901 
PRESENT PROBLEMS IN 
EVOLUTION AND HEREDITY. 
BY 
HENRY FAIRFIELD OSBORN. 
FROM THE SMITHSONIAN REPORT FOR 1892. 
WASHINGTON: 
GOVERNMENT PRINTING OFFICE. 
189 3 901 
PRESENT PROBLEMS IN 
EVOLUTION AND HEREDITY. 
BY 
HENRY FAIRFIELD OSBORN. 
FROM THE SMITHSONIAN REPORT FOR 1892. 
WASHINGTON: 
GOVERNMENT PRINTING OFFICE. 
1893. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY.* 
Henry Fairfield Osborn. 
In the past decade of practical research and speculation in biology, 
two subjects have oustripped in interest and importance the rapid prog- 
ress all along the line. These are, first, the life history of the repro- 
ductive cell from its infancy in the ovum onward, and second, the 
associated problem of heredity, which passes insensibly from the field 
of direct observation into the region of pure speculation. 
As regards the cell it was generally believed that the nucleus was an 
arcanum into the mysteries of which we could not far penetrate; but 
this belief has long been dispelled by the eager specialist, and it is no 
exaggeration to say that we now know more about the meaning of the 
nucleus than we did about the entire cell a few years ago. At that 
time the current solution of the heredity problem was a purely formal 
one; it came to the main barrier, namely, the relation of heredity and 
evolution to the reproductive cells, and leapt over it by the postulate 
of Pangenesis. The germ-cell studies of Balfour, Van Beneden, the 
Hertwig brothers, Weismann, Boveri, and others, have gradually led 
us to hope that we shall some day trace the connection between the 
intricate metamorphoses in these cells and the external phenomena of 
heredity, and more than this, to realize that the heredity theory of the 
future must rest upon a far more exact knowledge than we enjoy at 
present of the history of the reproductive cell both in itself and in the 
influence which the surrounding body cells have upon it. 
These advances affect the problem of life and protoplasm, whether 
studied by the physician, the anthropologist, or the zoologist, thus con- 
centrating into one focus opinions which have been formed by the 
observation of widely different classes of facts. As each class of facts 
bears to the observer a different aspect and gives him a personal bias, 
the discusson is by no means irenical, and it is our privilege to live 
through one of those heated periods which mark the course of every 
revolution in the world of ideas. Such a crisis was brought about by 
*The Cartwright Lectures for 1892; delivered before Alumni of the College of 
Physicians and Surgeons, February 12, 19, and 26, 1892. (From the Medical Record 
for February 20, March 5, April 23, and May 14, 1892.) 314 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
the publication of the theory of Darwin, in 1858, and, after subsiding, 
has again been aroused by Weismann’s theory of heredity, published 
in 1883. 
This is the situation I have ventured to present to you as Cartwright 
lecturer, not, of course, without introducing some conclusions of my 
own, which have been derived from vertebrate palaeontology, but which 
1 shall direct mainly upon human evolution. 
So far as theories need come before us now, remember that Lamarck 
(1792) attributed evolution to the hereditary transmission to offspring 
of changes (acquired variations) caused by environment and habit in 
the parent. Darwin’s latest view was that evolution is due to the 
natural selection of such congenital variations as favored survival, sup- 
plemented by the transmission of acquired variations. Weismann 
denies the transmission of acquired variations, or characters, entirely, 
and attributes evolution solely to the natural selection of tlie indi- 
viduals which bear the most favorable variations of the germ or repro- 
ductive cells. We must therefore clearly distinguish between “con- 
genital variations” which are part of our inheritance and “acquired 
variations” which are due to our life habits; the question is, are the 
latter transmitted? 
At the outset 1 would emphasize the extreme complexity of evolution 
by a few words upon variation, or in terms of medical science, upon 
anomalies. 
When we speak of a part as “anomalous” we mean that it varies at 
birth from the ordinary or typical form; it may be minute, as the small 
slip of a tendon, or large, as the addition of a complete vertebra to the 
spinal column. Wood has found that in the muscular system alone 
there are nine anomalies in the average individual. It is clear that 
the evolution of a new type, so far as the muscular system is concerned, 
must consist in the accumulation of anomalies in a certain definite 
direction by heredity. Thus the anomalous condition of one generation 
may become the typical condition of a very much later generation, and 
we observe the paradox of a typical structure becoming an anomaly 
and an anomalous structure becoming typical; for example, the supra- 
condylar foramen of the humerus was once typical, it is now anom- 
alous; the retardation in development of the wisdom tooth was once 
anomalous, it is now typical. 
The same principle applies to races which are in different stages of 
evolution; an anomaly in the white, such as the early closure of the 
cranial sutures, is normal in the black. jSTow the deductions of the 
Weismann school of evolutionists seem to be founded upon the prin- 
ciple ude minimis non curat, lex;'1'1 that we need only regard such major 
variations as can, ex hypothesis weigh in the scale of survival. Against 
this I urge that we must regard the evolution of particular structures, 
the components of larger organs, the separate muscles and bones for 
example, for the very reason that while in some cases they play a most PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
315 
humble role in our economy we cau prove beyond a doubt that they are 
in course of evolution. Minor variations in foot structure, which are 
possibly of vital importance to a quadruped whose very existence may 
depend upon speed, sink into obscurity as factors in the survival of 
the modern American. 
The evolution of man in the most unimportant details of his structure 
promises, therefore, to afford a far more crucial test of the Lamarckian 
vs. the pure natural selection theory, than in the domain of his higher 
faculties, for the reason that selection may operate upon variations in 
mind, while it taxes our credulity to believe it can operate upon varia- 
tions in muscle and bone. This is my ground for selecting the skele- 
ton and muscles for the subject of the introductory lecture. Never- 
theless, let us review variation in all its forms in human anatomy be- 
fore forming an opinion. Let us remember, too, that congenital and 
acquired variations are universal as necessities of birth and life; they 
are exhibited in the body as a whole—in its proportions, in the compo- 
nents of each limb, finally in the separate parts of each component, as 
in the divisions of a complex muscle. Thus the possibilities of trans- 
formism are everywhere. What is the nature and origin of congenital 
variations'? Their causes? Do they follow certain directions? Do 
they spring from acquired variations by heredity? These are some 
of the questions which are still unsettled. 
But striking as are the anomalies from type, the repetitions of type 
as exhibited in atavism and normal inheritance are still more so, and 
equally difficult to explain. Therefore our theory must provide both 
for the observed laws of repetition of ancestral form and the laws of 
variation from ancestral form, as the pasture-land of evolution. Add 
to these, that for a period in each generation this entire legislation of 
nature is compressed into the tiny nucleus of the fertilized ovum, 
and the whole problem rises before us in its apparent impregnability 
which only intensifies our ardor of research. 
LECTURE I.—TIIE CONTEMPORARY EVOLUTION OF MAN. 
The anthropologists and anatomists have en joyed a certain monopoly 
of Homo sapiens, while the biologists have directed their energies 
mainly upon the lower creation. But under the inspiring influences of 
the Darwinian theory these originally distinct branches have con- 
verged, and as man takes his place in the zoological system, compar- 
ative anatomy is recognized as the infallible key to human anatomy. 
For our present purpose we must suppress our sentiment at the out- 
set and state plainly that the only interpretation of our bodily struc- 
ture lies in the theory of our descent from some early member of the 
primates, such as may have given rise also to the living Anthropoidea. 
This is also the only tenable teleological vieAV, for many of our inher- 
ited organs are at present non-purposive, in some cases even harmful,— 
as the appendix vermiformis. 316 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
From tlie typical mammalian standpoint man is a degenerate 
animal; his senses are inferior in acuteness ; his upright position, while 
giving him a superior aspect, entails many disadvantages, as recently 
enumerated by Clevenger,* for the body is not fully adapted to it; his 
feet are not superior to those of many lower Eocene plantigrades; his 
teeth are mechanically far inferior to those of tlie domestic cat. In 
fact, if an unbiassed comparative anatomist should reach this planet 
from Mars he could only pass favorable comment upon the perfection 
of the hand and the massive brain. Holding these trumps, man has 
been and now is discarding many useful structures. I refer especially 
to civilized man, who is more prodigal with his inheritance than the 
savage. By virtue of the hand and the brain he is nevertheless the 
best adapted and most cosmopolitan vertebrate. The man of Nean- 
derthal or Spy, with retreating forehead and brain of small cubic 
capacity! was limited both in his ideas and his powers of travel; yet 
he was our superior in some points of osteological structure. But the 
period of Neanderthal was recent compared with that in which some 
of our rudimentary organs were serviceable, such as the vermiform 
appendix or the panniculus carnosus f muscle. These rudiments in 
turn are neogenetic when we consider the age of the two antique 
sense organs in the optic thalamus, the remnants of the median or 
pineal eye and the pituitary body, both of which were undoubtedly 
present, and probably useful, in the recently discovered Silurian fishes. 
I mention these vestiges of some of the first steps in creation to illus- 
trate the extraordinary conservative power of heredity (which is even 
more forcibly seen in our embryological development), partly also to 
show how widely our organs differ in age. Galton has compared the 
human frame to a new building built up of fragments of old ones; ex- 
tend this back into the ages and the comparison is complete. 
Development, balance, degeneration.—It is probable that none of our 
organs are absolutely static and that the apparent halt in the develop- 
ment of some is merely relative, as where a fast train passes a slow one. 
The numerous cases of arrested evolution in nature are always con- 
nected with fixity of environment, an exceptional condition with man, 
and we have ample evidence that some organs are changing more rap- 
idly than others. 
Adaptation to our changing circumstances is mainly effected by the 
simultaneous development and degeneration of organs which lie side 
by side, as in the muscles of the foot or hand; in terms of physiology, 
* Disadvantages of the Upright Position, article in American Naturalist, January, 
1884, vol. xviii. i). 1. 
tThe remarkable skulls and skeletons which have recently been discovered at 
Spy remove all doubts as to the normal, i.e., racial character of the famous Nean- 
derthal skull, which were entertained by Quatrefages and others. See Fraipont 
and Lohest, Archives de Biologie, 1887, p. 697. 
t This is an epidermal or twitching muscle in the quadrupeds. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
317 
we observe the hypertrophy of adaptive organs and atrophy of inadap- 
tive or useless organs. This compensating re-adjustment, whereby the 
sum of nutrition to any region remains the same during re-distribution 
to its parts, may be called metatrophism. It is the u gerrymander ” prin- 
ciple in nature. 
In practical investigation it is very difficult in many cases to deter- 
mine whether an organ is actually developing or degenerating at the 
present time, although its variability or tendency to present indi- 
vidual anomalies indicates that some change is in progress. I may 
instance the highly variable peroneus tertius muscle (Wood). The rise 
or fall of organs is so constantly associated with their degree of utility 
that in each case the doubt can be removed by a careful analysis of the 
greater or less actual service rendered by the part in question. Apart 
from the question of causation, it is a fixed principle that a part degen- 
erating by disuse in each individual will also be found degenerating in 
the race. 
Degeneration is an extremely slow process; both in the muscular and 
skeletal systems we find organs so far on the down grade that they are 
mere pensioners of the body, drawing pay (/. e., nutrition) for past hon- 
orable services without performing any corresponding work—the plan- 
taris and palmaris muscles for example. Of course an organ without a 
function is a disadvantage, so that the final duty of degeneration is to 
restore the balance between structure and function, by placing it hors 
de combat entirely. One symptom of decline is variability, in which the 
organ seems to be demonstrating its own uselessness by occasional 
absence. As Humphrey remarks: uThe muscles which are most fre- 
quently absent by anomalies are in fact those which can disappear with 
least inconvenience, either because they can be replaced by others or 
because they play an altogether secondary role in the organism.77 The 
stages downward are gradual; the rudiment becomes variable as an 
adult structure, then as a foetal structure; the percentage of absence 
slowly increases until it re-appears only as a reversion; finally the 
part ceases even to revert and all record of it is lost. This long strug- 
gle of the destructive power of degeneration, which you see is essen- 
tially an adaptive factor, against the protective power of heredity is 
the most striking feature of the law of repetition. (See Galton’s simi- 
lar principle of regression in anthropology.) 
A careful study of our developing, degenerating, rudimental, and 
reversional organs amply demonstrates that man is now in a state of 
evolution hardly less rapid, I believe, than that which has produced 
the modern horse from his small five-toed ancestor. As far as I can 
see, the only reason why our evolution should be slower than that of 
the ancient horse is the frequent intermingling of races, which always 
tends to resolve types which have specialized into more generalized 
types. Wherever the human species has been isolated for a long period 
of time, divergence of character is very marked, as will be seen in some 
of the races I refer to below. 318 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
To lighten the long eatalogue of facts, gathered from many authors, 
I shall frequently allude to habit, but will ask you to consider it for the 
time as associational rather than causal. Poucliet says: “Man is a 
creature of the writing table, and could only have been invented in a 
country in which covering of the feet is universal;” he should have 
added the “eating table.” From the average man our fashions and 
occupations demand the play of the forearm and hand, the independent 
and complex movements of the thumb and finger; the outward turning 
of the foot in walking. These are some of the most conspicuous features 
of modern habit. 
The skeletal variations *—In a most valuable essay by Arthur Thom- 
son upon “The Influence of Posture on the Form of the Articular Sur- 
faces of the Tibia and Astragalus in the Different Races of Man and 
the Higher Apes,” t we find clearly brought out the distinction between 
congenital variations and those which may be acquired by prolonged 
habits of life. It is perfectly clear from this investigation that certain 
racial characters, such as “ platycnemism ” or flattened tibia, which 
have been considered of great importance in anthropology, may prove 
to be merely individual modifications due to certain local and temporary 
customs. Thomson’s conclusions are that the tibia is the most variable 
in length and form of any long bone in the body. Platycneuiia is most 
frequent in tribes living by hunting and climbing in hilly countries, 
and is associated with the strong development of the tibialis posticus. 
The great convexity of the external condyloid surface of the tibia in 
savage races appears to be developed during life by the frequent or 
habitual knee flexure in squatting; it is less developed where the tibia 
has a backward curve, and is independent of platycneuiia. Another 
product of the squatting habit is a facet formed upon the neck of the 
astragalus by the tibia. This is very rare in Europeans; it is found in 
the gorilla and orang, but rarely in the chimpanzee. We must there- 
fore be on our guard to distinguish between congenital or hereditary 
skeletal characters which are fundamental, and “acquired” skeletal 
variations which may not be hereditary. The latter are of question- 
able value in tracing lines of descent, if not actually misleading; on 
the other hand, the teeth, as shown by Cope in his essay on “Lemurine 
reversion in human dentition,” have distinct racial patterns and are 
reliable indices of consanguinity, because their form can not be modified 
during life. 
The main features of present evolution in the backbone are the elab- 
oration of the spines of the cervical vertebra?, the increase of the spinal 
curvatures, the shortening of the centra of the lumbar vertebrae and 
* For recent general articles, see Blanchard, “L’Atavisme cliez l’Homme,” Rev. de 
Anthrop., 1885, p. 425; and Baker, “The Ascent of Man,” Proceedings of the American 
Association for the Advancement of Science, 1890. Also, Smithsonian Report for 1890, p. 
447. 
\ Journal of Anatomy and Physiology, 1889, p. 617. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
319 
shifting of the pelvis upward, whereby a lumbar vertebra is added to 
the sacrum and subtracted from the dorso-lumbar series. 
Cunningham has found that the division of the neural spines in the 
upper cervical vertebr a1 distinguishes the higher races from the lower.* 
The spine of the axis is always bifid, but the spines of the cervicals 
three, four, and five, are also, as a rule, bifid in the European, while 
they are single in the lower races. The same author shows that the 
bodies of the lumbar vertebrae are altering, by widening and shorten- 
ing, to form a firmer pillar of support, with a compensating increase in 
the length of the intervertebral cartilages.f In the child, the vertebrae 
present more nearly their primitive elongate compressed form. With 
this is associated an increase of the forward lumbar curvature (Tur- 
ner) the primitive (i. e.f Simian) curve was backward; even in the 
negroes the collective measurement of the posterior faces of the live 
lumbars is greater than the anterior, in the proportion of 106 to 100; 
whereas in the white the collective anterior faces exceed the posterior 
in nearly the same proportion—100 to 96. 
