, there are only two per- 
sistent and outstanding major structural differences 
between man and these apes, and those are the in- 
ability of man to oppose the big toe as he does his 
thumb-a feature associated with his erect position- 
and the relatively enormous size of the human brain, 
which is, in the adults of the higher living races of 
man, three times the size of the brain of any anthro- 
poid ape. Even in an Australian bushman, who 
belongs to one of, if not quite, the lowest of human 
races, or in a four-year-old child of any of the higher 
races, the brain is twice the size of that of an adult 
gorilla, whose body is as large as that of human 
adults. For the rest, however, the structural make- 
up of the anthropoid apes is extraordinarily like that 
of man. 
The gibbons, inhabiting southeastern Asia, stand 
more erectly when on the ground than the other an- 
thropoids, but have arms of such length that they 
are able to touch their hands to the ground as they 
stand. But they spend most of their time in trees, 
and feed on fruits, leaves and insects. In the same 
region we find the orang-utan, which walks, when on 
the ground, on the knuckles and sides of the feet. EVOLUTION OF VERTEBRATES 
215 
It also, however, prefers life in the trees, in which 
it builds nests for rest and concealment. The gorilla, 
the largest of the apes, and in details of structure 
perhaps most like man, attaining a height of five feet 
and weight of two hundred pounds, is a native of 
Africa, where it lives in families and subsists chiefly 
on fruits. Ever since the sensational tales of Paul 
du Chaillu, the gorilla has been popularly looked 
on as a ferocious animal quick to attack man. But 
Carl Akeley in recent intimate studies of the gorilla 
at home in the Belgian Congo reveals this great ape 
to be timorous and well disposed rather than fierce. 
Finally, in Africa also, is found the chimpanzee, 
which, in its various characteristics, including manner 
and mentality, taken altogether, most nearly of all 
the anthropoid apes approaches man. The capacity 
of the chimpanzee and orang-utan for being taught 
to imitate human behavior is well known to all fre- 
quenters of zoological gardens and vaudeville en- 
tertainments. 
The fossil remains of several now extinct an- 
thropoid apes have been found in Europe and else- 
where, and, most recently, in a single instance, in 
North America. Among these remains are those 
of one or more kinds which seem to have been even 
more closely similar to man than are any of the 
living anthropoids. As a matter of fact, no biolo- 
gists see in any of the living anthropoids a kind 216 
EVOLUTION 
which could be called a direct ancestor of human 
kind. Man and the present anthropoids are descen- 
dants, along distinct lines, of some now extinct com- 
mon ancestor. This ancestor has yet, if ever, to 
be found. CHAPTER XII 
THE EVOLUTION OF MAN 
In the popular use of the word "evolution" it is 
often, indeed, perhaps usually, made to mean ex- 
clusively the evolution of man. Man from the 
monkeys, that is the popular significance, and the 
popular damnation-when it is damned-of evolu- 
tion. From the biologist's point of view that is a 
wrong limitation and an unfortunate special conno- 
tation of the word. But even from his point of 
view the evolution of man is an inevitably included 
part of the meaning of evolution. For practically 
no biologist leaves man out of the evolutionary 
series. It would be inconsistent and even absurd 
for him to do so, because he lists man in all his 
classificatory textbooks, as we have just expressly 
stated in the close of the last chapter, as a verte- 
brate animal belonging to the class of mammals. 
Within this class he puts him in the order of pri- 
mates, and within this order the zoological classifier 
recognizes a special family, the Hominidae, repre- 
sented, in this geological time at least, by man 
alone. And he gives him a genus name, Homo, and 
species name, sapiens, combined to read Homo sap- 
217 218 
EVOLUTION 
iens, just as he gives such a binomial to every other 
known animal kind. He believes, on the basis of 
what seems to him incontrovertible evidence, that 
all other animal and plant kinds listed in his register 
of living creatures are the results of evolution. And, 
on the basis of similar evidence, he must and does 
accept logically and naturally this particular crea- 
ture, Homo sapiens, as having exactly the same 
status, as regards origin and blood relationship to 
other animal kinds, as have all the other animals 
which he knows. Homo sapiens, or man, may be 
the highest, the most interesting, the most impor- 
tant, the most anything, of all animal kinds, but 
that does not release him from his general relation 
to the evolutionary scheme of things. 
So evolution means to biologists, just as it does to 
laymen, the evolution of man-even though it means 
also much in addition to that. And the evolution of 
man means to them also the most important part of 
evolution-for they, too, are more interested in 
humankind than in any other kind of creature. 
Biologists might, therefore, as well frankly recog- 
nize that the interest of the great public in evolu- 
tion, so far as this public has any interest in it at 
all, is primarily, and often exclusively, an interest 
in the evolution of man. Hence, any biologist writ- 
ing or speaking of evolution must, if he wishes a 
general hearing, give a large attention to the spe- THE EVOLUTION OF MAN 
219 
cial subject of human evolution and its influence 
on our attitude toward the pressing problems of 
individual and societal life and fate. 
It is particularly this significance of evolution in 
its relation to our understanding of human endow- 
ment and possibilities, of human behavior and con- 
trol, of the relation of the individual to society and 
of society to the future evolution and fate of the 
race, that creates and holds the interest of people. 
If the evolution of man merely meant classifying 
him in zoological textbooks as a vertebrate animal 
of the class of mammals, and pointing out for him 
a long history as a constantly improving creature 
struggling from darkness into light, the public would 
let the classifiers have man to list and play with as 
an animal species developed by evolution, as much 
as they liked. 
But when this acceptance of man's evolution in- 
volves, as it at once does, the acceptance of such sig- 
nificant things as the surrender of the long-held 
conception of an immediate special creation of man 
by a special Creator, the recognition of a close 
genetic relationship of man with such animals as the 
anthropoid apes, and the control of man, in many 
aspects, by various natural laws to which all other 
creatures are subject, all of which acceptances bear 
importantly on our whole conception of philosophy 
and religion, then the public balks. Part of it simply 220 
EVOLUTION 
revolts and will have nothing to do with evolution. 
Part of it asks, dubiously, just how assured biologists 
are of human evolution. Finally, part of it, accus- 
tomed to see in science a method and a means of 
finding out the truth, and accustomed to accept the 
pronouncements of science as a basis of knowledge, 
declares itself ready to accept the evolution of man 
with all its implications. But it asks, rather nerv- 
ously, just what and how extensive these implica- 
tions are, and just how far their acceptance will 
modify our conception of the high estate of man; 
just how far they will despiritualize and materialize 
our understanding of humanness. What becomes 
of poetry, philosophy and religion, of inspiration, 
virtue, and God, when we accept the evolution of 
man? 
It would be a promise I could not fulfill, a pre- 
sumption I am not silly enough to dare, if I should 
say that I or any biologist or natural philosopher 
can satisfactorily answer these questions. We can- 
not. But it is our duty not to dodge these ques- 
tions, but to try to point out, as simply and clearly 
as may be, what approaches to their answers science 
is now in a position to make. It is especially needful 
to do this just now, because the public has been 
rather widely stirred recently by very positive anti- 
evolutionary statements and activities. These at- 
tacks have, as of old, mostly been made by the- THE EVOLUTION OF MAN 
ologians, and the evolution denied and banned by 
them is the evolution of man. For the rest of this 
book the evolution of man, therefore, will be my 
special subject. 
221 
The evidence for the evolution of man is gained, 
just as is the evidence for the evolution of the plants 
and other animals, from the study of his anatomy, 
physiology, embryology, paleontology and geo- 
graphical distribution. From all these sources there 
are impressive testimonies to man's oneness with all 
other life, to his origin by evolutionary processes 
from lower life forms, and to his ever continuing 
slow change and modification under the pressure 
of the always present major factors of life and 
evolution, such as variation, heredity, selection, and 
environmental influence, the resultant of whose in- 
fluences determines the character and evolutionary 
movement of living creatures. 
But there is one element in man's make-up and 
attributes, much more difficult to study and so far 
much less understood than most of his other attri- 
butes. It speaks less clearly concerning the reality 
and the manner of his evolution and, in fact, leaves 
open a most important opportunity for attack by 
those who would try to controvert the claims of the 
evolutionists. This element is his psychology; the 
manifestations of his mind and spirit. If man owes 222 
EVOLUTION 
all that he has to evolution he has at least been 
able to go so far beyond all other creatures in the 
quantitative development of mind and all that goes 
with mind, that he seems, to many, to have a mind 
and related capacities which are really qualitatively 
different from those of all other creatures, even the 
highest among them. And this has led to the de- 
velopment of a school of students of human life who 
accept the natural evolution of man's body, but call 
for another and different, presumably a supernatu- 
ral, explanation of his mind and spirit. This school 
includes some real scholars, but a larger number of 
laymen who wish to follow science as far as they 
can, but have an emotional urge to see in man's 
higher intellectual and especially his so-called 
spiritual capacities something quite beyond scientific 
explanation. So important is this matter of mind in 
any consideration of human evolution, that I want to 
devote a following chapter exclusively to a brief 
statement of the present status of the scientific man's 
study of the evolution of mind. Leaving this very 
important matter aside, then, for the moment, we 
may glance swiftly at some of the more or less 
familiar facts which reveal both the evidence for, 
and the character of, the evolution of man. 
In an earlier chapter (Chapter IV), special refer- 
ence was made to the striking similarity, indeed, 
fundamental identity, of the skeleton of man and THE EVOLUTION OF MAN 
223 
that of other vertebrates; the likeness being more 
and more close as one passes from the lower verte- 
brates (fishes) on up to the higher ones (mammals). 
Also, among the mammals this similarity, bone for 
bone, both in shape and disposition, grows more and 
more marked as one passes from the lower orders, 
as the Monotremes and Marsupials, to the highest, 
the Primates, which includes the monkeys and apes. 
This story of skeletal likeness or identity is repeated 
by the story of each of the other great organ sys- 
tems, the muscles, nerves and nerve centers, alimen- 
tary canal and tributary glands, the respiratory and 
circulatory systems. 
There are differences, to be sure, but the very 
differences reinforce in one's mind the unescapable 
conclusion of the origin of man's structure by an 
evolutionary process from the structure of the ver- 
tebrate animals. For these differences are not radi- 
cal, but comparative, and they are plainly associated 
with differences between the habits of man and the 
habits of these animals. Especially are many of 
these differences plainly associated with the erect 
posture of man. The gradatory steps culminating 
both in the change of the habit of going on all fours 
to that of going on the lower limbs, and in the change 
of structure, especially skeletal and muscular, made 
necessary by this change of habit, are beautifully 
shown by a comparison of the habits and structure 224 
EVOLUTION 
of the anthropoid apes with those of the lower 
mammal orders on one hand and those of man on 
the other. 