The lower region of the back is also the seat of one of the most inter- 
esting and important of the changes in the body, namely, the correlated 
evolution of the inferior ribs, the lumbar vertebne, and the pelvis,—to 
which embryology, adult and comparative anatomy, and reversion all 
contribute their quota of proof. In most of the anthropoid apes, and 
therefore presumably in the pro-anthropos, there were thirteen com- 
plete ribs and four lumbar vertebrae, while man has twelve ribs and 
five lumbars. Thus we may consider the superior lumbar of adult 
man as a ribless dorsal; not so in the human embryo, however, for 
Rosenberg § has found a cartilaginous rudiment of the missing thir- 
teenth rib upon the so-called first lumbar. Atavism contributes an 
earlier chapter in the history of this region, for Birmingham || reports, 
out of fifty cases examined in one year, two in which there were six 
lumbars, and in each the thirteenth rib was well developed; this is an 
interesting example of “ correlated reversion,” for as the pelvis shifted 
downward to its ancestral position upon the twenty-sixth vertebra, 
the thirteenth rib was also restored. The other ribs are in what the 
ancients styled a “state of flux;” our eighth rib has been so recently 
floated from the sternum that, and according to Cunningham,it re- 
verts as a true rib in twenty cast's out of a hundred, showing a decided 
preference for the right side. Regarding also the occasional fusion of 
the fifth lumbar with the sacrum and the unstable condition of the 
twelfth rib, which is by variation rudimentary or absent, Rosenberg 
makes bold to predict that in the man of the future the pelvis will shift 
another step upward to the twenty-fourth vertebrae, and we shall then 
* Journal of Anatomy and Physiology, 
1886, p. 636. 
t Ibid., 1890, p. 117. 
t Ibid, 1887, p. 473. 
§ Morpli. Jahrb., 1876. 
|| Journal of Anatomy and Physiology, 
1891, p. 526. 
IT Ibid., 1890, p. 127, 320 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
lose our twelfth, rib. The upright position, and consequent transfer of 
the weight of the abdominal viscera to the pelvis, may be considered 
the habit associated with this reduction of the chest; at all events, in 
the evolution of quadrupeds there is a constant relation of increase be- 
tween the size of the posterior ribs and the weight of the viscera, until 
the rib-bearing vertebrae rise to twenty and the lumbars are reduced 
to three.* It would be interesting to note the condition of the ribs in 
some of the large-bellied tribes of Africans in reference to this point. 
The coccyx has naturally been tlie center of active search for the 
missing flexible caudals. As is well known, the adult coccyx contains 
but from three to five centers, while the embryo contains from five to 
six. Dr. Max Bartels has made “Die geschwanzten Menschen” the 
subject of an exhaustive memoir upon cases of the reversion of the 
tail, wliile Testut records all the primitive tail muscles in various 
stages of reversion. Watson reports that the curvatores coccygia 
(depressores eaudse) occur only in 1 in 1,000 cases. 
This suggests a moment’s digression to consider the different phases 
of reversion. The thirteenth rib recurs by what Gegenbaur calls “ neo- 
gen etic reversion,”! for it is simply the anomalous adult development of 
an embryonic rudiment. Under neogenetic reversions many authors 
also include cases of the “arrested development,” or persistence of an 
embryonic condition to adult life, such as the disunited odontoid proc- 
ess of the axis vertebra, which happens to repeat a very remote an- 
cestral condition. I think such cases may illustrate a reversional 
tendency, although many cases of arrested development, such as 
anencephaly, have no atavistic significance whatever.! More rare 
and far more difficult to explain are the “ paheogenetic reversions,” in 
which the anomaly, such as the supracondylar foramen, reverts to an 
atavus so remote that the rudiment is not even represented in the 
embryo. 
The features of skull development are primarily the increase of the 
cranium and the late closure of the cranial sutures, in contrast with 
the more complete and earlier closure of the facial sutures. 
So far as I can gather, this seems to be another region where the 
white and colored races present reversed conditions; the early closure 
and arrest of brain development in the negroes is well known; the 
later closure among the whites is undoubtedly an adaptation to brain 
growth. In his valuable statistics upon the Cambridge students, 
Galton says: “Although it is pretty well ascertained that in the 
masses of the population the brain ceases to grow after the age of 
nineteen, or even earlier, it is by no means the case witli university 
students. In high honor men head growth is precocious, their heads 
predominate over the average more at nineteen than at twenty-five.” 
• In the elephant and rhinoceros, 
t Morph. Jahrb., Bd. vi, p. 585. 
t Anencephaly, it should he said, is frequently associated with numerous rever- 
sions. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY 
321 
Many of the cases of arrested closure of facial sutures are rever- 
sional, as they correspond with the adult condition of other races, such 
as the divided malar, or as Japonicum. The human premaxillary, a 
discovery with which Goethe’s name will always be associated, is some- 
times partially, more rarely wholly, isolated; it is late to unite with the 
maxillary in the Australians, and has been reported entirely separate 
in a new Caledonian child (Deslongcliamps) and in two Greenlanders 
(Cams). The orbito-maxillary frontal suture, cited by Turner as a re- 
version to the pithecoid condition, is believed by Thomson, after the 
examination of 1,037 skulls, to be merely an accidental variation, with- 
out any deeper significance.* The development of the temporal bone 
from two centers, observed by Meckel, Gruber, and many others, is 
considered by Albrecht a reversion to the separate quadrate of the 
sauro-mammalia. This I think is in the highest degree improbable (see 
of Reversion”). The open cranial and closed facial sutures 
are apparently associated with our increasing brain action and decreas. 
ing jaw action; in one case the growth is prolonged and the sutures are 
left open, in the other, the growth is arrested and the sutures are 
closed. 
Is the lower jaw developing or degenerating? This question has re- 
cently been the subject of a spirited controversy between Mr. W. Platt 
Ball,t representing the Weisman school, and Mr. F. Howard Collins,f 
supporting Herbert Spencer’s view that a diminishing jaw is one of the 
features of our evolution which can only be explained by disuse. Mr. 
Collins find that, relatively to the skull, the mass of the recent English 
jaw is one-ninth less than that of the ancient British and, roughly 
speaking, half that of the Australian. He appears to establish the 
view that the jaw is diminishing. 
Closely connected with this is the evolution of the teeth; how are 
they tending? 
Flower § has shown, as regards the length of our molar series, that 
we, together with the ancient British and Egyptians, belong to a small- 
toothed or “microdont” race; the Chinese, Indians (North American), 
Malayans, and negroes in part, are intermediate or “ mesodout,” while 
the Andamanese, Melanasians, Australians, and Tasmanians are u ma- 
crodont.” While undersize marks the molars as a whole, the wisdom 
tooth is certainly in process of elimination; it has the symptoms of de- 
cline; it is very variable in size, form, and in the date of its appear- 
ance, is often misplaced, and is not uncommonly quite rudimentary 
(Tomes).j| Here is another instance where the knife-and-forkless races 
reverse our degeneracy, for in them not only is the last normal molar 
* Journal of Anatomy and Physiology, 1890. y>. 348. 
t Arc the Effects of Use and Disuse Inherited8 Nature Scries, 1890. 
+ The Lower Jaw in Civilized Races, 1891. 
0 Journal of the Anthropological Institute, 1880. 
I| brutal Anatomy, ]>. 416. 
n. Mis. 114- 21 322 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
(m. 3) large and cut long before the traditional years of discretion, but 
in the first two lower molars are found two intermediate cusps (Tonies)* 
which are variable or absent in us (Abbott); moreover, in the macro- 
dont races a surplus molar t (m. 4) is sometimes developed. Mummery 
reports nine such cases among 328 West Africans (Ashantis). As an 
instance of associated habit I may here mention that Dr. Lumlioltz, 
the Australian explorer, informs me that in adult natives the teeth are 
worn to the gum; in the absence of tools they are used in every occu 
pation, from eviscerating a snake to cutting a root. A tour of inspec- 
tion through any large collection of skulls brings out the contrast be- 
tween the sound and hard-worn molars of the savage, and the decayed 
and little-worn molars of the white. 
Upon the descent theory, the reduction of teeth in the progenitor of 
man began as far back as the Eocene period, for not later than that 
remote age do we find the full complement of three incisors and four 
premolars in each jaw; now there are but two remaining of each. 
Baume, a high authority, believes he has discovered eleven cases of a 
rudimental reversion of one of these lost premolars f not cutting the 
jaw. Not infrequently both these missing teeth occur by reversion. 
It is difficult to conceive of reversion to such a remote period, yet it 
is supported by other evidence. An embryonic third incisor has, I 
believe, been discovered. As long ago as 1863 Sedgwick § recorded a 
ease of six upper and lower incisors in both jaws, and appearing in 
both the milk and permanent dentitions; this anomaly was inherited 
from a grandparent, a striking instance of hereditary reversional ten- 
dency. We might consider that these cases of supernumerary teeth 
belong in the same category as polydactylism, or additional fingers, 
which are not atavistic, but for the fact that they do not exceed the 
typical ancestral number, whereas the fingers do. 
We owe to Windle || a careful review of the incisor reversions, in 
which he shows that the lost, incisors re-appear more frequently in the 
upper than the lower jaw (coinciding with the fact that the lower teeth 
were the first to disappear in the race); he considers that the lost 
tooth was the one originally next the canine, and concludes by adding 
our present upper outer incisor to the long list of degenerating organ 
He supports this statement by measurements and by citing cases in 
which it lias been found absent. Yet the reduction of the jaws is ap- 
parently outstripping that of the teeth, if we can judge from the 
* Dental Anatomy, p. 416, 
+ This tooth lias been found in several other macrodont. tribes (Australians, Tas- 
manians, Neo-Caledonians), Fontan. 
X Odontologisclic Forschungen, p. 268. This rudiment is found between the first and 
second normal premolars. 
§ British and Foreign Medico Chirurgical Review, 1863. 
|| Journal of Anatomy and Physiology, 1887, p. 85. 
5F Baumc believes that the missing incisor is the primitive median one, while Tur- 
ner believes it is the second. The fossil record supports Windle. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
323 
frequent practice among American dentists of relieving the crowded 
jawr by extraction. 
We now turn to the arches and limbs. Flower has pointed out that 
the base of the scapula is widening in the higher races, so that the 
u index,” or ratio of length to breadth, is quite distinctive. Gegenbaur 
associates this with the development of the scapulo humeral muscles 
and the greater play of the humerus as a prehensile organ. 
In general, the arm increases in interest as wTe descend toward the 
hand, both in the skeleton and musculature, because here we meet with 
the first glimpses of facts which enable us to form some estimate of the 
rate of human evolution. The well-known humeral torsion (connected 
with increased rotation) ascends from 152° in the polished stone age to 
134° in the modern European. The intercondylar foramen, or perfora- 
tion of the olecranon fossa, is exceptionally well recorded •* ft is found 
in 30 per cent of skeletons of the reindeer period; in the dolmen period 
it fell to 24 per cent; in Parisian cemeteries between the fourth and 
tenth centuries it is found in 5.5 per cent; it has now fallen to 3.5 per 
cent. The condylar foramen, occasionally forming a complete bridge 
of bone above the inner condyle and transmitting the median nerve 
and brachial artery, is known as the “ entepieondylar ” foramen in com- 
parative anatomy, and is one of the most ancient characters of the 
mammalia; it reverts paheogenetically in 1 per cent of recent skeletons, 
but much more frequently in inferior races (Lamb). In the wrist bone 
is sometimes developed another extremely old structure—the oscentrale. 
Grubert reported its recurrence at 0.25 per cent approximately. This 
is a case of neogenetic reversion, for Leboucqf shows that there is a 
distinct centrale in every human carpus in the first part of the second 
month, which normally fuses with the scaphoid by the middle of the 
third month. 
The divergence of the female from the male pelvis is an important 
feature of our progressive development; it is proved by the fact that, 
as we descend among the lower races it becomes increasingly difficult 
to distinguish the female skeleton from the male, for the pelves of the 
two sexes are nearly uniform. Here it seems to me is a most interest- 
ing problem for investigation. Arbuthnot Lane’s§ view's of the mechan- 
ical causes of this divergence, which are strongly Lamarckian, may be 
weighed with the theory of survival of the fittest, for the large female 
pelvis is perhaps the best example that can be adduced of a skeletal 
variation which would be preserved by natural selection, for reasons 
which are self-evident . The third trochanter of the femur is believed 
by Prof. Dwight,!! of the Harvard Medical School, to be a true re- 
* See Blanchard, op. cit., p. 450. 
t Virchow’s Archiv, 1885, p.353. 
t Ann. do la Soc. do MM. do Gand, 1884. 
§ Journal of Anatomy and Physiology, 1888, p. 214. 
|| Ibid., 1890, p. 61. 324 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
version (1 per cent) in our race and not an acquired variation, as it 
is very frequently found among the Sioux, 50 per cent, Laplanders, 04 
per cent, and Swedes, 37 per cent; like the condylar foramen it is an 
ancient mammalian character. 
The foot is full of interest in its association of degeneration and de- 
velopment with our present habits of walking; the great toe is increas- 
ing and the little toe diminishing, causing the oblique slope from within 
outward which is in wide contrast with the square toes in the infant or 
in the lower races. In many races the second toe is as long as the first, 
and the feet are carried parallel instead of the large toe turning out. 
If anyone will analyze his sensations in walking, even in his shoes, he 
will be conscious that the great toe is taking active part in progression 
while the little toe is passive and insensitive. We are not surprised, 
therefore, to learn from Pfitzner* that we are losing a phalanx, that in 
many human skeletons (41.5 per cent in women and 31. per cent 
in men) the two end joints of the little toe are fused. The fusion 
occurs not only in adults, but between birth and the seventh year, and 
in embryos of between the fifth and seventh month. The author does 
not attribute this to the mechanical pressure of tight shoes because it 
is found in the poorer classes. lie considers it the first act of a total 
degeneration of the fifth toe. 
Variations in the muscles.—The evolution of the muscles of the foot- 
looks in the same direction. As you know, the large toe in many of 
the apes is set at an angle to the foot and is used in climbing. It is 
still employed in a variety of occupations by different races. Accord- 
ing to Tremlett,t the celebrated great toe of the Annamese, which nor- 
mally projects at a wide angle from the' foot, is contemptuously men- 
tioned in Chinese annals of 2285 b. c., the race being then described 
as the “ cross-toes.” The long flexor of the hallux is apparently de- 
generating, showing a tendency to fuse with the flexor communis; the 
abductors and adductors of this toe are also degenerating, the latter 
being proportionately large in children (Huge). The little toe exhibits 
only by reversion its primitive share of the flexor brevis (Gegenbaur); 
more frequently it varies in the direction of its future decline by losing 
its flexor brevis tendon entirely. Two atavistic muscles, the abductor 
metatarsi quintif (always present in the apes), and the peroneus parvus 
(Bisclioff), also point to the former mobility of the outer side of the foot. 
In general the bones of the foot are developing on the inner and degen- 
erating on the outer side, with loss of the lateral movements of the 
hallux and of all independent movements in the little toe. The associ- 
ated habit-is that the main axis of pressure and strain now connects 
the heel and great toe, leaving the outer side of the foot comparatively 
functionless. 
* See Humboldt, 1890; also Nature, 1890, p. 301. 
1 Journal of the Anthropological Institute, 1880, p. 461. 
t Darwin: Descent of Man, p. 42. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
325 
The variations in the muscular system mark oft' more clearly the 
regions of contemporary evolution, and therefore are even more instruc- 
tive than those in the skeleton. Muscular anomalies have however 
never been adequately analyzed. Even the remarkable memoir of M* 
Testut, “ Sur les Anomalies musculaires,” is defective in not clearly 
distinguishing between variations which look to the future, those which 
revert to the past, and those which are fortuitous, for the author is 
strongly inclined to refer all anomalies to reversion. 
The law of muscular evolution is specialization by the successive 
separation of new independent contractile bands from the large funda- 
mental muscles, while the law of skeletal evolution is reduction of 
primitive parts and the specialization of articular surfaces. The num- 
ber of muscles in the primates as a whole has therefore been steadily 
increasing, while the number of bones has been dimimishing. In man 
the number of muscles is probably increasing in the regions of the 
lower arm, and diminishing in every other region. The analysis is 
rendered very difficult by the fact that some muscles (e. r/., those con- 
necting the shoulder with the neck and back) revert to a former condi- 
tion of greater specialization when they were employed in swinging the 
body by the arms, and in quadrupedal locomotion; while other muscles 
(e. <7., those connecting the forearm and fingers) revert to a former 
simpler arrangement when the hand was mainly a grasping organ, and 
the thumb was not opposable. 
As in the skeleton, we find that muscular anomalies include (1) 
paheogenetic reversions, or complete restorations of lost muscles; (2) 
neogenetic reversions, or revivals of former types in the relations of 
existing muscles; (3) progressive variations, which either by degener- 
ation or specialization point to future types; (4) fortuitous variations, 
which cannot be referred to either of the above. 
Duval observes that the ftexor longus policis repeats in reversion all 
.the stages of its evolution between man and the apes, in which it is a 
division of the flexor profundus. Gruber and others have even observed 
the absence of the thumb tendon. This is true of all the new muscles. 
Of this Testut writes: 
“ Ne dirait-on pas, en le voyant s’eloigner si souvent de sonetat nor- 
mal, que la nature voudraitle remener a sa disposition primitive, lut- 
tant ainsi sans cesse contre Padaptation, et ne lui abandonnant qu’ a 
regret Pune de ses plus belles conquetes.” 
Speaking of the hand, Baker says: 
“On comparing the human hand with that of the anthropoids, it may 
be seen that this efficiency is produced in two ways—first increasing 
the mobility and variety of action of the thumb and fingers ; second, 
reducing the muscles used mainly to assist prolonged grasp, they be- 
ing no longer necessary to an organ for delicate work requiring con- 
stant re-adjustment.”* 
*“Thc Ascent of Man, ” Proceedings Am. Ahhoc. A dr. Sci. 1890, vol. xxxix, ]>. 853. 
Also, Smithsonian Report, 1890, p. 449.  PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
You have noticed the recent discovery that the grasping power of 
infants is so great that the reflex contraction of the fingers upon a 
slender crossbar sustains their weight; this power and the decided 
inward rotation of the sole of the foot and mobility of the toes are per- 
sistent adaptations. Our grasping muscle, the palmaris longus, is 
highly variable and often absent; like the plantaris of the calf, it has 
been replaced by other muscles, and its insertion has been withdrawn 
from the metapodium to the palmar fascia. In negroes we frequently 
find the palmaris reverting to its former function of flexing the fingers 
by insertion in the metacarpals. 
The rise of muscular specialization by degeneration is beautifully 
shown in the extensor indicis, which, while normally supplying the 
index only, reverts by sending its former slips to the thumb, middle, 
and even to the ring finger. Testut* believes that the extension power 
of the middle and ring fingers has declined, as the cases of reversion 
point to greater mobility; the extensor minimi digiti is distinct and 
highly variable (Wood), often sending a slip to the ring finger. 
The entire flexor group of the hand, excepting the palmaris, is appa- 
rently specializing. The demonstration by Windlet and Bland Sutton, 
that the origin of the flexors and extensors is shifting downward from 
their original position, is evidence of an adaptation to the short special 
contractions required of them. 
The abductor pollicisf is also progressive and variable (Wood); the 
reduplication of its inferior tendon, which is sometimes provided with 
a distinct muscle, apparently points to the birth of a second abductor. 
The opponens of the thumb is well established and constant. Variabil- 
ity seems to characterize both the developing and degenerating mus- 
cles; the latter are apt to be absent; it is rare that an important muscle, 
such as the extensor indicis, is absent, but such cases are reported. 
It is interesting to note that the lost muscles of the body are almost 
exclusively in the trunk or shoulder, and pelvic arches, and not in the 
limbs. It will be remembered that the human shoulder joint is excep- 
tionally rigid, whereas in the quadrupedal state it was a factor in pro- 
gression. Some of the muscular reversions in this quadrupedal region 
are the levator clavieuhe (I to (10, Macalister), traelielo-elavicularis, 
scalenus intermedins, acromio-basilaris (Champneys), transversus 
nucha1 (Gegenbaur). Apparently associated with the former swinging 
of the body by the fore limb in the arboreal life are the atavistic coraco- 
braclualis-brevis (Testut), the epitrochleo-dorsalis (Testut), and pecto- 
ral is tertius (Testut).§ 
Centers of variability.—As the literature is so readily accessible I 
will not multiply illustrations of the innumerable congenital variations 
* Sur les Anomalies Mnsculaires, p. 564. 
t Journal of Anatomy and Physiology, 1890, p. 72. 
t Or extensor ossis metaearpi pollicis. See Testut, ]). 553. 
$ Quain describes seventy anomalous muscles (Ant.f vol. i.). Testut describes a 
still larger number. PRESENT PROBLEMS IN EVOLUTION AND HEREDITV. 
327 
related to human evolution. 1 call attention to several important in- 
ductions. First, there are several centers in which both the skeletal 
and muscular systems are highly variable. Second, that the most con- 
spicuous variations, and therefore the most frequently recorded, are 
reversions. Third, that structure lags far behind function in evolution. 
The conclusions of Wood, and of Testut,* are that variability is inde- 
pendent of age or sex, of general muscularity, and of abnormal mental 
development. Wood found 981 anomalies in 102 subjects; of these, G23 
were developed upon both sides of the body, while 358 were unilateral. 
Of still greater interest are the statistics collected by Wood between 
1807-’08 in the dissecting room of King’s College, upon 36 subjects 
(18 of each sex). These show that there are more anomalies in the 
limbs than in the trunk; that anomalies are rare in the pelvis; that 
there were 292 anomalies in the anterior limbs to 119 in the posterior; 
that in both limbs the anomalies increase toward the distal segments, 
culminating in the muscles of the thumb, where they rise to 90 per 
cent, (mainly flexor longus pollicis, and abductor longus pollicis). 
These facts seem to prove conclusively that while variation is universal 
it rises to a maximum in the centers where human evolution is most 
rapid; here are Herbert Spencer’s conditions of unstable equilibrium. 
This has a direct bearing, as 1 shall show, upon our theory of heredity. 
Fortuitous congenital variations.—I have thus far considered only 
those variations which apparently have a definite relation to the course 
of human evolution. There is an entirely different class of congenital 
variations which may be described as fortuitous or indefinite because 
they do not occur in any fixed percentage of cases; they are liable to 
take any direction; they can not be considered reversional because 
they are not found in the hypothetical atavus, and there is not suffi- 
cient evidence to cause us to consider them as incipient features of our 
future structure. 
Some may not be truly congenital (i. e. springing direct from the 
germ cells) but may be merely deviations from the normal course of 
development. I may instance the variations in the carpus recorded by 
Turner f in which the trapezium and scaphoid unite, or the trapezoid 
and semilunar divide, or the astragalus and navicular unite (Ander- 
son). 
The best example of fortuitous congenital variations are seen in su- 
pernumerary fingers and vertebra;. The eighth cervical vetebra, bear- 
ing a rudimentary rib,! >s not a reversion because the most remote an- 
cestors of man have but seven cervicals. In cases where a rib is de- 
veloped upon the seventh cervical, however, the reversion theory is 
perhaps applicable because rib-bearing cervicals are relatively less re- 
* Sur les Anomalies Mnsculaires, p. 760. 
t Journal of Anatomy and Physiology, 1884, p. 245. 
X Arl). Lane: Journal of Anatomy and Physiology, 1885, p. 266.  PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
mot*1. The same distinction applies to poly dactyl ism. How absurd 
it is to consider a sixth finger atavistic, when we remember that even 
our Permian ancestors had but five fingers. 
We can not however class, as purely fortuitous a variation which 
occurs in a definite percentage of cases presenting twenty four differ- 
ent varieties, but occurring in the same region. Such is the much-dis- 
cussed* musculus sternalis, a muscle extending vertically over the 
origin of the pectoral is from the region of the sterno-mastoid to that 
of the obliquus externus. Testut lightly applies his universal rever- 
sion theory, and ms this muscle is not found in any mammal considers 
it a regression to the reptilian presternal (Opliidia) ! Turner also con- 
sidered it as reversional in connection with the panniculus carnosus, 
the old twitching muscle of the skin, which plays so many freaks of 
reversion in the scalp and neck; this view is negatived by the fact that 
this muscle is innervated by the anterior thoracic (Cunningham, Shep- 
herd) which would connect it with the pectorial system, or by the inter- 
costal nerves (Bardeleben). Although the high percentage of recur- 
rence in the sternalis in anenceplialous monsters (90 percent according 
to Shepherd) supports the reversion view, it is offset by the high per- 
centage (4 per cent.) in normal subjects, for this is far too high for a 
structure of such age as the reptilian presternal. Cunningham has 
advanced another hypothesis, first suggested by the frequency of this 
anomaly in women, that this is a new inspiratory muscle, having its 
origin in reversion, but serving a useful purpose when it recurs, and 
therefore likely to be perpetuated. 
These fortuitous variations, as well as variations in the proportions 
of organs, play an important part in the present discussion upon hered- 
ity, for it is believed by the Weismann school that such variations, if 
they chance to be useful, will be accumulated by selection and thus 
become race characters. 
The limits of reversion.—There is such a wide difference of opinion 
upon the subject of reversions that it is important to determine what 
are some of the tests of genuine reversions. How shall we distinguish 
them from indefinite variations or from anomalies like the sternalis 
muscle, which strain the reversion theory to the breaking point? 
Testut, t Duval,and Blanchard take the extreme position that almost 
all anomalies reproduce earlier normal structures, and that the excep- 
tions may be attributed to the incompleteness of our knowledge of 
comparative anatomy. I may here observe that popular as the descent 
theory has recently become in France, neither these anthropologists 
nor the palaeontologists show a very clear conception of the phyletic 
or branching element in evolution. If they do not find a muscle in the 
primates they look for it in other orders of mammals. Now, since 
these other branches diverged from that which gave rise to man at a 
* See Turner, Shepherd, and Cunningham: Journal of Anatomy and Physiology. 
t Sur les Anomalies Musculaires, p. 4. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
329 
most remote period, 1 lie discovery of a similar muscle may be merely a 
coincidence; it is by no means a proof of reversion. 
The first test of reversion is therefore the anatomy of the atavus 
and this is derived partly from the palaeontological record of the 
primates, partly from the law of divergence, viz., that features which 
are common to all the living primates were probably also found in the 
stem form which gave rise to man; finally, from the comparative anat- 
omy of the living anthropoidea. 
The second test is whether a structure passes the limits of reversion 
as determined by cases of atavism in which there can be no reasonable 
doubt. Two of these phenomena have recently been discussed, which 
seem to extend the possibilities of reversion back to structures which 
were lost at a very remote period. I refer to papers by Williams and 
Howes. Williams* has analyzed 166 recorded cases of polymastism; 
he finds that supernumerary nipples of some form occur in two per 
cent, and that in all except four of the cases examined the anomalies, 
tested by position, etc., support the reversion hypothesis. In the 
living lemurs, which form a persistent primitive group of monkeys, we 
find that the transition from polymastism to bimastism is now in 
progress by the degeneration of the abdominal and inguinal nipples; it 
is fair to assume that the higher monkeys also lost their abdominal 
nipples at a primitive stage of development, and therefore that cases 
of multiple nipples indicate reversion to a Lower Eocene condition! 
Howes t has recently completed a most interesting study of the u in- 
tranarial epiglottis,” or cases in which the epiglottis is carried up into 
the posterior nares, as in young marsupials and some cetacea, to sub- 
serve direct narial respiration. This has now been observed to occur 
by reversion in all orders of mammals, including the monkeys and 
lemurs. One case has also been reported by Sutton of its occurrence 
in a human fetus. This is apparently a human reversion to a structure 
much older than the age of the lemurs. 
The third test is the inverse ratio to time. It would seem, a priori, 
that the percentage of recurrence of atavistic structures should decrease 
as the extent of time elapsing since the structure disappeared increases. 
This law’ is apparently established in the case of the condylar and 
intercondylar foramina, and if we examine all the percentages which 
have been established we see at once that they bear a ratio to time; 
compare the relative frequency of the ischio-pubic (50 per cent), dorso- 
epitrochlearis (5 per cent), and levator-clavicnhe (1.66 per cent) mus- 
cles with the periods which have elapsed since their past service. This 
is why it is so important to establish percentages for all our atavistic 
organs; fuller statistics will not only bear upon heredity, but I can 
conceive of their application to the extremely difficult problem of 
estimating geological time. We must, of course, establish as a stand- 
* Journal of Anatomy and Physiology, 1891, ]>. 224. 
t Ibid, 1889, p. 587.  PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
art! cases of congenital variation in 'which the frequency of recurrence 
has been steadily declining in the same race between two known periods 
of time—an available structure is the intercondylar foramen or supra- 
trochlear foramen, as recorded by Blanchard, Shepherd, and others. 
The reversional tendency is hereditary. There are many cases, both 
of reversions (as in the teeth) and indefinite variations being hereditary, 
that is, re-appearing in several generations, or skipping a generation 
and recurring in the second. 
Summary.—There are clearly marked out several regions in the human 
body in which evolution is relatively most rapid, such as ti.e lower 
portion of the chest, the upper cervicals, the shoulder girdle in its rela- 
tion to the trunk, the lower portion of the arm and hand, the outer 
portion of the foot. We notice that these regions especially are centers 
of adaptation to new habits of life in which new organs and new rela- 
tions of parts are being acquired and old organs abandoned. 
We observe also that all parts of the body are not equally variable, 
but these centers of evolution are also the chief centers of variability. 
The variations here are not exclusively, but mainly, of one kind; they 
rise from the constant struggle between adaptation and the force of 
heredity. Here is a muscle like the extensor indicis attempting to give 
up an old function and establish anew one; it maintains its new func- 
tion for several generations, and then goes back without any warning 
to a function which it had thousands of years ago. Thus the force of 
reversion strikes us as a universal factor. 
Now the singular fact about reversion is the frequent proof it affords 
of what Gal ton has called “particulate inheritance.” When the ex- 
tensor indicis reverts, all the muscles around it may be normal; there- 
fore we are obliged to consider each of these muscles as a structure by 
itself, with its own particular history and its own tendencies to develop 
or degenerate. Thus it is misleading to base our theory of evolution 
and heredity solely upon entire organs; in the hand and foot we have 
numerous cases of muscles in close contiguity, one steadily developing, 
the other steadily degenerating. Reversion very rarely acts upon many 
structures at once; when it does, we have a case of diffused anomaly, 
some repetition in the epidermis, or in the entire organism of a 
lower type. 
Yet in spite of reversion and the strong force of repetition in inherit- 
ance, the human race is steadily evolving into a new type. We must, 
it seems to me, admit that an active principle is constantly operating 
upon these particular structures, guiding them into new lines of adapta- 
tion, acting upon widely separate minor parts, or causing two parts, 
side by side, to evolve in opposite directions, one toward degeneration, 
the other toward development. 
I may now recall the two opposed theories as to what this active 
principle is: 
The first, and oldest, is that individual adaptation, or the tendencies PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
General regions of evolution. 
Developing organs. 
Degenerating organs. 
Reduction. 
Variable. 
Rudiments 
(occasionally absent). 
Reversions 
(from embryo). 
Reversions 
(from germ cells). 
Skull and jaw. 
Cranium. 
Facial sutures. 
Premaxillary. 
Spinal curvatures. 
Infer, maxilla. 
Hyoids. 
Lumbars and pelvis. 
Coracoid. 
Cervicals lumbars, and coccyx. 
Female pelvis. 
4th and 5tli caudals. 
5tli and 6th caudals. 
Caudals-)-. 
Lower ribs. 
8th rib. 
12th rib. 
13th rib. 
7th cerv. ribs. 
Scapula and lower humerus. 
Scapula. 
Condylar for. 
Clavicle. 
S. trochlear, for. 
Outer side of pcs. 
Hallux. 
4th and 5th digits 
Terminal phalanx of 5th 
3d trochanter. 
Tibia. 
of pes. 
dig. of pes. 
Centrale-manus. 
Intermedium, pes. 
Incisors, lateral sup. 
3d molar. 
3d molar. 
? Third premolar. 
4th premolar. 
Flex. prof, and perf. 
Extensor indicis. 
Panniculus carnosus 
X 
Epitroclil. dors. 
Flex. long, pollicis. 
Acromio-basilar. 
Abd. long, pollicis. 
Pyramidalis. 
Levator clavicula-. 
Pect. tertius. 