The story told by the vestigial and retrogressive 
structures alone in man's body is a brief for the evo- 
lution of man that is practically unanswerable. It 
is a brief particularly confusing to the special crea- 
tionist. How can an all-wise, all-powerful, special 
Creator ever be held responsible for creating man 
with a host of useless and degenerating parts in his 
body? As pointed out in the earlier chapter on the 
evidences of evolution found in comparative anatomy 
and embryology, anatomists now list nearly two 
hundred cases of vestigial and retrogressive struc- 
tures in the human body. Their explanation by evo- 
lution is a reasonable one. Man, in assuming an 
upright position with a body inherited from lower 
mammals going on all fours, and in his other 
modification of habits and character of functions, 
has not completed his adaptive structural evolution 
and is now in process of losing or modifying parts of 
his inherited body structure so that it may conform 
with his new habits. 
In this earlier chapter, too, are told some of the 
striking facts of man's embryology. They reveal 
how each human being, in his individual develop- 
ment from single fertilized egg cell to complex adult 
body of trillions of differentiated cells associated THE EVOLUTION OF MAN 
225 
into organs and organ systems, passes through a 
series of stages in which he is, as regards the char- 
acter of his bodily make-up, first invertebratelike, 
then fishlike, then amphibianlike, then partly reptile- 
like and partly birdlike, then characteristically mam- 
malian, and only finally of that particular and pe- 
culiar type of body which differentiates him from all 
other animals as man. He repeats, or recapitulates in 
his embryonic development, the major steps by which 
he has arisen by evolution from lower animal forms. 
His heart is first a single-chambered tubular organ 
like that of the adult lowest vertebrates. Then it 
becomes partially divided into two successive cham- 
bers, an auricle and a ventricle, and now resembles 
the adult heart of the fishes. The auricle next 
divides into two cavities, and now this embryonic 
human heart of three chambers resembles the fully 
developed heart of the next highest vertebrate class, 
the amphibians. Later, the ventricle also divides 
into two cavities, and thus the four-chambered heart 
characteristic of the higher vertebrates and man is 
reached. Similarly, the red blood cells of the human 
embryo are, when first formed, large and nucleated. 
In this stage they resemble the red blood cells of 
adult fishes and amphibians. Later, the embryonic 
human blood cells become similar in structure to 
those of adult reptiles. Finally, sometime before 
the birth of the human embryo, these blood cells 226 
EVOLUTION 
become, as they do also in all other mammals, non- 
nucleated and biconcave. Thus it is evident that the 
human heart and the cells of the human blood pass, 
during the embryonic development of each human 
being, through stages representing the different adult 
conditions of the heart and blood in successively 
higher vertebrate classes. 
Similar stories are told, in more or less detail, 
by the other organs and organ systems. The facts 
of human embryology alone are enough to indicate 
both the reality and the general course of the evo- 
lution of man. There is no other explanation of 
them but evolution which does not make human 
reason to be a travesty of itself. 
But besides the sources of knowledge concerning 
the evolution of any plant or animal group, or of 
man, which lie in the study of embryology and com- 
parative anatomy, there are also the sources afforded 
by the study of paleontology and geographical dis- 
tribution. What evidences and revelations of the 
evolution of man do these sources afford? Are 
there human fossils that throw light on human evo- 
lution? Does the present distribution of the various 
races and types of living man help us to an under- 
standing of the course of man's evolutionary devel- 
opment? 
The historian speaks of modern history and an- 
cient history. But all of the history of the historian THE EVOLUTION OF MAN 
227 
seems to the geologist and biologist to be only very 
modern history, indeed. It is only the history of 
man during the last few thousand of his many thou- 
sand years of existence on earth. It tells the tale of 
what manner of man lived, and of his achievements 
during the five or six thousand years just preceding 
this year. But the paleontologist and anthropologist 
find irrefutable evidence that man, of one kind or 
another, has existed on this earth for certainly no 
less than one hundred thousand years, and probably 
for several hundred thousand years. This evidence 
is that of the actual remains (fossils) of these early 
men and of a great mass of things made by them, 
their tools and weapons and utensils, their orna- 
ments and works of art. These human fossils and 
artifacts have been found in such relation to various 
geological formations and intermixed with the 
fossil remains of such various extinct animals well 
known to paleontologists, that no doubt can exist of 
the truly ancient period of existence of these pre- 
historic human beings thus revealed to our knowl- 
edge. 
Although the findings of such fossils and human 
artifacts began as long ago as the middle of the last 
century, the large majority of them have been made 
since the beginning of this century. And since 1900 
the rapidly succeeding finds have been so numerous 
that there is now an impressive array of material 228 
EVOLUTION 
available to the student of human prehistory. On 
the basis of this material, most of it of indisputable 
authenticity, the anthropologist can picture with 
surprising detail the character and behavior of our 
human-and near-human-ancestors of the last sev- 
eral hundred thousand years. He can distinguish 
among several definite types of human creatures, 
living at definitely distinguishable times during the 
present geological epoch, called Quaternary, scat- 
tered through that first and longer part of it called 
Pleistocene, or Glacial time, and that latter and 
much shorter part of it called Recent, or Post-Glacial 
time. He sees a gradual change in succeeding 
periods from a more bestial type of man to a more 
human type; from an ape-man type to the present 
type. He sees the evolution of man as it has so far 
run its course. 
Most of the finds, constituting the evidence so far 
available to students of man's prehistory, have been 
made in Europe and the British Isles. But one of 
the most important was made in Java and another 
in Rhodesia in Africa, while North America, from 
which no really ancient human or anthropoid fossil 
had ever before been recovered, recently has made 
its first contribution to the genealogy of earliest man 
by uncovering a relic of an anthropoid ape of unusu- 
ally interesting character. 
It has been for a long time a matter of some won- THE EVOLUTION OF MAN 
229 
der to anthropologists that North America has con- 
tributed little or nothing to the history of anciently 
prehistoric, that is, Glacial time, man. From time 
to time human relics claimed to be geologically 
ancient have been uncovered in this country, such as 
the Calaveras skull, the Lansing man, the Florida 
man and others, and most recently and perhaps most 
promisingly the Lagow sand-pit man from near 
Dallas, Texas. But none of these relics has yet 
succeeded in establishing for itself an age at all com- 
parable with that of the numerous European and 
English finds. The American relics seem to be only 
those of extra-early Indians belonging to a period 
well beyond the last glaciation. 
It is very different in Europe. From France we 
know of a half dozen or more skulls and skeletons 
which are all of Mid or Late Glacial time. There 
are similar relics from Belgium, Germany, Austria, 
Spain and England. These are all remains of so- 
called Neanderthal man. Then, there is the still 
older Heidelberg jaw. The age of this relic may 
be no less than 400,000 years. Perhaps no older, 
but perhaps of even more primitive type, is the Pilt- 
down man, or "dawn man," of Sussex, England. 
Also of very primitive type is the Rhodesian man, of 
which a few fossils are known. Plainly later than 
these, although still much older than any American 
relics, are the numerous skeletons and skulls of Cro- EVOLUTION 
230 
Magnon man found variously in Central and West- 
ern Europe. This man is probably the earliest type 
of true present-day Homo sapiens, but he lived from 
twenty to thirty thousand years ago. 
Oldest and most primitive of all the human or 
near-human relics are those skull parts, teeth and 
left femur found in 1891 in Java, and which are the 
basis for establishing the existence in Upper Plio- 
cene, that is, nearest Pre-Glacial time (or, at latest, 
in earliest Pleistocene or Glacial time) of a low 
ape-man type of creature which is called Pithecan- 
thropus erectus. For with all its simian characters 
of head, the character of its thigh bone indicates 
that it carried its hideousness erectly. 
To add to the evidence of these human or near- 
human fossils of indubitable antiquity, the anthro- 
pologist has still another kind of evidence of man's 
ancientness on this earth, an evidence much en- 
larged in recent years. This evidence lies in the 
existence of the results of early man's handiwork, a 
myriad examples of which have been found in situa- 
tions and under conditions that clearly prove their 
varying geological antiquity. So many of these 
artifacts of human origin have been found, and 
they are so characteristically various and are so 
consistently distributed as regards geologic time, 
that the anthropologists have been able to distin- 
guish a series of definable cultural stages in human THE EVOLUTION OF MAN 
231 
prehistory, each associated with different succeeding 
structural types of men. 
The less ancient of these discoveries include primi- 
tive bone and stone tools-not to refer to such 
comparatively recent things as the handiwork of the 
Neolithic and Metal ages-and various carvings on 
bits of mammoth ivory and reindeer horn, and 
numerous drawings of wild horses, mammoths, and 
other extinct mammals, as well as of ancient man 
himself, on the walls of limestone caves. 
The more ancient of these relics of human ac- 
tivity, found abundantly under conditions that show 
them to be contemporaneous with Glacial time and 
even earlier, are certain chipped flints adapted for 
use as simplest tools and weapons by various types 
of flaking in ways to produce cutting edges and 
convenient handholds. Some of these flaked flints 
have been found under conditions that seem to prove 
them older than any actual human fossils yet dis- 
covered. Some have been ascribed not only to 
Upper Pliocene time but even to older strata. If 
these oldest flints, called eoliths, are to be accepted 
as truly man-touched, they prove the existence of 
Tertiary man, which is to say that they carry man's 
antiquity back from Early Glacial time by another 
half million or more years. 
But a certain discussion, vigorous to acrimony, 
rages about these oldest chipped flints. One group EVOLUTION 
232 
of scholars holds that they may have been produced 
by natural causes through rough contacts with each 
other or other larger flint fragments. But the Nes- 
tor of American paleontologists and anthropolo- 
gists, Professor Henry Fairfield Osborn of the 
American Museum of Natural History, is convinced 
that the so-called Ipswich eoliths of England are the 
handiwork of Tertiary man. 
Under any circumstances, a great advance has been 
made in recent years in further proving the general 
course of the evolution of man. The characteristics 
of this evolutionary progress toward present-day 
humanness are, in physical changes alone, as summed 
up by Lull, an increase in stature and erectness of 
body, an increase in cranial capacity and perfection 
of the brain, especially in that portion of it which 
is concerned with the higher intellectual faculties 
and with speech, change in skull conformation, a 
heightening of the forehead and lessening of the 
brow ridges, a reduction of jaw power and dental 
arch, with resulting chin prominence, and changes in 
the teeth, such as a reduction of the canines and 
loss of the diastemata. 
As we survey the imposing array of human fossils 
now on exhibition before the wondering eyes of 
modern man running from ape-man Pithecanthropus 
through Heidelberg and Piltdown dawn man, on 
through Neanderthal and Cro-Magnon man up to THE EVOLUTION OF MAN 
233 
man of to-day, we can plainly see man's physical 
evolution, even as we see that of the horse in the 
series from little five-toed Eohippus of early Ter- 
tiary times, through later and larger four-toed 
Orohippus and still later and larger three-toed 
Mesohippus to one-toed Equus of to-day. 
Another kind of evidence for man's evolution is 
afforded by an examination of the various races or 
types of man existing to-day. Not all present men 
are alike. They differ, not merely individually but 
as groups. There are major varieties or types of 
present-day man. These varieties differ not only in 
their culture or kind of civilization, which is a varia- 
tion that can be partly explained, at least, as a more 
or less superficial difference due to various environ- 
mental factors impinging on each successive genera- 
tion; but they differ also in inherent and biologically 
heritable characters of structure and physiology. 