Cor. bracli. brev. 
Ischio-pubie. 
Shoulder to trunk. 
Triceps extensor 
Psoas parvus. 
Depressores caudm. 
surae. 
SeaDsorius. 
Trunk to femur. 
Glutteal group. 
Abd. and add. hal- 
Muscles, 5th toe. 
Plantaris. 
Abd. metars. 5th. 
Facial group. 
lucis. 
Palinaris. 
Peroneus parvus. 
* It is probable that some of these muscles are represented in the foetus. 
Partial table of characters in evolution. 332 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
established by use and disuse upon particular structures in the parent 
are in some degree transmitted to the offspring and thus guide tin* 
mam course of variation and adaptation. 
The second is that all parts of the body are variable, and that wher- 
ever variations take a direction favorable (that is, adaptive) to the sur- 
vival of the parent they tend to be preserved; where they take the op- 
posite direction they tend to be eliminated. Thus, in the long run, 
adaptive variations are accumulated and a new type is evolved. 
It is evident at once, from a glance over the facts brought forward 
in this lecture, that the first theory is the simplest explanation of these 
facts; that use and disuse characterizes all the centers of evolution; that 
changes of structure are slowly following our changesof function or habit. 
But while the first explanation is the simplest it by no means fol- 
lows that it is the true one. In fact, it lands us in many difficulties, 
so that I shall reserve the pros and cons for my second lecture upon 
heredity. The Lamarckian theory is a suspiciously simple explana- 
tion of such complex processes. 
LECTURE II.—THE DIFFICULTIES IN THE HEREDITARY THEORY. 
Nur muss icli noclimals betonen, dass nacli meiner Auffassung tier Anfang einer 
neuen Reiho erblicher Abweicliungen, also aucli der Eintritt einer nenen Art oline 
eine vorausgegangene erworbene Abweichung undenkbar ist.—Virchow. 
State of opinion.—The above quotation from one of the most eminent 
authorities of our times represents the unshaken conviction of a very 
large class upon one side of the question of transmission of acquired 
characters, which is met by equally firm conviction upon the other side. 
Herbert Spencer, whose entire system of biology, psychology, and 
ethics is based upon such transmission, says: “I will only add that, 
considering the width and depth of the effects which acceptance of one 
or other of these hypotheses must have on our views of life, mind, morals, 
and politics, the question which of them is true demands, beyond all 
other questions whatever, the attention of scientific men.71 * This shows 
that Spencer considers the matter still sub judice, and lest you may think 
1 am bringing before you an issue in which learning and experience 
are ranged against ignorance and prejudice, I have taken some pains 
by correspondence with a number of friends abroad to learn the pres- 
ent state of opinion. The two leading English and French authorities 
upon this subject express themselves doubtfully. 
Gralton’s mind is still wavering, as in his work of 1880 he says: 
“1 am unprepared to say more than a few words on the obscure, 
unsettled, and much-discussed subject of the possibility of transmitting 
acquired faculties. - - - There is very little direct evidence of its 
influence in the course of a single generation, if the phrase of ‘acquired 
faculties7 is used in perfect strictness and all inheritance is excluded 
that could be referred to some form of natural selection, or of infection 
before birth, or of peculiarities of nurture and rearing.77! 
*Nineteenth Century, 1889. 
t Natural Inheritance, 1889, p. 14. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
333 
Kibot, although in the center of the French Lamarckians, says: “Not- 
withstanding these facts the transmission of acquired modifications 
appears to be very limited, even when occuring in both of the parents.” 
Excepting from Kblliker; His, the Leipsic anatomist; Pfiiiger, the 
physiologist; Ziegler, in pathology; and He Vries, in botany, Weis- 
mann has not found much sympathy from his own countrymen in his 
opinion “that acquired characters cannot be transmitted; - - - 
that there are no proofs of such transmission, that its occurrence is 
theoretically improbable, and that we must attempt to explain the 
transformation of species without its aid.”* Besides Virchow t and 
Eimer, t Haeckel has expressed himself strongly against Weismaun. 
My colleague, Prof. Wilson, writes me (Munich, December 31,1891) 
that, while Weisinann’s modified theories as to the phenomena in the 
reproductive cells are pretty generally accepted, Hertwig, Hofer, Paully, 
Boveri, and others are pronounced advocates of the acquired-charac- 
ter- transmi ssion theory. 
In Pans Brown-Sequard, who was among the first to test this prob- 
lem experimentally by observing the inheritance of the effects of nerve, 
lesions, his assistant, Dupuy, Giard, Duval, Blanchard, and others are 
on the affirmative, or Lamarckian side. 
Physiologists generally have fought shy of the question, although I 
think in the end they wiU be forced to take it up with the morpholo- 
gists, and give us the physio-morphological theory of heredity of the 
future. Prof. Michael Foster, of Cambridge, and Prof. Burdon-San- 
derson, of Oxford, both write me that the question has hardly come 
into the physiological stage of inquiry at all. Yet in England Weis- 
mann has found his strongest supporters among some of the naturalists: 
Wallace, Laukester, Thiselton Dyer, Meldola, Poulton, Howes, and 
others; while, excepting Windle, the anatomists, including Mivart and 
Lawson Tait, with Sir William Turner as the most prominent, are all 
Lamarckians. Huxley, Romanes, and Flower are said to be doubtful. 
In this country the opinion of naturalists is directly the outgrowth of 
the class of studies in which each happens to be engaged. So far as I 
know every vertebrate and invertebrate paleontologist is a Lamarck- 
for in this field all evolution seems to follow the lines of inherited 
use and disuse; most of those engaged upon invertebrate zoology in 
cline to follow Weismaun. 1 have conversed upon this subject with 
many physicians, and find that without exception the transmission of 
acquired characters is an accepted fact among the profession. 
Exact statement of the problem.—It is important at the outset to 
* Biolog isches Cmtralblatt, 1888, pp. Go and 97. 
tUebcr den Transforinismus, Archivf. Anthropologic, 1889, p. 1. 
i Organic Evolution, upon the Law of Inheritance of Acquired Characters. Tubin- 
gen, 1888. Trans. 
§ See the writings of Hyatt, Cope, Ryder, Dali, Scott, and others. 334 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
state most clearly wliat is and wliat is not involved in this discussion. 
Weisinann* does not claim that the reproduction or germ cells are un- 
influenced by habit; on the other hand, he admits that most important 
modifications in these cells may and do result from changes of food, 
climate, from healthy or unhealthy conditions of the body ; also from 
infectious disease, where it is quite as possible that the microbes may 
enter the reproductive cells as any other cells of the body; from alco- 
holism, where the normal molecular action of the protoplasm of the 
germ cells may be disturbed, resulting in abnormal development, and 
there are some very interesting experiments which I shall cite on this 
point; from some nervous disorders which profoundly modify cell- 
function in all the tissues; in other words, ovum sanum in corpore mno. 
But to accept all this, and even to include all our rapidly increasing 
knowledge of the direct relation between such phenomena as produc- 
tion of deformities and determination of sex, and the influences of 
environment upon the ovum; or the influences of the mother upon the 
foetus—this is all aside from the real question at issue. 
It may be stated thus: Given G, the ova and spermatozoa, the germ 
cells or material vehicles of hereditary characters; $, the body or so- 
matic cells of all the other tissues conveying the hereditary characters 
of nerve, muscle, and bone; Tr, the variations in these body cells “ac- 
quired” during lifetime; given these factors,.the real question is: Do 
influences at work producing variations in certain body cells of the 
parent so affect the germ cells of the parent that they re appear in cor- 
responding body cells of the offspring? To take a concrete ease, will 
the increased use of the cells of the extensor indicis muscle in the 
parent so stimulate that portion of the germ cells which represents this 
muscle that the increment of growth will in any degree re appear in 
the offspring? 
This is what is required of heredity upon the Lamarckian hypothe- 
sis, and 1 think you will see at once that while this hypothesis sim- 
plifies the problem of evolution it in a corresponding degree renders 
more difficult the problem of heredity—for we have not the first ray of 
knowledge of what such a process involves. There is no quality more 
essential to scientific progress than common honesty; if we take a 
position let us face all its consequences; the more we reflect upon it 
the more serious the Lamarckian position becomes. 
In the present lecture let us first briefly review the progress of the 
science of heredity which has led up to the present discussion. Second, 
let us examine the evidence for and against the Lamarckian theory, 
and inquire how far natural selection can explain all the facts of evolu- 
tion. Third, let us examine the evidence for such a continuous relation 
between the body cells and germ cells as must exist if the Lamarckian 
theory is the true one. 
History of the heredity theory.—In a valuable summary of the past 
* See Essays upon Heredity and Kindred Biological Problems, 1889. Trans. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
335 
theories of heredity * J. A. Thompson distinguishes three general 
problems, which are often confused: 1st. What characters distinguish 
the germ cells from other cells of the body? 2d. How do the germ 
cells derive these distinguishing characters? 3d. How shall we inter- 
pret “particulate” inheritance, or the reappearance of single peculiari- 
ties in the offspring? 
The various theories may be grouped under two heads, “ Pangenesis 
of Germ cells” and “Continuity of Germ cells” according to the 
dominating idea in each. 
1. Pangenesis.—The idea pervading pan genesis was first expressed 
by Democritus that the “ seed ” of animals was derived by contributions 
of material particles from all parts of the bodies of both sexes, and that 
like parts produced like. Two thousand years later, Button revived 
this conception of heredity in his “ Molecules organiques.” In 1864 
Herbert Spencer suggested the existence of “physiological units,” 
derived from the body cells of the parent, forming the germ cells and 
then developing into the body cells of the offspring. 
It is interesting to note the course of Darwin’s thought upon this 
matter in his published works and in his “Life and Letters.” He was 
at first strongly opposed to the views upon evolution advanced by 
Button, by Erasmus Darwin, his grandfather, expanded by Lamarck, 
and now known as Lamarckian. But gradually becoming convinced 
that his own theory of natural selection could not account for all the 
facts of evolution, he unconsciously became a strong advocate of 
Lamarck’s theory, and contributed to it a feature which Lamarck had 
entirely omitted, namely, a theory of heredity expressly designed to 
explain the transmission of acquired characters. Darwin’s “provi- 
sional hypothesis of pangenesis ” f postulated a material connection 
between the body cells and germ cells by the circulation of minute buds 
from each cell; each body cell throws off a “gemmule” containing its 
characteristics, these gemmules multiply and become especially concen- 
trated in the germ cells ; in the latter they unite with others like them- 
selves; in course of development they grow into cells like those from 
which they were originally given off. (See Fig 1, Diagram n.) 
Galton, who has always been doubtful in regard to use inheritance, 
while advancing a theory of “continuity,” partly approved Darwin’s 
pangenesis idea in the cautious statement: “Each cell may throw off a 
few germs that find their way into the circulation and thereby have a 
chance of entering the germ cells.” t At the same time Galton contrib- 
uted very important experimental disproof of the existence of “gem- 
mules,” and in fact—of the popular idea of the circulation of hereditary 
characters in the blood, by a series of careful experiments upon the 
*See Proe, Roy. Soc. Edin., 1888, p, 93. 
t See Animals and Plants under Domestication,1875, vol. ii, p. 349. 
t Contemporary Review, vol. xxvii, p. 80-95. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
transfusion of blood in rabbits; he found that the blood did not con- 
vey with it even the slightest tendency to transfer normal character- 
istics from one variety to another. 
Firs? 
generation; 
Second 
generation 
Third 
generation. 
Maternal 
Paternal. 
Maternal 
Paternal.-. 
Maternal 
Paternal..,. 
Flu. 1. 
/. o.: fertilized ovum or embryo, containing maternal and paternal characteristics; 6', soma, or adult 
body, containing n, s, m, d, v, somatic cells of"the various tissues; and G, germ cells of the reproductive 
glands. 
I. Histogenesis.—Showing the successive rise G, and union /. o. of the maternal and paternal 
germ cells l>y direct histogenesis. 
II. Pangenesis.—Showing the tissues of the body contributing to the germ cells G, so that 
each/, o. is composed of elements from both the somatic and germ cells. 
III. Continuity.—Showing the division of the embryo'/; o., into somatoplasm, s (from which’ arise 
the body cells), and germ plasm, G (which passes direct to the germ cells), establishing a direct con- 
tinuity. 
Prof. Brooks, of the Johns Hopkins University, then contribute and 
original modification of pangenesis in which the functions of the ova and PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
337 
spermatozoa were sharply differentiated.* (1) He regarded the ovum 
as a cell especially designed as a storehouse of hereditary characteris- 
tics, each characteristic being represented by material particles of some 
kind; thus hereditary characters were handed down by simple cell divi- 
sion, each fertilized ovum giving rise to the body cells in which its hered- 
itary characters were manifested and to new ova in which these charac- 
ters were conserved for the next generation (this portion of Brooks’s 
theory is very similar to G-alton’s and Weismann’s). (2) The body 
cells have the power of throwing off “gemmules,” but this is exercised 
mainly or exclusively when its normal functions are disturbed, as in 
metatrophic exercise or under change of environment. (3) These gem- 
mules may enter the ovum, but the spermatozoon is their main center. 
According to this view the female cell is rather conservative and the 
male cell progressive; the union of these cells produces variability in 
the offspring, exhibited especially in the regions of the offspring cor- 
responding to the regions of functional disturbance in the parent. 
This hypothesis was well considered, and while that feature of it which 
distinguishes the male and female germ cells as different in kind has 
been disproved, and the whole conception of gem mules is now abandoned, 
the fact still remains that we shall nevertheless be obliged to offer some 
hypothesis to explain the facts disregarded by Weismann for which 
Brooks provides in his theory of the causes of variation. 
2. Continuity of germ cells.—The central idea here is an outgrowth 
of our more modern knowledge of embryogenesis and histogenesis, and 
is therefore comparatively recent; it is that of a fundamental dis- 
tinction between the “germ cells,*’ as continuous and belonging to the 
race, and the “body cells,” as belonging to the individual. Weismann 
has reftned and elaborated this idea, but it was not original with him. 
Richard Owen,t in 1849, Haeckel,f in 1860, liauber,§ in 1879, in turn 
dwelt upon the distinction which Dr. Jaeger, now of manufacturing 
fame, first clearly stated: 
“Through a great series of generations the germinal protoplasm re- 
tains its specific properties, dividing in every reproduction into an 
ontogenetic portion, out of which the individual is built up, and a 
phylogenetic portion, which is reserved to form the reproductive 
material of the mature offspring. This reservation of the phylogenetic 
material 1 described as the continuity of the germ protoplasm. - - - 
Encapsuled in the ontogenetic material the phylogenetic protoplasm 
is sheltered from external influences, and retains its specific and em- 
bryonic characters.” The latter idea has, under Weismann, been 
expanded into the theory of isolation of the geim cells. 
Galton introduced the term “stirp” to express the sum total of 
* The Laic of Heredity, 1883. 
t Se'j Parthenogenesis, in his Anatomy of Vertebraten 
\ Generelte Morphologic, vol. 11, p. 170. 
§Zool. Am., vol. ix, p. 166. 
H. Mis. 114 22 338 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
hereditary organic units contained in the fertilized ovum. His con- 
ception of heredity was derived from the study of man, and he sup- 
ported the idea of continuity in the germ cells in order to account for 
the law of transmission of “latent” characters; it is evident from this 
law that only a part of the organic units of the “stirp” become “patent” 
in the individual body; some are retained latent in the germ cells, and 
become patent only in the next or some succeeding generation. For 
example, the genius for natural science was “patent” in Erasmus Dar- 
win, grandfather of the great naturalist, it was “latent” in his son, 
and re-appeared intensified in his grandson, Charles Darwin. I have 
elsewhere* summed up as follows Gabon’s general results, which so 
remarkably strengthen the “continuity” idea: We are made up, bit 
by bit, of inherited structures, like a new building, composed of 
fragments of an old one, one element from this progenitor, another 
from that, although such elements are usually transmitted in groups. 