It is the custom of anthropologists and biologists 
to classify all existing men as of one species. That 
is, they call the major different types, such as Cau- 
casian, Negroid and Mongoloid, only different sub- 
species, and the major subtypes within each of these 
subspecies they call races. For example, the Nor- 
dics, Alpines and Mediterraneans are called races of 
the Caucasian subspecies. For all of these subspecies 
and races can intermate fertilely, and that is the EVOLUTION 
234 
biologist's principal physiological criterion of a 
species. 
But if differences in such constant structural char- 
acters as stature, skull conformation, skin color, 
form and color of hair, etc., were to be assumed to 
indicate species differences, as similar differences 
constantly are assumed to indicate different species 
in classifying the animals and plants, then we should 
unhesitatingly rank a number of the existing differ- 
ent races of man as different human species. As a 
matter of fact, the biologists and anthropologists do 
classify several of the different fossil types of man 
as of different human species and even genera. This 
classification is based, of course, exclusively on the 
marked structural differences apparent among these 
extinct types. The physiological criterion of fertile 
intermating cannot be made use of. 
But whether looked on as different species, or 
different varieties or races, the present living types 
of man are susceptible of classificatory separation on 
the basis of marked physical and, probably, mental 
differences. This separation can and does take the 
general form of constituting a series of human types 
representing different stages of human evolution. 
These stages correspond in some degree with stages 
recognizable in the series, as so far known, of pre- 
historic human kinds, running, that is, from lower 
to higher types of evolutionary development. Even THE EVOLUTION OF MAN 
235 
so conservative a scholar as Kroeber, leaning 
strongly toward an environmental rather than an 
hereditary explanation of present human varieties, 
recognizes that on a basis of fairly sound inherent 
physical differences "the vast bulk of mankind does 
fall naturally into three great divisions, each of 
which again subdivides into three or four principal 
branches, in regard to whose distinctness there is no 
serious difference of opinion. The scattering remain- 
der of races are allied sometimes to one primary 
stock, sometimes to another, but always with some 
special peculiarities." 
There is little difficulty in recognizing a distinct 
evolutionary difference between the dwarf negritos 
of equatorial Africa and Malaysia or the prog- 
nathous and beetling-browed Australians on the one 
hand, and the Nordic, Alpine and Mediterranean- 
Caucasian races of Europe on the other. The whole 
problem of the existence and geographical distri- 
bution of different races of living man is exactly the 
same kind of problem that constantly faces the 
faunistic naturalist in his study of any group of 
related kinds of lower animals and plants. It is 
an evolutionary problem. 
There should be added to this brief account of 
the evidences of human evolution at least a refer- 
ence to the interesting blood tests devised by Dr. 
George H. F. Nuttall of Cambridge (England) and 236 
EVOLUTION 
others, which indicate the chemical likeness of the 
blood of the anthropoid apes and that of man. 
These tests, called precipitin tests, are based on the 
discovery that if the fresh blood serum of any ani- 
mal is injected into the veins of a rabbit there will 
be produced in the rabbit's blood an antibody. This 
is analogous to the antitoxin which is produced in 
the blood of a horse by the injection of diphtheria 
virus. Now, if into blood, taken from an animal 
of the same species as that from which serum was 
originally injected into the rabbit's body, there be 
introduced a few drops of the drawn-off blood serum 
of the treated rabbit a white precipitate will be pro- 
duced. But if the rabbit serum is introduced into 
blood from another kind of animal, unless a kind 
closely related to the animal species from which the 
original injection of blood into the rabbit's body 
was made, there will be no precipitate. 
If we use human blood for the injection into the 
rabbit's blood it will respond in this way when, 
later, a few drops of rabbit serum are added to 
human blood. There will be a similar response 
although less marked if blood from an anthropoid 
ape is used, or, but still less markedly, if blood 
from a monkey be used. But there will be no pre- 
cipitin reaction if the blood of a horse or pig or 
other animal be used. 
If, however, blood from a horse be used for the THE EVOLUTION OF MAN 
237 
original injection into the rabbit's body, then the 
rabbit serum will produce a precipitin reaction in 
blood from another horse or from a donkey. The 
same is true if dog and wolf are used in the experi- 
ment. 
In other words, this blood test reveals a chemical 
similarity in the blood of closely related animals, 
and a dissimilarity in the case of that of widely re- 
lated animals. The fact that the human blood and 
the blood of the anthropoid apes both produce a 
nearly identical reaction is clear evidence of their 
close genetic, or "blood" relationship. CHAPTER XIII 
EVOLUTION OF MIND 
The human mind is the distinctive attribute of 
man. The nature and range of the powers of man's 
mind are what most importantly differentiate the 
human family from all other living creatures. Man's 
mind is of much more importance to him than his 
erect posture, his chin, his hand, his lack of hairi- 
ness or any other of those several characters which 
more or less sharply distinguish him from his nearest 
animal cousins. How has man come to have this 
master possession which places him so indisputably 
above and beyond all other animals? 
We can see something of the later progressive evo- 
lution of the human brain in the picture given us by 
the students of human embryology. We have seen 
how the embryologists have demonstrated that the 
developing human brain, especially that part of it 
forming the major portion of the cerebrum, passes 
through a series of successive stages roughly but un- 
mistakably corresponding to the adult brain condi- 
tions in fishes, amphibians, reptiles and the lower 
mammals. 
Also, we can see something of this evolution of 
238 EVOLUTION OF MIND 
239 
the human brain in the character of the cranial 
cavity of the various human and near-human crea- 
tures which lived in earlier geologic days than the 
present. When we compare the endocranial casts 
made from the fossil skulls and skull parts of Pithe- 
canthropus, the Piltdown man, the Rhodesian man, 
the Neanderthal man and the Cro-Magnon man with 
one another, and with that of man of to-day, we can 
detect a progressive expansion of those lateral and 
frontal territories of the brain which are especially 
associated with the increasing human powers of 
manual dexterity, discrimination, and mental con- 
centration. 
The anthropological studies, too, of the status 
of the mind in the different living subspecies of man 
-the Caucasian, the Mongoloid, and the Negroid 
-and in their races-the Caucasian Nordics, Al- 
pines and Mediterraneans, the Mongoloid Asiatics, 
Malaysians, Eskimos and Americans, and the 
Negroid Australians, Negritos, Melanesians, and 
African Bushmen-indicate that apart from differ- 
ences due to purely environmental and cultural con- 
ditions, there are some inherent differences in mind 
among living racial stocks. These differences are 
more or less gradatory in character and illustrate a 
progressive evolution from the relatively low mind 
type of the most primitive Negroids to the much 240 
EVOLUTION 
higher type possessed by the present Occidental 
Caucasians. 
But the progress of the brain and mind thus re- 
vealed is only a comparatively recent chapter in the 
whole story of the evolution of the human mind. It 
is only that chapter which treats of the progress of 
the human brain and mind since the appearance on 
earth of actual human or near-human beings. But 
the whole story goes back and down much farther 
than this. It goes back to the first living creatures 
appearing on earth; it goes down to the simplest 
ones now existing. 
In taking this broad and inclusive look at mind, 
we must first of all rid ourselves of our usual too 
anthropocentric attitude toward it. We must not 
think of mind as an exclusive attribute of man, for 
there are hundreds of thousands of species of ani- 
mals all possessing mind of one sort or another. 
Nor must we think, even, of that major character- 
istic of the human mind, intelligence, as something 
totally unshared by the lower animals. For not 
only the miscellaneous observations on the behavior 
of wild and domesticated animals in field and barn- 
yard and kennel made by naturalists and, indeed, 
by all of us, but the results of the carefully planned 
and controlled experiments of the genetic psycholo- 
gists and trained students of animal behavior, reveal 
varying positive degrees of intelligence in animals, EVOLUTION OF MIND 
241 
especially in the higher mammals, and also in birds, 
reptiles, amphibians and even fishes. 
But mind in the lower animals, and in man him- 
self, includes much besides intelligence. There are 
other organs of the body besides the brain whose 
functions contribute to the existence of mind. 
Indeed, if we look at mind in the broadest and fairest 
way we shall recognize it as existent in animals which 
have no brain at all, not even a special nervous sys- 
tem. Looking at mind as the total function of con- 
trol of animal behavior, and at the loci of mind as 
being in any structures of the animal body that help 
to determine this function, we shall come to a rather 
startling conclusion. This is, that mind and be- 
havior, in the simplest animals, are little more than 
phenomena which can be explained, or, at least, 
described, in terms of mere physics and chemistry, 
and are inherent in the very basic substance of which 
the bodies of these animals are composed. 
There is a whole host of living creatures, mostly 
minute one-celled, some of them called animals and 
others plants, according to their physiological char- 
acteristics, whose bodies contain no differentiated 
tissues or organs which may be called nerves or 
brains, and whose behavior seems wholly determined 
by two sets of physicochemical conditions. These 
are, first, those of their own fundamental physico- 
chemical make-up, and, second, those of the physico-  EVOLUTION 
242 
chemical character of their environment. These 
simple living creatures have an exceedingly limited 
range of behavior, and this behavior consists chiefly, 
if not solely, of inevitable mechanical responses to 
varying environmental influences. These responses 
are called tropisms, or reflexes, and after a sufficient 
number of experiments they can be learned and 
foretold. We find these simple creatures moving 
inevitably toward or away from light (positive or 
negative phototropism) ; toward or away from 
various chemicals (positive or negative chemo- 
tropism) ; in, or opposite to the direction of the pull 
of gravitation (positive or negative geotropism) ; 
in contact with, or avoiding contact with, solid sub- 
stance (positive or negative stereotropism) ; and so 
on; all strictly mechanistic behavior. Or, to substi- 
tute for the word behavior the name of that which 
presumably governs behavior, that is, mind (in our 
broad use of the word), we can say that among the 
lowest animals the reflex or mechanistic mind seems 
to be the only, or, at least, the principal kind of 
mind. 
When we move a step or two higher in the ani- 
mal scale, and find ourselves among the simpler 
many-celled animals, and especially when we reach 
the higher invertebrates, notably the insects, we still 
see tropisms or reflexes responsible for a consider- 
able part of behavior. But we also find another, EVOLUTION OF MIND 
243 
although closely related, type of behavior control 
much in evidence. This is instinct, and animals 
whose behavior is largely determined by instinct 
may be said to have instinct mind. 
This kind of mind may determine a behavior of 
great complexity and of most definite and specific 
advantage to its possessors in the struggle for ex- 
istence. Those many extraordinary adaptations in 
behavior displayed by the insects and many other 
invertebrates which constantly excite our wonder 
and admiration, are sufficient illustration of the great 
possibilities of instinct mind. For all these insects 
and other invertebrates have minds chiefly of the 
nature of instinct mind. There is in all of them an 
element, larger among the lower ones, less large 
among the higher ones, of the purely reflex and 
tropismic mind, and there may be among the highest 
a small element of the intelligence mind. But the 
element of instinct mind is by far the most important 
feature in the mental make-up of all those hundreds 
of thousands of invertebrates which constitute the 
overwhelming majority of living animal kinds. 