The hereditary congenital constitution thus made up is far stronger 
than the influences of environment and habit upon it. A large 
portion of our heritage is unused, for we transmit peculiarities we 
ourselves do not exhibit. The contributions from each ancestor can 
be estimated in numerical proportions, which have been exactly deter- 
mined from statistics of stature in the English race; thus the cpn- 
tributions from the “patent ” stature of the two parents together con- 
stitute one-half while the contributions by “latent” heritage from the 
grandparents constitute one-sixteenth, etc. One of the most important 
demonstrations by Galton is the law of regression; this is the factor 
of stability in race type which acts as gravitation does upon the pen- 
dulum; if an individual or a family swing far from the average 
characteristics of their race, and display exceptional physical or 
mental qualities, the principle or regression in heredity tends to draw 
their offspring back to the average. 
Now how shall we distinguish regression from reversion1? Very 
clearly, I think; regression is the short pull which tends to draw every 
variation and the individual as a whole back to the contemporary typ- 
ical form, while reversion is the long pull which draws the typical form 
of one generation back to the typical form of a very much earlier gen- 
eration. These forces are evidently akin, and in the shades of trans- 
ition from one type to another we would undoubtedly find a constant 
diminution numerically in the recurrence of characters of the older 
type, and thus “regression” would pass insensibly into “reversion.” 
Weismann has carried the idea of continuity to its extreme in his 
simple and beautiful theory of heredity, which is founded upon the 
postulate that there is a distinct form of protoplasm, with definite 
chemical and molecular properties, set apart as the vehicle of inherit- 
ance; this is the germ plasm, G, quite separate from the protoplasm 
of the body cells or somatoplasm, 8. Congenital characters arising in 
* Atlantic Monthly, March, 1891, p. 359. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
339 
the germ-cells are called blastogenetic, while acquired characters aris- 
ing in the body cells are somatogenetic. 
To clearly understand this view, let us follow the history of the ter* 
tilized ovum in the formation of the embryo. It first divides into soma- 
to-plasm and germ-plasm (see Fig. 1, Diagram in), the former supplies 
all the tissues of the body—n, s, m, d, v, nervous, muscular, vascular, 
digestive, etc.—with their quota of hereditary structure; the residual 
germ-plasm is kept distinct throughout the early process of embryonic 
cell division until it enters into the formation of the nuclei of the re- 
productive cells, the ova or spermatozoa. Here it is isolated from 
changes of function in the somato plasm, and in common with all other 
protoplasm is capable of unlimited growth by cell division without loss 
or deterioration of its past store of hereditary properties; these prop- 
erties are lodged in the nucleus of each ovum and spermatozoan, and 
these two cells, although widely different in external accessory struc- 
ture (because they have to play an active and passive part in the act 
of conjugation), are exactly the same in their essential molecular 
structure, and the ancestral characters they convey differ only because 
they come along two different lines of descent. When these cells unite 
they carry the germ-plasm into the body of another individual. Thus 
the somato plasm of each individual dies, while the germ-plasm is im- 
mortal; it simply shifts its abode from one generation to another; it 
constitutes the chain from which the individuals are mere offshoots. 
Thus the germ-plasm of man is continuous with that of all ancestors 
in his line of descent, and we have an explanation of the early stages 
observed in development in which the human embryo passes through 
a succession of metamorphoses resembling the adult forms of lower 
types. 
In order to emphasize, as it were, the passage of the germ-plasm 
from one generation to another without deterioration in its marvellous 
hereditary powers, Weismann added the idea of its isolation. Not only 
does he repudiate the pangeuesis notion of increment of germ-plasm 
by addition of gemmules, but he believes that it is unaffected by any 
of the normal changes in the somatic or body cells. As this continuity 
and isolation would render impossible the transmission of characters 
acquired by the somato plasm, Weismann began to examine the evi- 
dence for such transmission, and coming to the conclusion that it was 
insufficient, in his notable essay on u Heredity,” in 1883, he boldly at- 
tacked the whole Lamarckian theory and has continued to do so in all 
his subsequent essays. 
Being forced to explain evolution without this factor, he claimed 
that variation in the germ plasm was constanly arising by the union 
of plasmata from different lines of descent in fertilization, and that 
these variations are constantly being acted upon by natural selection 
to produce new types. He thus revived Darwin’s earlier views of 
evolution, and this in part explains his strong support by English nat- 
uralists. 340 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
It will be seen at once that there are a number of distinct questions 
involved. 
The matter of first importance in life is the repetition and preservation 
of type, the principle which insures the unerring accuracy and precision 
with which complex organs are built up from the germ cells; the force 
of regression and the more remote forces of reversion all work in this 
conservative direction; the theory of the preservation of these forces in 
a specific and continuous form of protoplasm is by far the most plausi- 
ble we can offer at present. 
The matter of second importance, but equally vital to the preserva- 
tion of races, in the long run, is the formation of new types adapted to 
new circumstances of life. I shall now attempt to show that the facts 
of evolution, while not inconsistent with the idea of continuity of the 
germ plasm, are wholly at variance with the idea of its independence, 
separation, or isolation from the functions of the body. This can be 
done by proving, first, that the theory of evolution solely by natural 
selection of chance favorable variations in the germ plasm is inade- 
quate; second, that the inheritance of definite changes in the somatic 
cells is also necessary to explain evolution, and therefore there must 
exist some form of force or matter which connects the activities of the 
somatoplasm with those of the germ plasm. 
In the following table are placed some of the facts of human evolu- 
tion which we have observed in the first lecture, and as they are part 
of inheritance, they also constitute the main external phenomena of 
heredity: 
Phenomena of heredity. 
Conservative (toward past type). 
Natural. 
Progressive (toward future type). 
a. Repetition of parental type. 
b. Regression (in many characters) to 
contemporary race type. 
c. Reversion (mainly in single charac- 
ters) to past race type. 
Fortuitous and in- ] 
definite. 
Variability. 
a. Definite variation in single charac- 
ters, by accumulation ==. 
b. Definite variation in many charac- 
ters (from contemporary race 
type). 
What are causes of these various phenomena? 
Factors of evolution.—The term u kinetogenesis ” has been applied 
to the modern form of the Lamarckian theory, for it is an application of 
kinetic or mechanical principles to the origin of all structures such as 
teeth,bone, and muscle. It would be fatal to this theory if it could be 
shown that the changes taking place in course of a normal individual 
life, under the laws of use and disuse, are iuadaptive, or do not corre- 
spond to those observed in the evolution of the race. 
The relative growth of Organs.—Ball,* in his long argument 
against Lamarckianism, claims that such is the case, and that use 
* Op. cit., p. 129. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
341 
inheritance would be an actual ev il: “Bones would often be modified 
disastrously. Thus the condyle of the human jaw would become 
larger than the body of the jaw, because as the fulcrum of the lever 
it receives more pressure. Some organs (like the heart, which is 
always at work) would become inconveniently or unnecessarily large. 
Other absolutely indispensable organs which are comparatively pas- 
sive or are very seldom used would dwindle until their weakness 
caused the ruin of the individual or the extinction of the species.” 
He later cites from Darwin* the “Report of the United States Com- 
mission upon the Soldiers and Sailors of the Late War,” that the 
longer legs and shorter arms of the sailors are the reverse of what 
should result from the decreased use of the legs in walking and 
increased use of the arms in pulling. A little reflection on Mr. Rail’s 
part would have spared us this crude objection, for whatever difficul- 
ties may arise from theoretical speculation as to the laws of growth, 
or from statistics, the fact remains that activity must increase adapta- 
tion in every part of the organism; otherwise the runner and the 
trotting horse should be kept off the track to increase their speed, 
the pianist should employ as little finger exercise as possible. If the 
growth tendencies in single organs are transmitted, it is evident that 
the adaptive adjustments between these tendencies will also be trans- 
mitted. 
The Feet.—In point of mechanical adaptation, man, with the single 
exception of his thumb and forearm, has not progressed beyond the 
most primitive eocene quadruped. The laws of evolution of the foot 
in the ungulate or hoofed animals, which have been especially studied 
by Kowalevsky, Ryder, Cope, and myself, affords a conclusive dem- 
onstration that the skeletal changes in the individual coincide with 
those which will mark the evolution of the race. In the earliest un- 
gulates the carpals and tarsals are disposed, as in man, directly above 
each other, with serial joints, as in diagram A, Fig. 2; in the course 
of evolution all these joints became interlocking, as in diagram B, 
Fig. 3; thus producing an alternation of joints and surfaces similar to 
those which give strength to masonry. In studying these facts Copet 
reached a certain theory as to the motion of the foot and leg in loco- 
motion. In trying to apply this, I found it could not be harmonized 
with all the facts, and I worked out an entirely different theory4 This 
I found subsequently coincided exactly with the results previously 
obtained by Muybridge, by the aid of instantaneous photographs, and 
summarized by Prof. Harrison Allen, of the University of Pennsyl- 
vania^ 
Tlie monodactylism of the horse was attained by the atrophy of the 
* Descent of Man, p. 32. 
t American Naturalist, 1887, p. 986. 
t See Trans, of American Philosophical Society, 1889, p. 561. Philadelphia. 
$The Muybridge Work at the University of Pennsylvania. Philadelphia, 1888. 342 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
lateral toes, and concentration of the major axis of body weight and 
strain upon the middle linger and toe. Man is also tending toward 
monodactylism in the foot by the establishment of the major axis 
through the large toe and atrophy of the outer toes. The present atro- 
phy of our small toe is as good a parallel as we can find of the changes 
which were occurring in the eocene period among the ancestors of the 
horse. 
The Teeth.—But how about the teeth, in which there is an absolute 
loss of tissue in consequence of use? This is another objection raised 
by Ball, Poulton, and others, which disappears upon examination. 
The dental tissues, while the hardest in the body, and, unlike bone, 
incapable of self-repair, are not only both living and sensitive, but, to 
Fig. 2. 
Fig. 3. 
Primitive ungulate foot.—Lines of 
vertical cleavage on either side of the 
middle toe, III. Spreading of toes would 
cause separation of the carpals. 
Recent ungulate foot.—No lines 
of vertical cleavage. All joints broken 
by enlargement of scaphoid, unciform, 
and radius, the hones resolving greatest 
impact in walking. Lateral toes, I, V, 
degenerate. > 
a very limited degree, plastic and capable of change of form. Ex 
hypothesis it is not the growth, but the reaction tendency which pro- 
duces the growth, which is transmitted. The evolution of the teeth, 
therefore, falls into the same category as bone.* In the accompanying 
iigures I have epitomized the slow transformation of the single-fanged 
conical reptilian tooth, such as we see in the serpents, into the low- 
crowned human grinder. We now know all the transition forms, so 
that we can homologize each of the cusps of the human molar with its 
varied ancestral forms in the line of descent. For example, the anterior 
lingual or inner cusp of the upper true molars traces its pedigree back 
to the reptilian cone. The anterior triangle of cusps, or trigon, seen in 
the mesozoic mammalia and persisting in the first inferior true molar 
of the modern dog, is still seen as the main portion of the crown of the 
human upper molars (pr, pa, me). To this was added, ages ago, the 
* See especially the papers of Ryder, Cope, and the writer, “ Evolution of Mam- 
malian Molars to and from the Tritubercular Type,” American Naturalist, 1889. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
343 
posterior lingual cusp, or hypocone, which, as Oope has shown, is ex- 
hibited in various degrees of development in different races and is an 
Fig. 4. 
Evolution of the cusps of the human lower molar.—pH, protoconld (anterior buccal cnspl; 
paA, paraconid; me1’, metaconid (anterior lingual cusp); hy\ hypoconid (posterior buccal); ew1, onto- 
conid (posterior lingual cusp); ms'1, mesoconulid (intermediate cusp). Diagram 1.—Reptilian stage. 
Diagrams 2-5.—Mesozoic mammals, first lower molars showing rise of ancestral cusps. Diagram (i.— 
Eocene carnivore (miacis), showing how the low tubercular crown m3 is derived from the high crown 
ml. Diagram 7.—Eocene monkey (Anaptomorphux), showing how the primitive anterior lingual cusp 
pa1' disappears. Diagram 8.—Human first molar with its ancestral cusps. 
important race index.* A glance through the diagrams shows that 
the development of the crown has been by the successive addition of 
new cusps. Without entering upon the details of evidence, which 
Lower molar. 
Upper and lower molars opposed 
'Fig. 5. 
Upper molar. 
Kef to i'i.ax of upper and lower molars in all mammals.—Eacli tooth consists of a triangle, 
trig on, with the protocone, pr, at the apex. The apex is on the inner side of the upper molars and on 
the outer side of the lower molars. 
would lie out of place here, I may say, briefly, that the new main cusps 
have developed at the points of maximum wear (/. e., use), and con- 
* The upper molars in many Esquimaux are triangular (as m rig. 6, diagram 11); 
in most negroes they are square (diagram 12). In our race they are intermediate. 344 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
versely in the degeneration of the crown, disuse foreshadows atrophy 
arid d i sappearai tee. 
Upon the whole, with some exceptions which we do not at present 
understand, the course of evolution of the teeth supports the evi- 
dence derived from the skeleton that, whether any true causal relation 
has existed or not, the lines of individual transformation in the whole 
fossil series preceded those of race transformation. 
Evolution op tup: human upper molars.—Diagram 9 —Anaptomorphus, a Lower Eocene monkey. 
Diagram 10.—An Upper Eocene monkey. Diagrams 11 and 12.—Human; 11, Esquimaux; 12. negro. 
See addition of “talon,” hy. to “ trigon ” composed of pa, pr, me. 
The rise of new organs.—We owe to Dr. Arbutlmot Lane a most 
interesting series of studies upon the influences of various occupa- 
tions upon the human body. He proves conclusively that individual 
adaptation not only produces profound modifications in the proportions 
of the various parts, but gives rise to entirely new structures. 
His anatomy and physiology of a shoemaker* shows that the life-long 
habits of this laborious trade produce a distinct type, which if ex- 
amined by any zoological standard would be unhesitatingly pro- 
nounced a now species—homo sartorius. The psychological analysis 
which a Dickens or Balzac would draw, showing the influences of the 
struggle for existence upon the spirit of this little tailor, could not be 
more pathetic than Dr. Lane’s analysis of his body. The bent form, 
crossed legs, thumb and forefinger action, and peculiar jerk of the 
head while drawing the thread, are the main features of sartorial habit. 
The following are only a few of the results: The muscles tended to 
recede into tendons, and the bony surfaces into which they were in- 
serted tended to grow in the direction of the traction which the 
muscle exerted upon them. The articulation between the sternum and 
the clavicle was converted into a very complex arthrodial joint, con- 
stituting almost a ginglymoid articulation. The sixth pair of ribs 
were ancliylosed to the bodies of the vertebrae, indicating that they 
had ceased to rise and fall with sternal breathing, and that respira- 
tion was almost exclusively diaphragmatic. The region of the head 
and first two vertebrae of the neck was still more striking: the trans- 
verse process of the right side of the atlas, toward which the head 
was bent, formed a new articulation with the under surface of the 
jugular process of the occipital bone, u a small synovial cavity sur- 
rounded this acquired articulation, but there Avas no appearance of a 
* Journal of Anatomy and Physiology, 1888, p. 595. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
capsular ligament;” the left half of the axis was united by bone to 
the corresponding portion of the third cervical; there was found a 
new upward prolongation of the odontoid peg of the axis, and a new 
accessory transverse ligament to keep it from pressing upon the cord. 
In short, u the anatomy of the shoemaker represents the fixation and 
subsequent exaggeration of the position and tendencies to change 
which Avere present in his body Avhen he assumed the position for a 
short period of time. 
Rate of inheritance.—This illustration serves also to emphasize the 
great contrast between the rapidity of individual transformation and 
the slowness of race transformation. Xo one would expect the son of 
this shoemaker to exhibit any of these acquired malformations. Yet 
Dr. Lane thinks he has observed such effects in the third generation 
by the summation of similar influences. 
All palaeontological evidence goes to shoAv that the effects of normal 
habits, if transmitted at all, would be entirely imperceptible in one 
generation. The horse, for example, has not yet completely lost the 
lateral toes which became useless at the end of the Upper Eocene period. 
This objection as to rate of evolution may be urged Avitli equal force 
against the natural selection theory. It is obvious that the active pro- 
gressive principle in evolution (whatever it is), must contend Avitli the 
enormous conservative power of inheritance, and this, to my mind, is 
one of the strongest arguments against the possibilities of the rise of 
adaptive organs by the selection of chance favorable variations in the 
germ plasm. 
Application to human evolution.—Principles underlying these illus- 
trations may now be applied to some of the facts in human evolution 
brought out in the first lecture. They show that if functional tenden- 
cies are transmitted we can comprehend the distinct evolution history 
of each organ; the rise and fall of two organs side by side; the definite 
and purposive character of some anomalies; the increase of variability 
in the regions of most rapid evolution; the correlation of development 
balance, and degeneration in the separate organs of the shoulder, hand, 
and foot. 
Yet e\ren granting this theory there still remain difficulties. The 
relation of use and disuse to some of the contemporary changes in the 
human backbone is rather obscure. I would hesitate to pronounce an 
opinion as to Avhether our present habits of life are tending to shorten 
thelumbars, increase the spinal curvatures, and shift the pelvis with- 
out making an exhaustive study of human motion. Among the influ- 
ences Avhicli Dr. Lane has suggested* as operative here are the wear- 
ing of heeled shoes and the increase of the cranium. He considers the 
additional or sixth lumbar vertebra as a new element rather than as a 
reversion, and Avorks out in some detail the mechanical effects of the 
* Journal of Anatomy and Physiology, 1888, p. 219. 346 
PRESENT PROP.LEMS IN EVOLUTION AND HEREDITY. 
presence of the fetus upon female respiration (i. e., in the sternal re- 
gion) and upon the pelvis. Now, if it be true that the pelvis is larger 
in the higher races than in the lower, I do not think that Dr. Lane can 
sustain his point, because in the lower races the fetus is carried for an 
equally long period, during a much more active life, and in a more con- 
tinuously erect position. Therefore, if these mechanical principles 
were operating, the pelvis in the modern lower races should be larger 
than in the higher. On the other hand, the form of the female pelvis 
in the higher races is one of the best established selecting or eliminat- 
ing factors, a large pelvis favoring frequent births and the preserva- 
tion of those family stirps in which it occurs. I mention this to show 
how cautious we must be in jumping to conclusions as to kinetogenesis. 
The transformism in all the external features of the skull, jaws, and 
teeth may be attributed to inherited tendencies toward hypertrophy 
or atrophy; but how about the convolutions of tlie turbinal bones or 
the complex development of the semicircular canals and cochlea of 
the internal ear and the many centers of evolution which are beyond 
the influences of use and disuse? These are examples of structures 
which fortify Weismann’s contention, for if complex organs of this 
character can only be accounted for by natural selection, why consider 
selection inadequate to account for all the changes in the body? 
Difficulties in the natural-selection theory.—The answer, I think, is 
readily given: We do not know whether use and disuse are operating 
upon the mechanical construction of the ear; we do know that the 
organ can be rendered far more acute by exercise; but even if it were 
true that habit can exert no formative influence, the ear is one of those 
structures which since its first origin has been an important factor in 
survival and may therefore have been evolved by natural selection. 
Now, the very fact that selection may have to care for variations in 
such prime factors in survival as the ear, renders it the more difficult 
to conceive that it also is.nursing the minutiae of variation in remote, 
obscure, and uncorrelated organs. 
Even in the brief review of human evolution in the first lecture I 
have pointed out eight independent regions of evolution, upward of 
twenty developing organs, upward of thirty degenerating organs. 
A more exhaustive analysis would increase this list tenfold. Now, 
where chance variation should produce an increase in size in all the 
developing organs, and a decrease in size of all the degenerating 
organs, and an average size in all the static organs, we would have all 
the conditions favoring survival. But the chances are infinity to one 
against such a combination occurring unless the tendencies of variation 
are regulated and determined, as Lamarckians suppose, by the in- 
heritance of individual tendencies. But may not the favorable vari- 
ations in the body be grouped to either out-weigh or under-weigh the 
unfavorable variations? This would be possible if combinations oc- 
curred; but we can readily see that combinations, such as we observe PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
347 
in the separate elements of the foot alone, completely neutralize each 
other so far as u survival” is concerned ; how the foot would neutralize 
the hand, or the foot and hand would neutralize the lumbar region.* 
It is this consideration of single organs, the observation of their in- 
dependent history, the rise of new compound organs by steady growth 
from infinitesimal beginnings of their separate elements, the combined 
testimony of anatomy and paleontology which force us to regard the 
theory of evolution by the natural selection of chance variations as 
wholly untenable. With the utmost desire to regard the discussion in 
as fair a spirit as possible, the explanations offered by the adherents of 
Weismann’s doctrine strike me as strained, evasive, and illogical.! 
We can however by no means undervalue or dispense with natural 
selection, which must be in continuous operation upon every character 
of sufficient importance to weigh in the scale of survival. I need 
hardly remind you that this selecting principle was first discovered in 
1813 by Dr. W. C. Wells, of Charleston, in connection with the immu- 
nity from certain tropical diseases enjoyed by negroes and mulattoes.J 
The eliminating factor in selection is illustrated almost daily in cases 
of appendicitis. I regret I have not had time to ascertain whether or 
not this disease is considered due purely to accident or to congenital 
variation in the aperture of the appendix, which favors the admission 
of hard objects. If so, modern surgery is only benefiting the individual 
to the detriment of the race by its efficient preventive operations; and 
every individual who succumbs to this disease can reflect with melan- 
choly satisfaction that he does so pro bono publico. 
Conclusions as to the factors of evolution.—The conclusions we 
reach from the study of the muscular and skeletal systems are there- 
fore as follows: 1. That individual transformism in the body is the 
main determinant of variations in the germ cells, and is therefore the 
main cause of definite progressive or retrogressive variations in single 
organs. 2. That evolution in these organs is hastened where all mem- 
bers of the race are subject to the same individual transformism. The 
contrast between the rate of individual transformism and race trans- 
formism is due to the strong conservative forces of the germ plasma. 
3. That evolution is most rapid where variations are of sufficient rank 
to become factors in survival. Then selection and use inheritance unite 
forces as active progressive principles opposing the conservative prin- 
ciple in the germ plasma. 4. That fortuitous and chance variations 
also arise from disturbances in the body or germ cells; they may be 
perpetuated, or disappear in succeeding generations. 
*1 have expanded this idea fully in recent papers upon the theory of evolution of 
the horse. See “Are Acquired Variations Inherited?” American Naturalist, Feb- 
ruary, 1891. 
1 See Weismann’s last essay, “Retrogressive Development,” in Nature, Biol. Mem., 
trans., in press. 
\ See Introduction of Darwin’s Origin of Species. 348 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
Applying these views to variation there should theoretically ap- 
pear to be just those two distinct classes of anomalies in the human 
body which we have seen actually occurring: First, those in the path 
of evolution, arising from perfectly normal changes in the somato-plasm 
and germ-plasm: second, those wholly unconnected with the course 
of evolution, arising fortuitously or from abnormal changes in the 
somato-plasm or germ-plasm; to this head maybe attributed the whole 
scale of deformities. Thus tranformism and deformism should be 
kept distinct in our minds. Nevertheless the facts of deformism con- 
tribute the strongest body of evidence which we can muster at present 
to prove that there does exist a relation between the somato-plasm and 
germ-plasm which renders transformism possible. 
The rela tions between the somato-plasm and perm -plasm.—"We ha ve 
seen reasons to take a middle ground as to the distinct specific nature 
of the body cells and germ cells, and this position is, I think, strength- 
ened the more broadly we extend our inquiry into all the fields of pro- 
toplasmic activity. 
There are three questions before us. 
1. What is the evidence that the germ-plasm and somato-plasm are 
distinct? 
2. What is the specific nature of the germ-plasm? 
3. What is the nature of the relations which exist between the two? 
1. The separation of the germ-plasm is in the regular order of evolu- 
tion upon the principles of physiological division of labor. The unicel- 
lular organisms combine all the functions of life in a single mass of pro- 
toplasm, that is, in one cell. In the rise of the multi cellular organisms 
the various functions are distributed into groups of cells, which spe- 
cialize in the perfecting of a single function. Thus the reproductive 
cells fall into the natural order of histogenesis, and the theory of their 
entire separation is more consistent with the laws governing the other 
tissues than the theory which we find ourselves obliged to adopt, that 
while separate they are still united by some unknown threads with the 
other cells. 
The morphological separation of what we may call the race proto- 
plasm becomes more and more sharply defined in the ascending scale 
of organisms. Weismann’s contention as to the absolutely distinct 
specific nature of the germ-plasm and somato-plasm has however to 
meet the apparently insuperable difficulty that in many multi cellular 
organisms, even of a high order, the potential capacity of repeating 
complex hereditary characters, and even of producing perfect germ 
cells, is widely distributed through the tissues. 
For example, cuttings from the leaves of the well-known hot-house 
plant, the begonia, or portions of the stems of the common willow tree, 
are capable of reproducing complete new individuals. This would in- 
dicate either that portions of the germ plasm are distributed through 
the tissus of these organisms, or that each body cell has retained its 
potential quota of hereditary characters. PRESENT PROBLEMS IX EVOLUTION AND HEREDITY. 
349 
Among the lower animals we find the same power; if we cut a hydra 
or bell animalcule into a dozen pieces, each may reproduce a perfect 
new individual. As Ave ascend in the animal scale the power is con- 
fined to the reproduction of a lost part in the process known asrecres- 
eence. As you well know, in the group to Avliich the frog and sala- 
mander belong, a limb or tail, or even a lower jaw, may be reproduced. 
The only logical interpretation of these phenomena is that the heredi- 
tary powers are distributed in the entire protoplasm of the organism, 
and the capacity of reproduction is not exhausted in the original for- 
mation of the limb, but is capable of being repeated. There has been 
considerable discussion of late as to the seat of this power of recrescence. 
It seems to me not impossible that in the vertebrates it may be stored 
in the germ cells, and it would be very interesting to ascertain experi- 
mentally whether removal of these cells would in any way limit or 
affect this power; we know that such removal in castration or ovari- 
otomy sometimes profoundly modifies the entire nature of the organ- 
ism, causing male characters to appear in the female, and female char- 
acters to develop in the male. 
So far as man is concerned it has been claimed by surgeons that 
genuine recrescence sometimes occurs ; for example, that a new head is 
formed upon the femur after exsection; but my friend Dr. Y. P. Gibney 
informs me that this is an exaggeration, that there is no tendency to 
reproduce a true head, but that a pseudo-head is formed, which may 
be explained upon the principle of regeneration and individual trans- 
form ism by use of the limb. • 
Pfliiger’s opinion is that recrescence does not indicate a storage of 
hereditary power, that there is no pre-existing germ of the member, 
but that the re-growth is due to the organizing and distributing power 
of the cells at the exposed surface, so that, as new formative matter 
arrives, it is built up gradually into the limb. This view would reduce 
re crescence to the level of the regeneration process which unites two 
cut sections of the elements of a limb in their former order. It is partly 
opposed to tin1 facts above referred to, which seem to prove the dis- 
tribution of the hereditary power. Yet it seems to me quite consistent 
to consider these three processes—a, reproduction of a new individual 
from every part; />, recrescence of a new member from any part; c, re- 
generation of lost tissues—as three steps indicating the gradual, but 
not entire withdrawal of the reproductive power into the germ cells. 
I have not space to consider all the grounds Avliich support the view 
of the separation of the germ cells in man. Some of the more proini 
nent are: the very early differentiation of these cells in the embryo, 
observed with a few exceptions in all the lower orders of animals, and 
advancing so rapidly in the human female that several mouths before 
birth the number of primordial ova is estimated at seventy thousand, 
and is not believed to be increased after the age of two and a half 
years. The most patent practical proof is that we may remove every 350 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
portion of the body which is not essential to life and yet the power of 
complete reproduction of a new individual from the germ cells are un- 
impaired. Among .the many reasons advanced for pensioning the 
crippled soldiers of our late war you never hear it urged that their 
children are incapacitated by inheritance of injuries. The strongest 
proof however rests in the evidence I have already cited from hered- 
ity of the extraordinary stability of the germ cells, which is the safe- 
guard of the race. 
2. The specific nature of the germ-plasm must be considered before 
we consider its relations. Wherein lies the conservative power of the 
germ-plasm, and in what direction shall we look for its transforming 
forces'? You see at once that marvellous as is the growth of cells in 
other tissues, the growth of the germ cell is still more so. 
We find it utterly impossible to form any conception of the contents 
of the microcosmic nucleus of the human fertilized ovum, which is less 
than one twenty-five-hundredths of an inch in diameter, but which is 
nevertheless capable of producing hundreds of thousands of cells like 
itself, as well as all the unlike cells of the adult organism. We can only 
translate our ideas as to the possible contents of this nucleus in the 
terms of chemistry and physics.* 
Spencer t assumed an order of molecules or units of protoplasm lower 
in degree than the visible cell units, to the internal or polar forces of 
which, and their modification by external agencies and inter-action, he 
ascribes the ultimate responsibility in reproduction, heredity, and 
adaptation. This idea of biological units seems to me an essential part 
of any theory; it is embodied in Darwin’s “ gemmules,” in Haeckel’s 
“ plastidules;” yet, as Lankester says the rapid accumulation of bulk is a 
theoretical difficulty in the material conception of units. In the direc- 
tion of establishing some analogy between the repetition power of hered- 
ity and known function of protoplasm, Haeckel % and Hering§ have 
likened heredity to memory, and advanced the hypothesis of persist- 
ence of certain undulatory movements; the undulations being suscep- 
tible of change, and therefore of producing variability, while their ten- 
dency to persist in their established harmony is the basis of heredity. 
Berthold, Gautier, and Geddes|| have speculated in the elaboration of 
the idea of metabolism; the former holding the view that “inheritance 
is possible only upon the basis of the fundamental fact that in the 
chemical processes of the organism the same substances and mixtures 
of substances are reproduced in quantity and quality with regular 
periodicity.” 51 
* See Kay Lankester, Nature, .July 15, 1876. 
t Principles of Biology, vol. i., p. 256. 
XPerigenesis dvr Plastidule oder die Wellenzeugung dcr Lebenstheilchcn. Jena, 1875. 
§ Ueber d. Geddehtniss als ein eallgemeine Function d. organischen Materie. Vienna, 
1870. 
|| See also Thomson, op. cit., |>. 102. 
51 Berthold: Studien iiber Protoplasma-Meehanik. 1 .eip,sic, 1886. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
351 
I have merely touched upon these speculations to show that the un- 
known factors in heredity are also the unknown factors in operation in 
living- matter. All we can study is the external form, and conjecture 
that this form represents matter arranged in a certain way by forces 
peculiar to the organism. These forces are exhibited or patent in the 
somatic cells; they are potential or latent in the germ cells. 
The last stage of our inquiry is as to the mode in which the action of 
habit or environment upon the somatic cells can be brought to bear 
upon the germ cells. 
The nature of the relation between the body cells and germ cells.—I 
have already shown that we are forced to infer that such a relation 
exists by the facts of evolution, although these facts show that the 
transmission of normal tendencies from the body to the germ cells is 
ordinarily an extremely slow process. 
Virchow* says every variation in race character is to be traced back 
to the pathological condition of the originator. All that is pathologi- 
cal is not diseased, and inheritance of a variation is not from the influ- 
ence upon one individual necessarily, but upon a row of individuals. 
This is in the normal condition of things. In the abnormal condition 
the rate of transmission may be accelerated. 
Does this transmission depend upon an interchange of material par- 
ticles, or upon an interchange of forces, or both1? 
There are three phenomena about which there is much skepticism, to 
say the least, which bear upon the question of a possible interchange 
of forces between the body and the germ-cells. These are the inherit- 
ance of mutilations, the influence of previous fertilization, and the in- 
fluence of maternal impressions. They are all in the quasi-scientific 
realm, which embraces such mental phenomena as telepathy. That is, 
we incline to deny them simply because we can not explain them. 
Mutilations.—Since the publication of Weismann’s essays the sub- 
ject of inherited mutilations has attracted renewed interest. 1 would 
first call attention to the fact that this matter has only an indirect bear- 
ing, for a mutilation is something impressed upon the organism from 
without ; it is not truly u acquired;” the loss of a part by accident pro- 
duces a sudden but a less profound internal modification of the organism 
than the loss of a part by degeneration. Most of the results are nega- 
tive; many of the so-called “ certain ” cases prove upon investigation 
to be mere coincidences. Weismann t himself experimented upon white 
mice, and showed that 901 young were produced by five generations of' 
artificially mutilated parents, and yet there was not a single example of 
a rudimentary tail or of any other abnormality in this organ. The cases 
of cleft ear lobule have recently been summed up.f Israel reports two 
cases of clefts in which the parent’s ears were normal. Schmidt and 
* “TTelter den Transformismus,” Arvhivf. Anthropologic, 1888, j>. 1. 