Proud as we are of our own mind, in which the ele- 
ment of intelligence plays so large and important a 
part, and familiar as we are with seeing this element 
play a smaller or greater role in the minds of the 
higher vertebrates, we should not forget the fact 
that an immensely larger number of animals have 244 
EVOLUTION 
their behavior controlled by instinct than by intelli- 
gence. And these instinct-minded animals live suc- 
cessful lives. 
When one contemplates the variety, precision and 
successful achievement of honeybee behavior or ant 
behavior, or of the behavior of the solitary wasps, 
made familiar to us by Fabre's entertaining accounts, 
we hesitate to accent the fact that all of this is the 
achievement of the instinct mind-of a mind which 
does not learn, which permits of almost no varia- 
tion in its determination of behavior, and which is 
entirely inherited and almost identical in degree of 
development in each of all the myriad individuals of 
any given species. Yet reflexes and instinct account 
for all their marvelous behavior. 
Yet at the same time we are struck, and almost 
shocked, by the limitations of the instinct mind. Its 
possessor does not need to be taught or to learn by 
experience the elaborate and successful behavior 
which is an essential part of its life activities. But, 
on the other hand, it cannot learn. It cannot add 
to its mental possession anything not given it by 
heredity. With any serious modification in environ- 
ment the possessor of instinct mind alone is lost. 
It may show remarkable inherited adaptation but it 
shows no individual adaptation, or extremely little. 
The solitary wasp mother must find, depending on 
her species, just this or that kind of insect or spider EVOLUTION OF MIND 
245 
victims with which to stock her nest burrow for the 
food of her children. She must dig this burrow in 
just such and such a way, in just such and such a 
place. She must sting her victims in precisely such 
a way as to paralyze but not kill them. She must 
drag them into the nest burrow in precisely one par- 
ticular way. If anything happens to disarrange the 
elaborate series of successive performances, she must 
begin all over again, or give up entirely. 
Thus, though the instinct mind enables its pos- 
sessor to do remarkable things, it has remarkable 
limitations. It has had an extraordinary develop- 
ment in animal evolution, but this development has 
not led to the highest form or type of mind. In- 
stinct has not developed into intelligence or reason. 
It has run a course of its own, a course that has, 
indeed, carried it quite away from intelligence, but 
has led it very far along its own line. The behavior 
of a honeybee is more complex and of much more 
specialized adaptative character than the behavior 
of the animals with low grades of intelligence, but 
the behavior of the animals with higher grades of 
intelligence has far greater possibilities than that of 
the honeybee. 
The highest type of intelligence mind is that pos- 
sessed by man. Indeed, so much more capable and 
so much higher is the mind of man than that of any 
other animals possessed of intelligence, that many 246 
EVOLUTION 
persons, thinking especially of its capacity for mem- 
ory, inference and constructive imagination, main- 
tain that it has elements in it qualitatively different 
from those entering into the mental make-up of other 
animals. The evolutionist, however, does not feel 
forced to admit this. He recognizes a great quanti- 
tative difference between the human mind and that 
of any other animals, even the highest of the 
mammals. But he believes this difference to be es- 
sentially only quantitative, not qualitative. How- 
ever, he is open to proof to the contrary. 
The evolutionist-psychologist who considers mind 
as not merely the functioning of the brain but as all 
that goes to determine the many various perform- 
ances in or of a human individual, finds, in his analy- 
sis of the human mind, a certain proportion of 
reflexes, a certain proportion of instinct, a certain 
proportion of unconscious brain activity, and a cer- 
tain proportion of conscious intelligence. In the 
knee jerk he sees a reflex: in the child's suckling he 
sees an adaptive, life-saving instinct: in dreams and 
many performances not the result of conscious in- 
tent, he sees the brain working unconsciously, and in 
activities consciously chosen and carried out he sees 
the element of intelligence and reason. A great 
American medical scholar has recently declared that 
man's proud possession of the intelligence mind does 
not lift him so high above the lower animals as EVOLUTION OF MIND 
247 
he is wont to think. Even when he is awake, this 
scholar declares, man is only a quarter conscious of 
what his body is doing. Three quarters of the 
energy created by the food man eats and the air he 
breathes is spent without his knowing it. 
Also, there is no doubt that man is not alone, or 
peculiar, in possessing an element of intelligence in 
his mind. Some students of the behavior of the 
animals are so irritated by the popular and conceited 
assumption that man alone is an intelligent creature 
that they are led to such sharp expressions as the 
one recently published by Dr. Hornaday, the vet- 
eran naturalist and present director of the New 
York Zoological Gardens, in his book, The Mind 
and Manners of Wild Animals. Here he declares 
that "some animals have more intelligence than 
some men, and some have far better morals!" 
Without necessarily accepting this dictum, we can 
all accept Dr. Hornaday's proofs, on the basis of a 
host of miscellaneous observations, of the existence 
of an intelligence element in the minds of many 
mammals and even lower vertebrates. His book is 
a fascinating collection of authentic accounts of in- 
telligent and reasoned behavior on the part of va- 
rious wild animals in field, forest and zoological 
gardens. Indeed, there are few of us but can tell 
our own stories of intelligent behavior on the part 
of pet dog, cat, horse, even chicken or canary. And 248 
EVOLUTION 
the carefully planned and often repeated laboratory 
experiments of the professional students of animal 
behavior reveal indisputable instances of intelligent 
reasoned behavior on the part of representatives of 
all the great vertebrate classes, even including the 
fishes. 
But man certainly stands preeminent among living 
creatures in his high development of mind. And 
this height is reached by his large possession of 
intelligence, not by any unusual development of in- 
stinct. The seat of this intelligence is the brain, 
especially the forebrain, or cerebrum, and man's 
brain is, even in the most primitive of living human 
kinds and the most ancient of true human species, 
larger than the brain of any other animal except the 
elephants and larger whales. The average size of the 
brain of present-day man is nearly three times that 
of the gorilla, which is the anthropoid ape most 
comparable with man in size. Almost midway in 
size between these two was the brain of Pithecan- 
thropus, the ape man of Java. This creature had a 
very limited forebrain and was probably governed 
in its reactions to environment, and in its general 
behavior, more by reflexes and instinct than by 
intelligence. 
True man has had a brain of the size of that 
possessed by present-day humankind ever since the 
days of Cro-Magnon man who lived in Europe EVOLUTION OF MIND 
249 
twenty-five or thirty thousand years ago. Cro- 
Magnon man was probably the first race of the 
present-day human species {Homo sapiens}. Before 
him, man of different geologic periods was, as far as 
known fossils show, of kinds sufficiently different 
from present-day man to be fairly ranked as different 
species (as Homo neanderthalensis, Homo heidel- 
bergensis, etc.}. In these early species of man, liv- 
ing from one to three or four hundred thousand 
years ago, the brain, especially the forebrain, was 
distinctly smaller and of less frontal development 
than in Homo sapiens, the present-day human 
species. 
There is a difference in brain size, too, among 
living human races. Such primitive races as the 
Bushmen, Negritos, Veddas, Australians, and Tas- 
manians (these latter having become extinct within 
historic times) have a brain running about ten per 
cent less in size than the average brain of European 
Caucasians. However, most of these primitive liv- 
ing races are of smaller body stature than the Cau- 
casians, and cranial capacity is somewhat related to 
bodily size. Also, there is considerable overlapping, 
for the larger-brained individuals of primitive races 
reach the average brain size of Caucasians, and 
smaller-brained Caucasian individuals have a brain 
no larger than that of some Negritos. But on the 
whole it can fairly be said that there are distinct 250 
EVOLUTION 
and characteristic differences in brain size among 
different living races of man. 
These differences indicate an evolution of the 
human brain, hence of the human mind, within the 
human genus, in the period of the existence of this 
genus on earth. The question, therefore, naturally 
arises, is this evolution still going on and is it to go 
on through future time? 
Anything like a positive answer to this question 
is very difficult, perhaps impossible, to make. If 
the human brain has not increased perceptibly in 
size since the time of Cro-Magnon man, twenty-five 
thousand years ago-and it has not-and if inher- 
ent human mental capacity has not increased percep- 
tibly since the days of the Egyptians of six thousand 
years ago, or of the Greeks of Homer's time-and 
this is generally admitted-it is easy to see that the 
anthropologist cannot say positively that the evolu- 
tion of the human mind is still going on. And if 
he cannot say this, equally he cannot say that it will 
go on in future time. 
But, on the other hand, that anthropologist or 
psychologist who would presume to declare, taking 
into account the brief period, from a geologic and 
evolutionary point of view, during which no percept- 
ible biological evolution of the human brain and 
mind has been apparent, that no such evolution was 
in course, and that the human mind had reached its EVOLUTION OF MIND 
251 
limit of development, would be a brave-or foolish 
-person. President Angell of Yale University, a 
psychologist of high standing, has well stated the 
position of the conservative scholar in a recent essay 
on the evolution of intelligence: 
"Is the evolutionary process at an end, so far as 
concerns the human brain and human intelligence? 
In the nature of the case no dogmatic reply can be 
offered with confidence, and one must fall back upon 
the probabilities of the case. I cannot altogether 
sympathize with the somewhat definite negative 
opinion occasionally advanced, for such negation has 
its chief justification in the vast extent of time 
throughout which little or no demonstrable advance 
has occurred in the organization of the human brain 
and therefore presumably in human intelligence. 
One cannot challenge the fact that for many thou- 
sands of years there has been little or no such change; 
but, on the other hand, the period of time for which 
we have such evidence, twenty or thirty thousand 
years, is so trifling compared to the total life of 
the race and the total duration of life itself on this 
planet, that a prediction based on such a relatively 
insignificant segment of man's history seems highly 
precarious. Assuming some extra-mundane observer 
of the primeval slime out of which organic life has 
come, it would certainly have seemed to such an 252 
EVOLUTION 
one grotesque to predict such changes as have actu- 
ally come to pass, and particularly as regards intel- 
ligence. Similarly it is entirely impossible to surmise 
at what point progress beyond present human capaci- 
ties may occur, but to conclude with any certainty 
that such further progress will not occur, much more 
that it cannot occur, seems hardly warranted." 
The evidences of evolution in the human mind, 
since the appearance, in Early Glacial time, of true 
human beings, are not limited to inferences which 
may be drawn, however fairly and certainly, from 
the increasing size of the brain during this period 
and from the varying size of the brain in different 
living human races. The anthropologists who study 
prehistoric man as well as those who study present- 
day man have a great mass of data before them. 
This they have made partially accessible to the gen- 
eral public, and the public can now trace the grada- 
tory or evolutionary steps in the development of 
human capacity to make and use weapons, tools, and 
ornaments, to construct habitations and means of 
transportation, to carve and draw and paint, to re- 
pair wounds and fight disease, to develop social re- 
lations and societal organization, to develop religion; 
in a word, to become civilized. 
Along with the actual fossil remains of Glacial 
and Post-Glacial man, hundreds of thousands of ac- EVOLUTION OF MIND 
253 
tual examples of man's handicraft have been found 
and studied and classified. These have enabled the 
modern student to recognize and define a long series 
of cultural stages in human evolution. 
For a long time, certainly no less than one or two 
hundred thousand years, perhaps longer, man had 
no implements but stone ones, and these almost all 
composed of pieces of flint variously shaped by chip- 
ping and flaking, but showing in the process of shap- 
ing and finishing different degrees of perfection. 
For a long period these stone weapons and tools 
were made by simply knocking off coarse flakes from 
a piece of flint, the core thus rudely shaped serving 
as the implement. These cores were all about alike. 
Then followed a period in which more varied core 
shapes were made. Then came the step of making 
some of the chipped-off flakes useful by sharpening 
them by a retouching by pressure on one side. Then 
this retouching became more skillful and the flake 
implements were much improved; later they were 
retouched on both sides. Finally prismatic flakes 
were formed by blows transmitted through a point. 
By the time man of the Old Stone, or Paleolithic, 
Age had reached the stage of making and using re- 
touched flakes of flint, he had also begun, in a most 
simple way, to make and use tools made from the 
bones of the wild animals he killed for food. Then 
came the use of horn. Bone heads for javelins and 254 
EVOLUTION 
spears, bone awls or pins, bone polishers, and later 
bone hammers and chisels or wedges were made. 
Reindeer antlers provided material for various horn 
implements such as harpoons and throwers. 
Man of the Old Stone Age, at least after the first 
one or two hundred thousand years of his existence, 
undoubtedly used wood and skins and crude shell or 
other ornaments. He had fire, but there is no evi- 
dence extant that he made dwelling houses or tents. 
He lived under rock shelters and in caves. He 
made carvings out of bone and horn and mammoth 
ivory, and later, at least, made drawings on the 
rock surfaces of cave walls. He colored some of 
these drawings by using ocher of various tfints. 
He had, indeed, a veritable art. He had certain 
burial practices, which suggest a simple religion. 
The end of the Paleolithic Age, which was prob- 
ably no less than 300,000 years after man had be- 
gun to exist, sees him with simple tools of consider- 
able variety, sees him using fire, cooking food, 
wearing clothes, living in definite shelters, capable 
of a simple but true art, and probably possessing 
some kind of religion. Starting as "animals among 
animals," he had come this far-in 300,000 years! 
Then came man of the New Stone, or Neolithic, 
Age, with his smooth, polished stone implements 
and a much greater variety of tools. With him 
came the bow and arrow, pottery, living in houses EVOLUTION OF MIND 
255 
in communities, the domestication of plants and ani- 
mals. But this was the time of Cro-Magnon man 
with a brain.as large as ours to-day. This was only 
30,000 years ago. After him came man of the 
various Metal ages, and then man of historic time. 
In those 30,000 years of Neolithic and Metal and 
historic ages has come all that man has to-day which 
he did not have at the end of the Old Stone Age, 
that is, 300,000 years after he first appeared on 
earth. What an acceleration of development! 
Why did human civilization move so slowly in those 
old days, and so rapidly in the new ones? There 
are two reasons. One is, that the biological evolu- 
tion of human brain and mind from their condition 
in earliest prehistoric man to that in Cro-Magnon 
man took, as all biological evolution takes, much 
time. The second is, that when there was finally 
reached a certain stage in this evolution, a new kind 
of human evolution, which we may call societal to 
distinguish it from the strictly biological evolution, 
depending on social inheritance as distinct again 
from true biological inheritance, became possible. 
And this social evolution is capable of extremely 
rapid development and did develop extremely 
rapidly. The next chapter will be devoted to a spe- 
cial discussion of this matter. 
Before, however, we leave the subject of the bio- 
logical evolution of the human mind, and its illus- 256 
EVOLUTION 
trations and consequences, we should note again that 
the anthropologists find among living human races 
different stages of human civilization associated with 
and undoubtedly partly, at least, dependent on ex- 
isting racial differences in the biological evolution of 
brain and mind. These stages show, as already 
pointed out in the last chapter, much parallel- 
ism with the various successive stages in time of 
prehistoric and historic man. There are human 
races living to-day which exist in a condition similar 
to that of the later periods of man of the Stone 
Age. There are still others living under circum- 
stances like those of the metal ages which imme- 
diately preceded historic time. We thus have in 
the varying living human races a picture of the 
evolution of humankind confirming that which we 
gain from a study of the successive prehistoric races 
of man. CHAPTER XIV 
SOCIAL INHERITANCE AND SOCIETAL 
EVOLUTION 
The word "inheritance" has come to have a 
double meaning in connection with human affairs, 
and hence its use is clouded by some misunderstand- 
ing. We speak of inheritance in connection with 
the passing on of money and goods from parents to 
children, and with the passing on from group to 
group and from generation to generation, by teach- 
ing, precept and example, of acquired and accumu- 
lated knowledge and customs and beliefs. This is 
social inheritance. Through it man is capable of 
transmitting knowledge and ideas, by various means, 
to such an extent that, as Julian Huxley has put it, 
"the experience of Moses, Archimedes and Charle- 
magne, of Jesus, Newton, and James Watt is modi- 
fying our behavior of to-day." But we speak also of 
a child's inheritance from parents and ancestors of 
eye and hair color, of bodily size and facial con- 
tour, of resistance and nonresistance to disease, of 
general mental capacity and of particular mental 
traits; in a word, of inherent structural, physiologi- 
257 258 
EVOLUTION 
cal and mental characteristics. This is biological in- 
heritance, or heredity. 
These two kinds of inheritance are fundamentally 
different and play different roles in evolution, and 
these roles vary in extent and importance in the 
evolution of different organisms. Among the plants 
and lower animals inheritance is almost exclusively 
biological. Among the higher animals, social in- 
heritance appears and assumes a varying degree of 
importance. In some of the birds and mammals 
the parents seem to pass on to their young, by a 
certain amount of teaching, the beginnings of that 
knowledge necessary for carrying on certain life- 
saving behavior. 
But, taken altogether, there is little social inherit- 
ance and societal evolution among the animals other 
than men. On the contrary, as important as biologi- 
cal inheritance is among human beings, social in- 
heritance, from one point of view, is even more 
important. Or, to avoid the dilemma of attempting 
to compare the values of these two equally indis- 
pensable and mutually complementary factors which 
have made man what he now is, we may better say 
that however important biological inheritance has 
been, and is, and will continue to be in human evolu- 
tion, we could never have attained without social 
inheritance, by this time, nor perhaps in any time, SOCIETAL EVOLUTION 
259 
the high and dominant position in nature which we 
have attained. 
This human evolution, dating it from the time 
when man was already human, or near-human, has 
been rapid. Of course "rapid" as thus used is a 
comparative term. As a matter of fact it has taken, 
as has been pointed out in an earlier chapter, prob- 
ably a half million, certainly more than a hundred 
thousand, years for man to climb from the structural 
and cultural stage of earliest Glacial time to the 
stage of man of to-day. But this period is not a 
long one in geologic and evolutionary history. It 
is, indeed, a very short one. It may be roughly 
divided into two periods of very unequal length; a 
first, or Paleolithic period, when the evolution of 
man was almost wholly biological in character, and 
moved with the characteristically slow pace of bio- 
logical evolution in general, and a second, or Neo- 
lithic-Metal Age-Historical period beginning only 
twenty or thirty thousand years ago, when the new 
factor of societal evolution based on social inherit- 
ance entered into human evolution and speeded it 
up enormously as regards its cultural and achieving 
phase. 
Changes in man due to biological evolution have 
been slight, and apparently not at all progressive or 
advantageous, perhaps indeed even retrogressive, 
since the time of Cro-Magnon man twenty thousand 260 
EVOLUTION 
years ago. Cro-Magnon man had a body in every 
way as well developed as ours, a brain as large, and 
probably a mental capacity as great, as ours. With 
the weakening of the natural selective processes 
which have come about as an incident of our growing 
altruism and our use of elaborate weapons, varied 
tools and machines, specialized houses, preventive 
and curative medicine and what-not other means for 
mass and individual self-protection and the amelio- 
ration of untoward natural conditions, the human 
body and its inherent physical capacities have almost 
certainly retrograded rather than advanced since the 
time of Cro-Magnon man. And there is no reason 
to believe that its inherent mental capacities-not 
its possibilities of mental achievement-have in- 
creased since that time. 
Anthropologists are often asked, especially by 
those who would find arguments against man's evo- 
lutionary origin and development, whether they 
claim that man of to-day is natively superior to the 
early Greeks and the earlier Egyptians and Meso- 
potamians six or seven thousand years ago. And 
the question might well be broadened to include Cro- 
Magnon man of twenty thousand years ago. The 
answer would have to be the same. It is, "No." 
But that does not mean that man has not had an 
evolutionary advance of startling character since that 
time. He has had this advance, but it is an ad- SOCIETAL EVOLUTION 
261 
vance in societal evolution, based on the rapid ac- 
quirement and accumulation of knowledge and its 
cumulative passing on from generation to generation 
by social inheritance. 
This element of societal evolution, and the accel- 
eration which it produces in human evolution, is 
apparent even in the prehistory of man. On examin- 
ing the myriad articles of prehistoric human handi- 
craft which have been found, and noting their rela- 
tion to geologic time, it is easy to see that in the 
series of cultural stages in the life of prehistoric 
man, the earlier and cruder of these stages were of 
much longer duration than the later more rapidly 
succeeding and obviously higher ones. The short 
Neolithic time produced a much larger variety and a 
much greater refinement of weapons and tools and 
utensils and ornaments than were produced in the 
ten or twenty times longer period of Paleolithic 
time. And with the oncoming of the Metal Ages, 
each successively shorter, and of early Historic time, 
this acceleration of cultural development, due to the 
larger and larger influence of social inheritance and 
societal evolution, was more and more marked. 
The importance of societal evolution in human 
development is also, of course, clearly revealed 
among living races of mankind by a comparison 
of the various cultural stages now represented among 262 
EVOLUTION 
them and an analysis of the varying degree to which 
societal evolution is active in each of them. 
Professor Breasted, the well-known American 
archaeologist, was pursuing certain archaeological 
investigations in the Valley of the Tigris during part 
of the Great War period. He found there some 
ancient mural decorations revealing many details of 
the condition of life among the inhabitants of this 
valley four thousand years ago. And he noticed 
that there was little difference between those details 
and those of the life of the native people living there 
to-day. Then, suddenly, a British expeditionary 
force swept into the region with all of the parapher- 
nalia and methods devised by modern science for 
transport, personal comfort, and highly effective 
warfare. The contrast of a highly developed cul- 
tural stage and a crudely primitive one was striking. 
Yet there was little contrast in the inherent physical 
and mental capacities of these two groups of human 
beings. But in one the social inheritance and so- 
cietal evolution factor was highly active and had pro- 
duced its striking results, while in the other it had 
not yet played the role in human development to the 
extent possible to it. 
The difference between the early Egyptians and 
Greeks, on the one hand, and modern man, on the 
other, is a difference in societal evolution. Modern 
man is not better endowed with body or brain than SOCIETAL EVOLUTION 
263 
were the great men of Greece, but he is better en- 
dowed with accumulated knowledge and the material 
results of the applications of science. He can 
achieve much more than the Greeks could in those 
lines of human activity depending on accumulated 
scientific knowledge and perfected instruments of 
power and precision. Similarly, the marked differ- 
ences in the extent to which societal evolution has 
advanced among living groups of peoples are recog- 
nized by our classification of a whole range of cul- 
tures, from those of "barbarians" to those of "civi- 
lized" peoples. 