1 Biological Memoirs, p. 432. 
t Journal of Anatomy and Physiology, 1891, p. 433. 352 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
Ornsteiu report affirmative eases. His shows that an affirmative ease, 
eited by V. Zwieeiki, is merely an inherited peculiarity. The entire 
evidence is unsatisfactory, and upon the whole, is decidedly negative. 
Not so however in cases where the mutilation results in a general 
disturbance of the normal functions of different organs, as in the ex- 
periments conducted by Brown-Sequard* upon guinea-pigs, in which 
we see ‘‘acquired variation” intensified. In these, abnormal degener- 
ation of the toes, muscular atrophy of the thigh, epilepsy, exoplithal- 
mia, etc., appeared in the descendants of animals in which the spinal 
cord or sciatic nerve had been severed, or portions of the brain removed. 
It was also shown that the female is more apt to transmit morbid states 
than the male; that the inheritance of these injuries may pass over 
one generation and re-appear in the second; that the transmission by 
heredity of these pathological results may continue for five or six gen- 
erations, when the normal structure of the organs re-appears. These 
cases, which are incontestable, at first sight appear to establish firmly 
the transmission of acquired characters; they were so regarded by 
Brown-Sequard. These lesions act directly upon the organs, and the 
abnormal growth of these organs appears to be transmitted. But can 
they not be interpreted in another way, namely, that the pathological 
condition of the nerve centers has induced a direct disturbance in those 
portions of the germ cells which represent and will develop into the 
corresponding organs of the future offspring? 
Previous fertilization.—Consider next the influence exerted upon 
the female germ cell by the mere proximity of the male germ cell, as 
exhibited in the transmission of the characteristics of one sire to the 
offspring of a succeeding sire, observed in animals, including the 
human species, also in plants. The best example is the oft-quoted 
case of Lord Morton’s mare, which reproduced in the foal of a pure 
Arab sire the zebra markings of a previous quagga sire. 
Some physiologists t have attempted to account for these remark- 
able indirect results from the previous fertilization or impregnation, 
by the imagination of the mother having been strongly affected, or 
from interchange between the freely inter communicating circulation 
of the embryo and mother, but the analogy from the action in plants 
(in which there is no gestation but early detachment and development 
of the fertilized cells) strongly supports the belief that the proximity 
of male germ cells acts directly upon the female cells in the ovary. 
All that we can deduce from these facts is that in some manner the 
normal characteristics and tendencies of the ova are modified by the 
foreign male germ cells without either contact or fertilization. 
Maternal impression.—The influence of maternal impressions in the 
* Comptes-Bendns, March 13, 1882. These experiments have been confirmed by 
Obersteiner. 
1 See the eases cited by Hi hot, and Darwin: Animals and Plants Under Domestica- 
tion, vol. i, p. 437. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
353 
causation of definite anomalies in the foetus is largely a matter of in- 
dividual opinion - 
It is denied by some high authorities, led by Bergman and Leuck- 
liart.* Most practitioners, however, believe in it, and 1 need hardly 
add that it is a universal, popular belief,! supported by numerous 
cases. I myself am a firm believer in it. The bearing which the sub- 
ject has upon this discussion is this: If a deviation in the develop- 
ment of a child is produced by maternal impression, we have a proof 
that a deviation from normal hereditary tendencies can be produced 
without either direct vascular or nervous continuity. 
We see an analogy between the experiments of Brown-Sequard, the 
influence of the previous sire, and the maternal influence. Neither, in 
my opinion, directly supports the theory of transmission of acquired 
characters, for they do not prove that normal changes in the body cells 
directly react upon the germ cells; they all show that the typical 
hereditary development of single organs may be diverted by living 
forces which have no direct connection with them according to our 
present knowledge. 
What the nature of these forces is I will not undertake to say, but 
I believe we must admit the existence of some unknown force, or rather 
of some unknown relations between the body cells and germ cells. 
A year ago, recognizing fully the difficulty of advancing any theory 
of heredity which would explain the transmission of acquired characters, 
I came to the following result: “ It follows as an unprej udiced conclusion 
from our present evidence that upon Weismann’s principle we can 
explain inheritance but not evolution, while with Lamarck’s principle 
and Darwin’s selection principle we can explain evolution, but not, at 
present, inheritance. Disprove Lamarck’s principle and we must assume 
that there is some third factor in evolution of which we are now 
ignorant.” 
In this connection it is interesting to quote again from my colleague, 
Prof. E. B. Wilson, lie writes that the tendency in Germany at 
present is to turn from speculation to empiricism, and this is due 
partly “ to the feeling that the recent wonderful advances in our 
knowledge of cell phenomena have enormously increased the difficulties 
of a purely mechauico-physieal explanation of vital phenomena. In 
fact, it seems that the tendency is to turn back in the direction of the 
vital-force conception. - - - As Boveri said to me recently, uEs 
gibt zu viel vorstand in der Natur uni eine rein meehanisclie Erklaruug 
der Saelie zu ermoglichen.” 
In the final lecture we turn to the forces exhibited in the germ cells. 
Handerworterbuch der Physiologic, Wagner, Artikel “ Zeugung,” Leuckhart. 
tSee Medical Record, October 31, 1891, an article by Joseph Drzewiecki, m. i>. 
H. Mis. 114 23 354 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
LECTURE III.—HEREDITY AND THE GERM CELLS. 
According to the general law* the germ cell was considered as 
matter potentially alive and having within itself the tendency to as- 
sume a definite living form in course of individual development. The 
nucleus must be extraordinarily complex, for it contains within itself 
not only the tendencies of the present type, but of past types far 
distant. The supposition of a vast number of germs of structure is 
required by the phenomena of heredity; Migeli has demonstrated that 
even in so minute a space as one one-thousandth cubic millimeter, 
400,000,000 micellae must be present. 
The study of heredity will ultimately center around the structure and 
functions of the germ cells. The precise researches of Galton show 
that the external facts of heredity, questions of average and of proba- 
bilities, of paternal and maternal contributions to the offsprings, are 
capable of being reduced to an exact science in which mathematical 
calculations will enable us to forecast the characteristics of the coming 
generation. 
There will still remain however a large residuum of facts which will 
present themselves to a mathematician like Galton, as fortuitous, or in- 
exact, such as the physiological conditions of reversion; the course of 
pre-potency, by which the maternal or the paternal characteristics pre- 
vail in parts or in the entire structure of the offspring; the material 
basis of latent heritage upon which reversion depends, and which com- 
pels us to hypothecate either an unused hereditary substance or a return 
to an older disposition of the forces in this substance; the nature and 
determination of sex. These apparently chance phenomena must also 
be due to certain fixed laws, and by far the most promising routes to 
discovery have already been taken by Van Beneden, the Hertwig 
brothers, Boveri, Maupas, and others. 
They have attacked the problem of the relation of the germ cells to 
the heredity on every side, and by the most ingenious and novel meth- 
ods, which are familiar enough in various branches of gross anatomical 
and physiological research, but seem almost out of the limits of applica- 
tion to minute microscopic objects. For example, the Hertwig brothers 
have ascertained the influence of various solutions of morphine and 
other drugs of the alcohols, and of the various degrees of temperature 
upon the ovum and spermatozoon during the conjugation period, with 
results which are highly suggestive of the causes of congenital mal- 
formations, anomalies, and double births. The Hertwigs and Boveri 
have succeeded in robbing ova of their nuclei and watching the results 
of the subsequent entrance of spermatozoa. In order to further test 
the relations of the nucleus to the remainder of the cell, Verworn has 
experimented along the same line with extirpations of every kind from 
the single cells of Infusoria. Of equal novelty are the recent studies of 
*See Huxley, Article “Evolution/' Enc. Britannica, vol. vizi, p, 746. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
355 
Maupas upon the multiplication and conjugation of the Infusoria, giv- 
ing us a host of new ideas as to the cycle of life, the meaning of sex, 
and the origin of the sexual relation. 
In all this research and in the future outlook there are two main 
questions: 
1. What is the hereditary substance? What is the material basis of 
heredity, which spreads from the fertilized ovuins to every cell in the 
body, conveying its ancestral characteristics? Is there any substance 
corresponding to the hypothetical idioplasm of Nageli? 
2. What are its regulating and distributing forces? How is the he- 
reditary substance divided and distributed ? How tar is it active or 
passive ? 
I may say at the outset that the idioplasm of Nageli, a purely ideal 
element of protoplasm which he conceived of as permeating all the tis- 
sues of the body as the vehicle of heredity, has been apparently mate- 
rialized in the chromatin or highly coloring materials in the center of 
the nucleus. This rests upon the demonstration by Van Beneden and 
others that chromatin is found not only in all active cells, but is a con- 
spicuous element in both the ovum and spermatozoon during all the 
phenomena attending conjugation. 
Fig. 7.—Typical cell division, showing the distribution op chromatin.—(From Parker after 
Carnoy.) A-C, arrangement of the chromatin in threads; O-K, formation of the chromatin rods 
and loops; F. splitting of the loops; G-H, retraction of the chromatin into the two daughter cells. 
Secondly, that while the chromatin is apparently passive, it is played 
upon by forces resident in the clear surrounding' protoplasm of the 
nucleus, but chiefly by the extra nuclear arclioplasm, which seems to 
constitute the dynamic and mechanical factor in each cell. This, un- 
like the chromatin, only comes into view when there is unusual activ- 
ity, as during cell-division, and is not evident (with our present histo- 
logical technique, at least), when the cell is arrested by reagents in 
any of the ordinary stages of metabolism. 
The distribution of hereditary substance.—I may first review some 
of the well-known phenomena attending the distribution of the chro- 
matin substance to the tissues. 356 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
I have borrowed from Parker, figures by Carnoy, to illustrate the rest- 
ing and active stages of the cell, and from Watase, a Japanese student 
of Clark University, figures representing the high differentiation of 
the cell contents during division (Figs. 8, 9). They bring out the active 
and passive elements of the typical cell. 
The phenomena of karyokinesis which attend the division and dis- 
tribution of the hereditary substance throughout the whole course of 
embryonic and adult development are well illustrated in Carnoy’s fig- 
ures (Fig. 7). First we have the quiescent period, in which the eliro- 
Fig. 8.—Before division. Differentiation of 
THE CYTOPLASM AND NUCLEUS DURING CELL DIVI- 
SION of asquid embryo, Loligo. (After Watase). 
M, The nuclear membrane; F, Achromatin or 
nucleoplasm; C, Cytoplasm, or protoplasm out- 
side of the nucleus; A-A, The two centrosomes 
of archoplasm; B, Extra nuclear arclioplasmic 
filaments; E, Intra nuclear archoplasn ic fila- 
ments attached to n, n', the chromatin rods. 
Fig. 9.—After division. Interior of a daughter- 
cell in the squid. (After Watase.) Division has 
just taken place and the daughter nucleus, N, shows 
the chromatin coil. The daughter centrosome is 
just forming two new centrosomes, A-A, by direct 
division. 
matin presents the appearance of a coiled, tangled thread; surround- 
ing this is the clear nucleo plasm (or achromatin) bounded by the nu- 
clear membrane; the extra-nuclear substance, or cyto-plasm, is appar- 
ently undifferentiated. As soon as cell division sets in, however, ra- 
diating lines are seen in the cyto plasm above and below the nucleus; 
these are called the archo-plasmic filaments by Boveri, since they pro- 
ceed from what is now believed to be the dynamic element, the archo- 
plasm (Fig. 8). As the activity becomes more intense the filaments are 
seen to diverge from a center—the archo-plasmic centrosome—which lies 
just without the nucleus at either pole; this radial display of cell 
forces suggested the term “asters” to Fol, and “spheres attractive” to 
Van Beneden. The behavior of the chromatin, or hereditary substance, PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
357 
under these archo-plasmic forces, is beautifully sliown in Oarnoy’s dia- 
grams (Fig. 7). First, the nuclear wall breaks up, then the chromatin 
coil unfolds into lines of vertical striation which become thread-like, 
hence the term mitosis, and then more compact, until finally a number 
of distinct vertical rods, chromatin rods, or chroma,monies are found. 
A remarkable and significant fact may be noted here, that the num- 
ber of chromasomes varies in the cells of different species, and even 
in the cells of different varieties (as in the thread-worm of the horse— 
Ascaris megalocephala), but is constant in all the cells of the same va- 
riety through all stages; thus the same number of chromasomes ap- 
pear in the first segmentation of the fertilized ovum as in the subse- 
quent cell division in the tissues. 
Oarnoy next indicates the vertical splitting of each rod into a loop 
or link preceding the horizontal splitting; thus we may conceive of a 
thorough re distribution of the chromatin before it passes into the 
daughter-cells. The split loops are each retracted toward a centro- 
some, suggesting to some authors a contractile power in the archo-plas- 
mic filaments, each chromosome being apparently withdrawn by a 
single filament. But as the chromasomes separate, the filaments also 
appear between them, and are variously termed “interzonal,” “verbin- 
dungs faden,” “filaments reunissant;” there is therefore some differ- 
ence of opinion as to what the mechanics of the chromosome divisions 
really are. The chromatin is now retracted into two coiled threads, 
each the center of the daughter nucleus with a single eentrosome be- 
side it. But as the line of cleavage is drawn between the two cells 
(Fig. 9), the single eentrosome in each cell divides so that each 
daughter-cell is now complete with its chromatin coil and two archo- 
plasmic centrosomes. This process has been beautifully described by 
Watase.* 
It thus appears that both the chromatin and archo-plasm are perma- 
nent elements of the cell, such as we formerly considered the nucleus; 
the apparently passive chromatin is divided with great precision by the 
active archo-plasm, then the archo-plasm simply splits in two to resume 
the cleavage function. 
Fertilization—the union of hereditary substances.—Before looking at 
the host of questions which fertilization suggests, let us review a few 
of the well-known phenomena preparatory to tlie union of the germ 
cells in order to give greater emphasis to the importance of recent dis- 
coveries. 
First, the ovum is a single cell, the typical structure of which, with 
its nucleus and cytoplasm, is generally obscured by a quantity of food- 
material, surrounded by a rather dense cell wall. The ovum is said to 
be ripened or “matured” for the reception of the spermatozoon, by the 
extrusion of two small “polar bodies,” containing both chromatin and 
*See Marine Biological Laboratory Lectures, 1889. Boston: Ginn & Co. 358 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
hyaline protoplasm, and separating off by karyokinetic division. After 
maturation is complete, a single spermatozoon normally penetrates; 
then a reaction immediately sets in in the cell wall of the ovum which 
prevents other spermatozoa from entering. The head of the spermato- 
zoon and the nucleus of the ovum now fuse together to form a single 
nucleus, which it is obvious contains the hereditary substance of two 
individuals. This is the starting point of the segmentation or distri- 
bution process above described, and it follows that the fertilized ovum 
at this stage must contain its typical complement of chromatin, archo- 
plasm, etc., for the whole course of growth to the adult. 
How shall we connect these phenomena of fertilization with the 
facts of heredity! The most suggestive enigma in connection with 
the fertilization process has been the meaning of the two polar bodies, 
especially since Yan Beneden demonstrated that they contained chro- 
matin! For twenty-five years, speculation has been rife as to why the 
ovum should extrude a portion of its substance in two small cells; why 
not in one cell! why not in a larger number? Thanks to the intense 
curiosity which these polar bodies have aroused, and to the great va- 
riety of explanations which have been offered for them, we have ar- 
rived to-day at a solution which links the higher animals with the 
lower, breaks down the supposed barrier between the sexes, and ac- 
cords with the main external facts of heredity. 
It seems to me best to disregard the order of discovery, and to state 
the facts in the most direct way. First, a few words as to the specu- 
lations upon the meaning of the polar bodies. 
The early views of fertilization* were naturally based upon the ap- 
parent significance of this process in the human species, in which the 
sexes are sharply distinguished from each other in their entire struc- 
ture, and the reproductive cells are also widely differentiated in form, 
the ovum large and passive, the spermatozoon small and active. The 
readiest induction was to regard these elements as representing dis- 
tinct physiological principles, corresponding to the essential sexual 
characteristics—in short, as male and female cells, the former vitalizing 
and rejuvenating the latter. Thus one of the earliest definite “polar- 
body” theories was that the ovum was hermaphrodite, containing both 
male and female principles, and that it was necessary to get rid of the 
male substance before the spermatozoon could enter. 