Raphael Zon has written a fascinating account of 
the age-old warfare of man and the forests. First, 
man was dominated by the forests, then he struggled 
on more and more even terms with them, and now he 
dominates them. At least, he does where he has 
developed a high degree of societal evolution. But 
there are still culturally primitive peoples who con- 
tinue in the stage of domination by great forests. 
A similar story could be written about man's rela- 
tion to nature in a score and more of phases. His 
relation to the Tropics and the Arctics and to heat 
and cold in general; to the oceans and the deserts; to 
the bowels of the earth and the air and winds over 
it; to coal and ores and building stones; to the chemi- 
cal elements, to magnetism and electricity; to the 
plants and animals and to the various races of his 264 
EVOLUTION 
own species-each could be the subject of a fas- 
cinating story of the all-powerfulness of the social 
inheritance and societal evolution factor in his gen- 
eral evolutionary development. 
And there is another story or group of stories, as 
yet only partly written, which reveals in some meas- 
ure his interesting relations to the mysteries of his 
own body and mind. Man has always turned his 
eyes in upon himself and upon human nature in gen- 
eral as much as, or more than, he has turned his eyes 
toward the many phases of nature about him. His 
at first slowly growing and later ever more swiftly 
growing knowledge of his own bodily make-up and 
physiology; his struggles to understand the still 
deeper mysteries of his mind and his spirit-these 
play no less a part in his cultural development than 
his attempts to learn and master the secrets of that 
wide nature about him of which he is truly but a 
part, but of which he is unique in being a conscious 
and rationalizing part. 
In any study of the beginnings of human civiliza- 
tion the relations of culture to speech and language 
call, perhaps, for first consideration. Is it possible 
for one to exist without the other? "Actually, of 
course," says Kroeber, in his recent important book, 
Anthropology, "no such case is known. Specula- 
tively, different conclusions might be reached. It is 
difficult to imagine any generalized thinking taking SOCIETAL EVOLUTION 
265 
place without words or symbols derived from words. 
Religious beliefs and certain phases of social organ- 
ization also seem dependent on speech; caste ranking, 
marriage relations, kinship recognition, law and the 
like. On the other hand, it is conceivable that a con- 
siderable series of inventions might be made, and the 
applied arts might be developed in a fair measure by 
imitation, among a speechless people. Finally, 
there seems no reason why certain elements of cul- 
ture, such as music, should not flourish as success- 
fully in a society without as with language." 
"On the whole, however," he continues, "it would 
seem that language and culture rest, in a way which 
is not yet fully understood, on the same set of facul- 
ties, and that these, for some reason that is still more 
obscure, developed in the ancestors of man, while 
remaining in abeyance in other species." 
In the examination of certain of the fossils of 
early Paleolithic man, it has been noted that the 
conformation of the jaws and the smallness of the 
bony areas to which certain special muscles used in 
speaking are attached, suggest that these earlier 
human beings were restricted in their speaking pos- 
sibilities, or at least made limited use of speech. On 
the other hand, those areas of the brain cortex in 
which the nervous activities connected with speech 
are most centralized in present man, are fairly well 
developed in these early men, as is shown by casts 266 
EVOLUTION 
of the skull interiors, which conform closely to the 
brain surface. Fossil man, then, apparently had the 
language faculty, and probably spoke. But the his- 
tory and causes of the development in incipient man 
of the group of traits that may be called the faculties 
for speech and civilization, remain, as Kroeber 
points out, one of the darkest areas in the field of 
knowledge. 
In the study of the beginnings of human civiliza- 
tion there are and probably must remain serious gaps 
because of the lack of preserved materials to illu- 
minate this study. While stone implements are 
abundant from the time of the very earliest human 
fossils, implements of wood or articles of clothing 
do not appear until much later. But this may be 
because of their lack of preservation rather than 
their lack of early existence. However, metal im- 
plements, which could have endured from early 
times, as well as stone and bone and horn imple- 
ments, are not found of older date than the Metal 
Ages five or six thousand years ago. So it seems 
reasonable to assume that they did not exist in the 
older Stone Ages. Drawings on cave walls and 
carvings of horn and bone go back only to a certain 
period long subsequent to the time of the earliest 
human fossils and implements. 
Altogether, despite gaps and the one-sidedness of 
information due to possible lack of preservation of SOCIETAL EVOLUTION 
267 
certain kinds of early human handicraft, and the 
preservation of others, much has been learned of the 
beginnings of human civilization, while of course 
much more is known of its progress after the earliest 
days of human existence. Where archaeology can 
take the place of paleontology in revealing the rec- 
ords of prehistoric man, and the ethnologic study 
of primitive still living human groups can add its 
parallel testimony to that of archaeology, we have a 
fairly clear and continuous story of the later develop- 
ment of prehistoric human civilization. The migra- 
tions of early peoples, their religions and tribal and 
family relations and customs, their numerous 
legends, their gradually increasing domestication of 
animals and plants, and their developing and diversi- 
fying hunting, fishing, agricultural, industrial and 
housing advances, are all revealed with more or less 
fullness by the available records. Finally, where 
history can take the place of archaeology, the 
accounts of human civilizing processes and of civili- 
zation itself are comparatively complete. 
Now, in all this story of human development since 
the days when man was merely "animal among ani- 
mals," it is societal evolution rather than biological 
evolution which appears as the determining factor. 
And yet-and this is of fundamental importance- 
there has been a constant relation between man's 
biological and his societal evolution which must never 268 
EVOLUTION 
be overlooked, and which, as an inevitable and con- 
tinuing relation, must be constantly regarded as we 
seek for light on the possibilities and probabilities 
of future human evolution. For, on the one hand, 
societal evolution could never have played the part 
it has played in human development unless and until 
man's biological evolution had carried him to such a 
stage of mental and peculiar physical development 
as to make possible conscious thought and the accu- 
mulation and transference of knowledge by speech 
and writing; for these are the basis of social inherit- 
ance and societal evolution. And, on the other hand, 
as soon as societal evolution came well into existence, 
the further biological evolution of man could be 
and has been more or less controlled, and its direc- 
tion, consciously or unconsciously, determined by 
his societal evolution. CHAPTER XV 
THE HUMAN FUTURE 
We are living in a period of feverish "time-bind- 
ing" activity. We are bringing together, as never 
before, the yesterdays with to-day. The archaeolo- 
gists are opening, before the expanding eyes of the 
world, the tombs of Egyptian Pharaohs which were 
sealed thirty centuries ago. The anthropologists 
are uncovering less conspicuous, but more significant, 
relics of prehistoric types of man who lived all the 
way from tens to hundreds of thousands of years 
ago. We are truly learning the human past. 
But what of the human future? Can we project 
our vision into the centuries to come and see man 
then? Have our intensive studies of man of yes- 
terday and to-day given us any knowledge that we 
can use in helping us to picture man of to-morrow; 
to say whither man now is tending? 
Man has an insistent urge to speculate about his 
future, most keenly about his future as individual, 
but also, with lively interest, about his future as race 
or species. We guess; we try to find clues; we wel- 
come, and sometimes accept all too readily, declara- 
tions from any sources that have some seeming of 
269 270 
EVOLUTION 
authority. But in all this speculating, one type of 
man, the scientific type, tries to guard his specula- 
tions about the human future by holding to those 
same methods of inquiry which he has used with 
encouraging success in seeking answers to other great 
problems of nature. For the scientific man believes 
that man, for all of his high estate, is in and a 
part of nature, not above or out of it, and that 
human make-up and life and fate are to be studied 
as are other parts and phenomena of nature. He 
attempts, therefore, to get some glimpse of where 
man is tending in his evolutionary movement by 
studying the paths and the causes by which man has 
reached, from an earlier and different status, his 
status of to-day. 
Something of an outline of these paths and causes 
has been given in the earlier chapters of this book. 
We have had a fleeting picture of man slowly, very 
slowly at first, rising out of the welter of animal life 
from beastliness to humanness. "Animal among 
animals" in those earliest days of his first emer- 
gence in human, or near-human guise, he depended 
on brute strength in his struggle for existence, but 
with the advantage of already sharpened wits. 
Then, by virtue of these ever increasing wits, he 
added to his defensive and aggressive resources by 
devising simple weapons and tools made of the flint 
stones all about him and of the bones and horns of THE HUMAN FUTURE 
271 
the great mammals which were his competing asso- 
ciates. Then, as his brain grew in size and his men- 
tality in capacity, he came to depend ever more and 
more on this great advantage in his struggle to live 
and spread and adapt himself to varying natural con- 
ditions. He rapidly devised and used a myriad of 
new articles of handicraft and new means of dominat- 
ing and making use of natural forces and resources. 
And he developed that important new element, 
making for a rapid acceleration of evolutionary ad- 
vancement, to which we have referred-societal evo- 
lution in contrast with that strictly biological evolu- 
tion to which the plants and other animals are almost 
exclusively restricted for advance. Finally, he 
reached the status in which we know him to-day, 
with all his wonderful achievements and visions of 
others still more wonderful, his high development of 
the societal evolution factor and the recognition of 
his growing power, through this development, to 
play a conscious role in helping to determine his 
future fate. This is the outline of man's ascent from 
beastkind to first humankind and then to humankind 
of to-day. And now, seeing what has so far come 
to pass, conscious of our present status in nature, and 
certain that change, for better or worse, is inevitable, 
we utter that insistent inquiry: What of the human 
future ? 272 
EVOLUTION 
In attempting even the beginnings of a considera- 
tion of this inquiry, the distinction between those two 
major factors in human evolution which have been 
called biological evolution and societal evolution 
must be kept clearly in mind. But at the same time 
we must keep in mind the fact that although these 
major elements can be treated for the purposes of 
analytical discussion as more or less separable fac- 
tors, they are, in reality, closely intermingled and 
mutually interacting. 
The biological evolution of man, as of the plants 
and other animals, has been, is being to-day, and will 
be in the future, determined by the complex inter- 
action of such familiar fundamental. evolutionary 
factors as variation, heredity, selection, and influ- 
ence of environment. But man is able, consciously, 
to modify the natural working of some of these fac- 
tors not only with regard to himself, but also with 
regard to various plant and animal kinds which he 
has domesticated. He has unconsciously modified 
the conditions affecting the life of many wild plants 
and animals by acting as a powerful environmental 
agent. As such agent he has determined what shall 
be the environment, who shall be parents, what 
lines of variation and heredity shall persist and what 
be extinguished. 
All this influence exerted by man, both consciously 
and unconsciously, on the evolution of various plants THE HUMAN FUTURE 
273 
and animals, and on his own biological evolution, 
is one of the consequences of the varying form which 
his societal evolution has taken at various times in 
various places. It would not be difficult to estimate, 
with some reasonable approach to accuracy, the ex- 
tent and character of the results of this man-exerted 
influence as it has affected the fate of many plants 
and animal kinds. Consider the extinction of the 
American bison and passenger pigeon, the increase of 
the rabbit in Australia and the mongoose in Jamaica, 
the geographical restriction of the protozoan para- 
sites of malaria and yellow fever, the modifications 
through artificial selection and forced hybridization 
of the host of domesticated plants and animals. 
However, it would be very difficult, not to say 
impossible, to trace through the haze of prehistoric 
time and the growing complexity of human life, the 
character and extent of the results on man's biologi- 
cal evolution exerted by his early folkways, with 
their marriage restrictions, infanticide, sacrificial 
rites of magic and religion, food tabus, priestly medi- 
cal practice and malpractice, and the many other 
family and tribal customs which exercised, directly 
or indirectly, a certain degree of artificial selection 
within various human groups. 
But we can see in our life of to-day and in earlier 
historic time the reality of similar selective influences 
and, in some degree, the reality of their results. 274 
EVOLUTION 
Especially can we see very plainly a number of in- 
fluences that cannot but have, and must already have 
had, a seriously deleterious effect on the biological 
evolution of humankind. Think of the draining of 
the "blood of the best" by drastic war; the loss to 
the Teutonic stock by the Thirty Years' War, to the 
French stock by the Napoleonic Wars, to the Ameri- 
can stock by the Civil War and again to the French 
and German stocks by the Great War. Realize 
what the wearing out of women and children in the 
treadmills of modern industry must mean to the race. 
And, what, too, is resulting from the selective birth- 
rates in civilized nations where inferior stocks are 
increasing at the expense of superior stocks; and the 
undiscriminating altruism that keeps alive and ever 
breeding the hopelessly defective and unfit Jukes 
and Nams and Kallikaks. All these and other read- 
ily discernible unfortunate influences proceeding out 
of the present form of our societal evolution, have 
had or are having their inevitable effects on our 
biological evolution, effects that we must recognize 
as malign in their relation to the human future. 
The figures just published by Sir George Newman, 
chief medical officer of the Board of Education in 
England, in his annual report for 1922, show that 
more than forty per cent of the children in the ele- 
mentary schools of England and Wales are defective 
in some degree. Professor Karl Pearson, the emi- THE HUMAN FUTURE 
275 
nent vital statistician of the University of London, 
declares that one fourth of England's population is 
producing one half of England's next generation and 
that this fourth is that part of England's people 
most poorly endowed by both biological and social 
inheritance. 
Thoughtful students of human evolution have 
been more and more impressed with the growing 
extent of these hurtful influences and their effects 
on the human future. In two recent books by Pro- 
fessor S. J. Holmes of the University of California, 
called The Trend of the Race and Studies in Evo- 
lution and Eugenics, this feeling is expressed in 
strong, even poignant, terms. Professor Holmes 
makes a very gloomy forecast for the future of the 
human family, unless a change is produced in the 
conditions affecting its evolutionary course. Similar 
views are advanced by Professor E. M. East of 
Harvard University in his even more recent book 
Mankind at the Crossroads. 
We must give a serious attention to the situation. 
It is not sufficient, it is definitely dangerous, to brush 
aside these pessimistic utterances with an impatient 
resentment at feeling ourselves and our evolutionary 
fate examined cool-bloodedly by biologists from the 
same point of view as they examine the fate of ani- 
mal and plant kinds. It is dangerous to assume that 
a fate determined by natural factors and the fa- 276 
EVOLUTION 
miliar evolutionary processes may be accepted as 
inevitable in the case of plants and animals, but that 
we, all-powerful creatures of a superior kind, capa- 
ble of self-understanding and self-determination, are 
exempt from these biological controls which deter- 
mine the individual and racial fate of the lower 
creatures. 
That is to be fatally blind. All the history of the 
human race and of human groups tells us otherwise. 
We have originated and risen as have other animal 
species. And we are not exempt from the natural 
laws of life and evolution. Other animal groups 
and species have appeared and risen and persisted 
or fallen. We have appeared and risen and so far 
persisted-but we may fall. Indeed, when we ex- 
amine ourselves as subdivided into groups or nations, 
each with its own history of development and fate, 
we see that some of us have fallen. And in most 
of these cases of the fall of groups, analysis will 
reveal biological factors as potent determinants in 
these catastrophes. 
Fortunately, in our evolutionary rise there has 
been included such a development of mind that we 
alone among living creatures can know and under- 
stand something, at least, of the natural conditions 
and laws which control organic evolution. And 
we are able to cumulate this knowledge by social THE HUMAN FUTURE 
277 
inheritance, and thus to develop in high degree cer- 
tain possibilities in the way of biologic control. 
In that respect alone are we different, in our rela- 
tion to our evolution, from the plants and other 
animals. By this we are indeed given a certain 
control of our evolutionary fate. But this control 
is one not in despite of natural laws. It is a con- 
trol by virtue of the fact that we can understand 
these laws and make use of our understanding to 
adapt ourselves to them, to take advantage of them 
and use them in a conscious attempt to give ourselves 
the evolutionary fate which we desire. 
Thoughtful and informed men understand this, 
and on this understanding, and on the possibility of 
making everybody, or at least a governing majority, 
similarly understand this, depends our hope and our 
possibility of proving ill-founded the gloomy fore- 
bodings of such students of human biology and soci- 
ology as Holmes and East and the numerous other 
prophets of the decline of the race. 
There are encouraging signs of a widening in- 
terest and action in this matter. The modern 
eugenist disciples of Plato, with Galton at their 
head until his death, are now ever-increasingly nu- 
merous and active in all civilized countries. There 
is a growing tendency to temper altruism by intelli- 
gence. We find a great advance in guarding the 
public health, and a strong opposition to war as a 278 
EVOLUTION 
powerful agent making for a dangerous artificial 
selection, a "reversed selection," as it has 
been called. We see an American effort to safe- 
guard its stock from the effects of ill-advised race 
mixture resulting from indiscriminate immigration; 
the growing attempts to ameliorate the disastrous 
effects on women and children of selfish industrial 
methods; the active inquiry into the merits and 
demerits of birth control; the earnest and partly 
successful efforts to understand better the nature and 
distribution of intelligence; and, finally, the constant 
increase of general education in biological facts and 
principles. All these, and numerous other encour- 
aging present-day societal activities, which have a 
more or less direct influence on our biological evolu- 
tion, are to be put on the right side of the ledger 
account of the present conditions in human life to 
offset those sad entries on the other side which pro- 
vide the basis for the gloomy prognostications so 
much in evidence to-day. 
There are thus three all-important things to be 
kept in mind by those who already know them, and 
to be introduced into the minds of those who do not. 
First, that human evolution, hence the human future, 
is determined by two groups of causal factors, one 
group comprising those producing biological evolu- 
tion and the other those producing societal evolu- THE HUMAN FUTURE 
279 
tion. Second, that through societal evolution the 
course of biological evolution can be largely directed. 
And, third, that the determination of our societal 
evolution depends in a great measure on our own 
decisions and efforts. We may, by education, propa- 
ganda, and legislation, develop this societal evolution 
in such a way as to make it directly helpful or hurt- 
ful in its relation to our future through its imme- 
diate effects as an environmental agent, and indi- 
rectly helpful or hurtful through its influence on 
our biological, or fundamentally racial, evolution. 
These things are certain, and should be as widely 
and clearly understood as possible. Two other 
things about which there is less certainty should also 
be taken into consideration and intensively studied. 
One is the question as to whether or not there are 
elements in human life which tend to mold our 
societal evolution despite our own efforts to deter- 
mine it. The disconcerting way in which we con- 
tinue to indulge in destructive war, despite the 
knowledge of informed men of its societal and bio- 
logical danger and the desire of the great majority 
of civilized human beings to avoid it, and the im- 
potence with which we seem to face the pressing 
needs of post-war rehabilitation make us fear that 
we are in the grip of obscure but powerful forces 
which have their way with us despite our wishes. 
The other uncertain matter is the still open question 280 
EVOLUTION 
as to whether or not effects produced on individuals 
by environmental influences can be introduced in 
some degree into our heredity: in other words, 
whether it is really true, as most biologists hold, 
that no kinds of acquired characters can be inherited. 
These two matters need concentrated and prolonged 
scientific study. The results of such study may 
modify the attitude that we must take toward human 
evolution and hence the human future. 
But in the present state of our knowledge of the 
factors which enter into the determination of the 
fate of humankind we must leave these uncertain 
matters by the side, and fasten our attention on 
things that are certain. In the light which these 
give us, what shall we do to insure, as far as our 
conscious efforts can insure, and according to our 
understanding of human values, the human future we 
should like to have? How shall we make our chil- 
dren's land, and the land of our children's children 
a better land than ours? 
Let us selfishly, if it seems so, confine our atten- 
tion, for the moment, to America, to our own people. 
Let us think of the human future in terms of the 
American future. While many of the evolutionary 
problems of different peoples are common to all of 
them, some are particular to each people. For ex- 
ample, the problem of immigration, with its social 
and its biological, or racial, effects, is peculiarly an THE HUMAN FUTURE 
281 
American problem. So is the negro problem, which 
also has both social and biological phases. The 
problem of the decrease of good stocks through a 
selective birthrate is a problem common to all civi- 
lized peoples. So is the problem of the relation of 
a growing population to the food supply. There 
is, indeed, a host of problems posed to us by any 
examination of our life of to-day, which we must 
squarely face if we would look forward to our 
future with any attempt to help determine it. For 
this future will almost certainly not be deter- 
mined for us by sudden destructive catastrophe nor 
by sudden benevolent act of Providence, happening 
without some foreknowledge on our part or unre- 
lated to our present conditions and mode of life. 
Just as these conditions and mode have gradually 
and obviously grown out of our past conditions of 
life and manner of behavior, so the future will grow 
out of to-day. The future will be the effect of 
present causes; it is being shaped now. 
These causes lie in our present-day societal organ- 
ization and behavior and our present biological 
status. They will produce the future both through 
societal and through biological evolution. Now, 
we know enough to be sure that, as regards our so- 
cietal organization, we want such changes as will 
enable our people to be more widely and more sound- 
ly educated, more competently protected from dis- 282 
EVOLUTION 
ease and accident, more comfortably housed and 
wisely fed, more secure from harrowing class strug- 
gle, more encouraged and stimulated to live ac- 
cording to the Golden Rule. And we also know 
enough to be sure that we want our societal evolu- 
tion to be of a kind which will influence our bio- 
logical evolution to move along eugenic, not dys- 
genic, lines; to develop an American stock of 
inherently sounder bodies and higher intelligence. 
We do not want the American blood to be drained 
of its best elements by destructive war, or to be 
diluted and discolored by indiscriminate mingling 
with poorer blood. We do not want the inferior 
elements in our racial stock to increase while the 
superior elements decrease. We do not want our 
inherently mentally defective and our hereditarily 
infirm to multiply until our asylums outnumber our 
universities and colleges. We do want the opposite 
of all this. We want everything in our societal or- 
ganization and behavior which directly or indirectly 
affects our biological evolution-and most of it does 
-to affect this evolution in such a way as to make 
it move forward and upward, not backward and 
downward. 
We cannot change this biological evolution into 
radically new lines; we can only go forward or back- 
ward along a path already well determined. The 
human species has attained such a specialization of THE HUMAN FUTURE 
283 
structure, physiology and psychology that the stage 
of generalization, from which many different paths 
lead out, any one of which may be followed, has 
long ago been passed. Our evolution has been 
more and more canalized. We can go farther, or 
we can stop, or we can go backward. But we can- 
not branch out laterally. We cannot undertake new 
lines of development. 
This canalization of evolutionary movement by 
the attainment of a high degree of specialization 
is a familiar matter to the biologist and paleontolo- 
gist. He knows many examples in the biological 
history of plant and animal kinds, of the ever in- 
creasing success of specialized species in the face 
of a particular environment, and later their slow 
extinction because their specialization has not been 
of a kind to be successful in the face of a changing 
environment. He recalls the rise and fall of the 
mighty reptilian monsters of the inland American 
seas of Triassic, Jurassic and Cretaceous times. As 
their environment slowly changed these highly spe- 
cialized monsters faded out. They were beyond re- 
adapting themselves to the new conditions. 
But the fate of extinction through overspecializa- 
tion is not indicated for man. For one of the happy 
features in man's specialization is that of a great 
capacity for adaptation through his possession of 
a mind of intelligence and reason and, by virtue of 284 
EVOLUTION 
it, a capacity for dominating nature and using nat- 
ural resources to his advantage. He can over- 
come cold and heat, humidity and dryness, although 
naturally very susceptible to them; he can move 
across the oceans or through the air although he 
has no fins or wings; he can conquer forests and 
swamps, deserts and mountains; distance and dark- 
ness; he converts energy of one kind into another 
and makes new combinations of chemical elements 
for his use. In a word, he adapts himself to nature, 
not by changes of structure but by exercise of wits, 
and he modifies nature to suit himself. This par- 
ticular character of his high specialization can save 
him from the fate that so often attends over- 
specialization among plants and animals. 
But, nevertheless, he cannot diverge widely from 
the canalized path of evolution which the character 
of his biological specialization has determined for 
him. So he must bring all his great capacity for 
understanding and for acting to bear on the problem 
of making this already determined evolutionary path 
lead him to the Happy and beneficent future which 
he so much desires and in the achieving of which 
he has so important a personal role to play. In a 
word, the problem of the human future, both societal 
and biological, is a problem of which man holds 
the solution very largely in his own hands. It de- 
pends on his own intelligent conscious endeavor THE HUMAN FUTURE 
285 
to solve this problem happily. There are different 
possible human futures. The one which man wants 
and works for is the one which men can have. 
We can say the same of the American future. 
It is only a part, to be sure, of the general human 
future, but it is a part the particular features of 
which can be determined by the American people. If 
they determine this wisely they will influence for 
the good not only their own but the general human 
future. Circumstances have combined to give 
America a peculiar place among the present peoples 
of the earth. They have given her an unusual 
power to work for the good of the race. This 
unusual power gives her an unusual responsibility. 
To a peculiar and perhaps hardly sufficiently recog- 
nized degree, the human future depends on the in- 
fluence which America will, for good or ill, inevitably 
exert during the next few years. Let her take sci- 
ence as a handmaiden and make her serve in the 
cause of assuring human capacity and happiness. 
Let her direct the evolution of her own people in 
the way she would have it go; for the means to do 
this are in her own hands. If she does this she 
will not only do greatly for herself, but she will go 
far toward assuring the best evolution of all human- 
kind. INDEX 
Acquired characters, 116 
Adaptations of, birds' bills, 
30 
birds' feet, 29 
birds' wings, 31 
Algae, 160 
Alligators, 202 
Ammonites, evolution of, 77 
Amoeba, 172 
Amphibians, 200 
Animals, evolution of, 170 
Apes, 213 
anthropoid, 214 
fossil, 215 
Aristotle, 12 
Articulates, 187 
Artificial selection, 136 
Ascidians, 196 
Associations of plants and 
animals, 39 
Augustine, 14 
Australia, peculiarities of 
animals of, 88 
Bacteria, 163 
Baer, von, Karl, 54 
Bergson, 107 
Birds, 204 
kinds of, 25, 28 
of Galapagos Islands, 86 
Blood tests, revealing rela- 
tionship of man to apes, 
235 
Bonnet, 15 
Brain, development of hu- 
man, 58 
of man, 248 
Bryan, W. J., 94 
Buffon, 15 
Burbank, Luther, 138 
Camel, evolution of, 76 
Campbell, Douglas, 155 
Cats, varieties and origin, 
138 
Cattle, varieties and origin, 
139 
Centipedes, 187 
Chickens, varieties and ori- 
gin, 141 
Chimpanzee, 215 
Chordata, 196 
Coecilians, 201 
Comparative anatomy, 48 
Conifers, 147 
Corals, 177 
Crabs, 33 
Crayfish, 187 
Crocodiles, 202 
Cro-Magnon man, 229, 259 
287 288 
INDEX 
Cross-pollination of plants 
by insects, 149 
Cuvier, 16 
Cycads, 148 
Darwin, Charles, 17, 18, 83, 
85, 86, 87, 112, 132, 
134 
Darwin, Erasmus, 15 
Darwinism, defined, 21 
Darwin's explanation of evo- 
lution, 98 
Diatoms, 166 
Dogs, varieties and origin, 
138 
Domestication of animals 
and plants, 137 
Ducks, varieties and origin, 
141 
Earthworms, 187 
East, E. M., 275 
Echinoderms, 184 
Elephant, evolution of, 76 
Embryology, 54 
of man, 55 
Empedocles, 11 
Environment, response to, 
145 
Eugenics, 274 
Evidences of evolution, 47 
Evolution, defined, 1 
explanations of, 94 
factors of, 109 
growth of idea of, 11 
Factors of evolution, 109 
Ferns, 153 
Ferris, H. B., 55 
Fishes, 198 
Flagellata, 165 
Flints, evidence of evolution 
of man, 231 
Fossils, 67 
human, 71 
Fungi, 156 
Galapagos Islands, life on, 
85 
Galton, 121, 277 
Geese, varieties and- origin, 
141 
Geographical distribution of 
plants and animals, 79 
Geographical realms of life, 
83 
Geological chronology, table 
of, 73 
Gibbon, 214 
Glacial man, 229, 252 
Gorilla, 215 
Gregory, 14 
Gulick, 104 
Guyer and Smith, 118 
Haeckel, 19, 54, 160 
Heart, development of hu- 
man, 60 
Heredity, 120 
human, 131 
Mendelian, 124 INDEX 
289 
Hermit crabs, 34 
Hogs, varieties and origin, 
140 
Holmes, S. J., 275 
Honey bee, varieties and ori- 
gin, 142 
Hooker, 86 
Horse, evolution of, 75 
Horses, varieties and origin, 
139 
Human anatomy revealing 
evolution of man, 222 
Human brain, 248 
Human embryology reveal- 
ing evolution of man, 
224 
Human fossils revealing evo- 
lution of man, 227 
Human future, 269 
Human heredity, 131 
Human paleontology, 226 
Human specialization, 282 
Huxley, 18, 19, 66 
Inheritance, biological, 257 
in silkworms, 125 
of acquired characters, 
117 
social, 257 
Insects, 187, 191 
instincts of, 244 
Instinct, 243 
Intelligence, 245 
of animals, 247 
Invertebrates, evolution of, 
170 
Isolation, an evolution fac- 
tor, 103 
Jelly fishes, 177 
Jelly fishes, colonial, 180 
Jordan, David Starr, 84, 
103 
Kammerer, 117 
Kolliker, von, 100 
Korschinsky, 101 
Kroeber, A. L., 264 
Lamarck, 15, 17, 20, 116 
Lamarck's explanation of 
evolution, 97 
Lancelet, 197 
Leeches, 187 
Life, origin of, 109 
Linnaeus, 16 
Liverworts, 155 
Lizards, 202 
Lyell, 66 
Mammals, 207 
Man, brain of, 248 
embryology of, 55 
evolution of, 217 
influence on Nature, 272 
prehistoric, 227 
Mayor, A. G., 135 
Medusae, 178 
Mendel, 121, 123 290 
INDEX 
Mendelian combinations, 102 
Mendelian heredity, 124 
Mind, evolution of, 238, 
252 
Mollusks, 192 
Monera, 164 
Monkeys, 212 
Monterey cypress, 25 
Monterey pine, 24 
insects attacking, 40 
Mosses, 155 
Mutations, 100 
Natural selection, 112, 
133 
Neanderthal man, 229 
Negro problem, 281 
Neolithic man, 254 
Newman, H. H., 91 
Newman, Sir George, 274 
Notochord, 197 
Oceanic islands, life on, 85, 
88 
Orang-utan, 214 
Orchids, cross-pollination by 
insects, 150 
Origin of life, 109 
Origin of species, Darwin's, 
16, 17 
Orthogenesis, an evolution 
factor, 106 
Overproduction of young, 
112 
Paleolithic man, 229, 252 
Paleontology, 66 
Parasitic plants, 151 
Pasteur, no 
Pavlov, 118 
Pearson, Karl, 274 
Peridineae, 166 
Pigeons, varieties and origin, 
141 
Plants, catching insects, 
I51 
distinguished from ani- 
mals, 167 
evolution of, 147 
parasitic, 151 
plasticity of, 168 
Plato, 277 
Polyps, 36, 177 
Population and food supply, 
281 
Prehistoric man, 227 
Primates, 212 
Protozoa, 172 
colonial, 174 
Races of man, 233 
Rat, distribution of, 81 
Recapitulation theory, 54 
Reflexes, 242 
Reptiles, 202 
Romanes, 103, 104 
Salamanders, 201 
Sand plates, 184 INDEX 
291 
Sea anemones, 36, 177 
Sea cucumbers, 184 
Seashore, variety of life on, 
33 
Sea squirts, 196 
Sea urchins, 184 
Segregation, an evolution 
factor, 144 
Selection, 132 
artificial, 136 
natural, 112, 133 
sexual, 134 
Sheep, varieties and origin, 
140 
Silkworms, inheritance in, 
I25 
varieties and origin, 142 
Skeleton of vertebrates, sim- 
ilarity of parts of, 49 
Snails of Hawaii, 104 
Snakes, 202 
Social inheritance, 257 
Societal evolution, 258 
Speech, 264 
Spencer, Herbert, 19 
Spiders, 187 
Sponges, 37, 175 
St. Hilaire, Geoffry, 15 
Star fishes, 184 
Survival of the fittest, 114 
Toads, 201 
Tortoises, 202 
Tropisms, 242 
Turtles, 202 
Tyndall, no 
Variation, an evolution fac- 
tor, 112 
fluctuating, 115 
Variations on ladybird 
beetles, 99 
Variety of life, 43 
Vertebrates, evolution of, 
195 
Vestigial structures, 51 
in human body, 52 
Volvocales, 165 
Vries, de, Hugo, 100 
Wagner, Moritz, 103 
Wallace, Alfred Russell, 19, 
83, 85, 150 
Weismann, 117 
Wiedersheim, 52 
Worms, 187 
Yucca, pollinated by Pro- 
nuba, 150 
Zon, Raphael, 263 
FINIS