As Yon Siebold and Leuclcart had demonstrated that some ova re- 
produce partlienogenetically, that is without fertilization by spermato- 
zoa, Weismann turned to such forms for the solution of this problem, 
and was surprised to find that partlienogetic ova only extrude one 
polar body. This led him to attach one meaning to the first polar body, 
and another meaning to the second, which he viewed as designed to 
reduce the heredity substance in the ovum without regard to sex. 
Thus both this and the older theory conveyed alike the idea of reduc■ 
*See also the introduction of Weismann’s last essay, “Amphimixis.” PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
359 
sion, but with an entirely different supposition as to the nature of the 
material reduced or eliminated. 
Maupas on Conjugation among the Infusoria.*—Among the newer 
researches which throw light upon this old problem, those of Maupas 
are certainly the most brilliant. After a most exact and arduous re- 
search, extending over several years, he collected his results in two 
memoirs, published in 1889 and 1890. 
His experiments were first directed upon the laws of direct multi- 
plication by fission, which revealed a complete cycle of life in the single- 
celled Infusoria and showed that after a long period this mode of 
reproduction becomes less vigorous, then declines, and finally ceases 
altogether unless the stock is rejuvenated by conjugation of individ- 
uals from different broods. In other words, these broods of minute 
organisms grow old and die unless they are enabled to fertilize each 
other by an exchange of hereditary substance altogether analagous to 
that observed in the higher multicellular organisms. 
The cultures were made in a drop of water upon a slide, and feeding 
was adapted either to the herbivorous or carnivorous habits of the 
species. Under these conditions it was found that the rate of fission 
or direct multix>li cation varied directly with the temperature and food, 
rising in some species (Glaucoma scintillans) to five bipartitions daily. 
With the optimum of conditions this rate, if sustained for thirty-eight 
days, would produce from a single individual a mass of protoplasm 
equivalent to the volume of the sun. This rate is however found to 
be steady for a time, and then the offspring decline into “senescence,” 
in which they appear at times only one-fourth the original size, with 
reduced buccal wreaths and degenerate nuclear apparatus. This is 
reached sooner in some species than in others; StylonieMapustulata 
survives three hundred and sixteen generations or fissions, while Lcu- 
cophrys patula persists to six hundred and sixty generations. Finally, 
even under the most favorable conditions of environment, death ensues. 
Not so where conjugation is brought about by mingling the offspring 
of different broods in the same fluid, as in the natural state. Maupas 
soon discovered that exhaustion of food would induce conjunction be- 
tween members of mixed broods. He thus could watch every feature 
of the conjugation process, and determine all the phases in the cycle of 
life. These differed, as in the longevity of the species. In Stylonicliia, 
for example, “immaturity” extended over the first one hundred bipar- 
titions: “puberty,2’ or the earliest phase favorable to conjugation, set 
in with the one hundred and thirtieth bipartition; “eugamy,” or the 
most favorable conjugation phase, extended to the one hundred and 
seventieth; then “senescence” set in, characterized by a sexual hyper- 
esthesia in which conjugation was void of result or rejuvenesence, 
owing apparently to the destruction of the essential nuclear apparatus. 
*Sur la multiplication des Infusoires Cilids, Archie, etc Zoologic experimentata, Ser. 
2, yoI. vi., pp. 165-273; Le Rcijeunissement Karyogamique dies les Cities, vol. yii, pp. 
149-517. See also Hartog, Quart. Jour. Microscop. Science, December, 1891. 360 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
Conjugation begins with the approach of two individuals, and adhe- 
sion by their oral surfaces. There is no fusion, but an immediate trans- 
formation in the cell contents of each individual sets in, concluding 
Fig. 10.—Tee Conjugation of Infusoria. (From Weismann, after Maupas.) 1, Two infusoria copulating; M, raegauucleus; m, micronucleus; 
2-5, Successive divisions of micronuclei; 6, The migration of one of the persisting micronuclei from each infusorian into the other; 
7, Union of the interchanged micronuclei. 
with an interchange of nuclear substance. In each cell Maupas dis- 
tinguishes between the (M) m(ganucleus (Fig. 10, the macronucleus, 
nucleus, endoplast of authors), which presides over nutrition and PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
361 
growth and divides by constriction, and the (m) micronucleus (paranu- 
cleus, nucleolus, of authors), which presides over the preservation of 
the species. The latter contains chromatin; it is the seat of rejuve- 
nescence, the basis of heredity, it divides by mitosis, showing- all the 
typical stages of karyokinesis excepting the loss of the cell membrane. 
The transformation in each of these copulating cells first affects the 
centers of hereditary substance, viz, the micro-nuclei; they divide three 
times; thus the micronuclear substance is reduced to one-fourth of its 
original bulk. It is contained in two surviving micronuclei (the others 
being absorbed or eliminated), one of which migrates into the adjoin- 
ing cell; the other remains stationary. This migration is followed by a 
fusion of the migrant and stationary micro nuclei; this fusion effects a 
complete interchange of hereditary substance, after which the two in- 
fusoria separate and enter upon a new life cycle. Meanwhile the me- 
ganucleus breaks up and is reconstituted in each fertilized cell. 
Maupas gathers from these interesting phenomena additional proof 
that the chromatin of all cells bears the inherited characteristics and 
that the cyto plasm and nucleo plasm, or achromatin, is the dynamic 
agent, because the micronuclei bearing the chromatin are the only struct- 
ures which are permanent and persistent, all the other structures— 
nucleo-plasm, archo-plasm, etc.—being replaced and renewed. The re- 
duction of the chromatin is purely quantitative, the eliminated and fer- 
tilizing micronuclei being exactly equivalent; after the chromatin has 
been quartered the cell becomes incapable of further activity until it is 
reinforced by chromatin from the copulating cell. 
No distinction between the sexes in heredity.—The three laws 
which underlie these phenomena are: (1) That fertilization consists in 
the union of the hereditary substance of two individuals. (2) That 
before the union the hereditary substance in each is greatly reduced. 
(3) That there is no line between male and female, the conjugating cells 
are simply in a similar physiological condition wherein a mingling of 
hereditary characteristics affords a new lease of life. As Maupas says: 
“Les differences appelees sexuelles portent sur des faits et des 
phenomenes purement accessoires de la fecondation. La fecondation 
eonsiste uniquement dans la reunion et la copulation de deux noyaux 
semblables et equivalents, mais provenus de deux cellules distiuctes.” 
In this conclusion as to the secondary and superficial, rather than 
fundamental, difference between the two sexes, Maupas simply con- 
firms the views of Strassburger, the botanist, Hensen, It. and O. Hert- 
wig, Weismann, and others, namely, that sex has evolved from the 
necessity of cell conjugation; that even in the higher forms the cells 
born by the two sexes are absolutely neutral so far as sex is concerned, 
the wide difference of form of the germ cells is a result of physiological 
division of labor—the mass and yolk of the ovum having been differ- 
entiated to support the early stages of development while the sperma- 
tozoon has dispensed with all these accessories and acquired an active 362 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
vibratile form for its function of reaching and penetrating the ovum. 
The evidence of the Infusoria is paralleled among some of the plants, 
in which conjugation between entirely similar cells is observed. 
The causes finally determining sex may come surprisingly late in 
development, and according to the investigations of Diising and the 
experiments of Yung* and of Giron are directly related to nutrition. 
High feeding favors an increase of the percentage of females, while, 
conversely, low feeding increases the males. In Yung’s experiments 
with tadpoles the following results were obtained: 
Females. 
Males. 
Normal percentage 
57 
43 
High nutrition 
92 
8 
Geddes expresses tliis principle in physiological terms of metabolism, 
that anabolic (constructive) conditions produce females, while katabolie 
(destructive) conditions produce males. 
I think we may now safely eliminate the factor of sex from our cal- 
culations upon the problem of heredity, and thus rid ourselves of one 
of the oldest and most widespread fallacies. We shall thus, in using 
the terms “paternal’’ and “maternal” imply merely the distinction 
between two lines of family descent. 
The theory of reduction.—This leads us back to the significance of 
the polar bodies. Van Beneden’s discovery that these bodies con- 
tained chromatin led gradually to the view that they were not frag- 
ments of the ova, but represented minute, morphologically complete 
cells. Butsehli showed that they were given off independently of, and 
prior to, the contact of the spermatozoon, and, finding in the leeches 
that the first polar body subdivides to form two bodies, he considered 
them as formed by true cell division, and containing both nucleoplasm 
and chromatin. Giard independently reached a similar opinion, as- 
signing an atavistic meaning to the polar cells. Whitman, in 1878, 
advanced the idea that they represented vestiges of the primitive 
mode of reproduction by fission, while Mark described them as “abor- 
tive ova.” 
At this point speculation subsided until it was revived by Weis- 
mana’s attempt to connect these bodies with his theory of heredity,! 
already referred to. The whole history is clearly given in R. Hert- 
wig’s masterly memoir upon Ovo and Spermatogenesis in the Xema- 
fodes.| Taking advantage of Boveri’s discoveries in staining tecli- 
*See Gedd.es and Thomson: The Evolution of Sex, 1891; also, Dosing: Die Regu- 
liernng des Geschlechtsverkiiltnisses bei d. Yernielirnng der Menschen, Tiere und 
Pilauzen, Jen. Zeit. f. Natur., Bd. 17, 1884. 
tOn the Number of Polar Bodies an 1 their Significance in Heredity, 1887. 
fEi und Samenbildung bei Nenn.todeu, Archiv.f. Mikr. Anat., Bd. 26, 1890. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
nique, and stimulated by Weismann’s prediction that spermatozoa 
would also be found to extrude polar bodies, this author examined all 
stages in the peculiarly favorable germ cells of the thread-worm of the 
horse (Ascaris megalocephala). 
He made the surprising discovery that ova and spermatozoa are 
formed in a substantially similar manner by repeated divisions, the 
single difference being that the last products of division among the 
sperm cells are effective spermatozoa, capable of development in fer- 
tilization, while th e last products of division in the ovary are, first 
Fig. 11. 
The Maturation of Ova, or Formation of Polar Bodies in Asoaris. (From Weismai.n afte 
Hertzwig.) A, original germ-cell in embryonic germ-layer—4 chromatin rods: B, Ovum mother-cell— 
8rods; CD, First polar body extruded; F, Splitting of first polar body. Ovium still contains 4 rods; 
F, Second polar body extruded : Ovium mature with 2 rods. 
the true ova, and, second, the abortive ova (polar cells), incapable of 
development. In both ova and spermatozoa the nucleus contains but 
one half the chromatin which a typical nucleus contains; in the case 
of A. megalocephala each of the germ cells contains but two chroma 
somes while the normal body cells contain four. The manner iu which 
this maturation of the germ cells for conjugation is brought about is 
beautifully shown in these diagrams, taken from Weismann’s essay, 
“ Amphimixis.” You observe that the number of chromasomes in the 
primary getm cells is four (Figs. II and 12, A). Then are formed by 
subdivision the ovum and sperm “mother cells,” in which the chro- 
matin substance is doubled, so that we observe eight chromasomes 
The mother cells then divide and the chromatin is reduced to tour 
rods, a second division rapidly follows whereby the chromatin is reduced 364 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
to two rods, or half the original quantity. These last divisions take 
place by karyokinesis, but, as Hertwig points out, they differ from 
typical karyokinesis in the fact that the divisions follow so rapidly 
upon each other that the vesicular resting period of the nucleus is 
omitted. Thus, he suggests, is prevented an overaccumulation of 
chromatin substance prior to the fusion of the ovum and sperm. 
Fig. 12. 
Spermatogenesis in ascaris.—(From Weismann after Hertwig.) A, original germ cell—4cliro- 
matin rods; B, spernt mother cell—8 rods; C-D, first daughter cells with 4 rods each; E-F, forma- 
tion of second daughter cells, or mature spermatozoa, with 2 rods each. 
It is evident that the polar cells are rudimentary ova, which do not 
possess the yolk mass, etc., essential to development, and are divider 
off at a very late stage, sometimes after the egg has left the ovary, 
but are in other respects analogous to the spermatozoa. The reason 
these polar-cells have not disappeared altogether in either plants or 
animals is that they originally possessed a deep physiological impor- 
tance. As the first polar cell subdivides and forms two, it follows that 
from both ovum and sperm mother cells four daughter colls are formed, 
each containing half the chromatin substance of a normal nucleus. In 
the ovary three of these daughter cells abort and the fourth forms a 
true ovum; in the sperm gland, however, all four daughter cells form 
spermatozoa. 
We may thus consider the polar-cell problem as in all probability set- 
tled; the whole process is probably an inheritance or survival of a 
primitive condition in which all four ova, like the four spermatozoa, 
were fully functional. 
The relation between the chromatin and heredity.—We have just 
seen that the last stages in the preparation of the ova and spermato- 
zoa for conjugation result in halving the number of rods in the original 
germ cells. Now, as Hertwig and Weismann point out, one point is PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
365 
still left in doubt. Why is the chromatin substance doubled in the 
mother-cells so that two successive sub divisions are necessary to reduce 
it to half the original quantity! Hertwig has not attempted to answer 
this question, as he prefers to wait for further research. Weismann, 
however, who is unfortunately cut oft' from research by failing eyesight, 
has offered a speculative solution to this problem which he trusts may 
guide future investigation. 
This leads me to say a few words in regard to liis conception of the 
relation of the chromatin to heredity. (1) His ffrst premise is that in fer- 
tilization there is not a fusion of chromatin, but that a certain independ- 
ence is preserved between the maternal and paternal elements, based 
upon the observed fact that the two pairs of rods do not fuse but lie side 
by side, and upon the assumption that these pairs are kept distinct in 
each cell through all the subsequent stages of embryonic and adult devel- 
opment. If this is the case, the hereditary substance contributed by the 
father would remain separate from that contributed by the mother, 
throughout. (2) u Each of these pairs would be made up of the col- 
lective predispositions which are indispensable for the building up of 
an individual, but each possesses an individual character, for they are 
not entirely alike. I have called such units * ancestral plasms,’ and 
I conceive that they are contained in numbers in the chromatin of the 
mature germ cells of living organisms, also that the older nuclear rods 
are made up of a certain number of these. - - - Obviously these 
units can not become infinitely minute; however small they may be 
they must always retain a certain size. This follows from the extremely 
complicated structure which we must without any doubt ascribe to 
them.” These units are not, however, ultimate, they are in turn ex- 
tremely complex, and are composed of countless biological units of the 
kiiul conceived of by Nageli and others. (3) The reduction of the chro- 
matin only acquires a meaning when taken in connection with the 
above supposition of distinct ancestral plasms, and has no meaning 
if we accept IIertwig’s view that there is a complete fusion of ma- 
ternal and paternal germ plasm. This meaning is that reduction in 
the maturation of germ cells is sui generis, it does not divide the ances- 
tial plasms into two similar groups, but one daughter-cell receives one 
set of germ plasms or hereditary predispositions, and another daughter 
cell receives another; reduction is thus differential. According to this 
view the four sperm and ovum daughter-cells would each contain a 
different set of ancestral plasms. (4) The fact that the chromatin sub- 
stance is doubled in the sperm and ovum mother-cells, so that we ob- 
serve double the number of rods characteristic of the species, is to be 
explained as an adaptation to the requirements of natural selection, 
for tins doubling and subsequent double division render possible an 
infinite number of combinations (as many, in fact, as there are individ- 
uals) for selection to operate upon. 
This explanation of Weismann’s is an example of his apotheosis of 366 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
the theory of natural selection. Every process is made to suit this 
theory, which, as we have seen in the first and second lectures, is, in 
his opinion, the exclusive factor of evolution. But this very high de- 
gree of mingling and re-mingling of ancestral pre-dispositions would be 
fatal to evolution, for after a combination favorable to survival had 
been established in one generation it would be broken up into a new 
combination, perhaps unfavorable, to survival, in the next generation. 
This entire essay upon “Amphimixis,” or the theory of mingling of re- 
duced hereditary substance, will, I believe, mark a turning point to 
decline in Weismann’s influence as a biologist. His whole reasoning 
is now in a circle around the natural selection theory. 
The meaning of conjugation.—Weismann looks upon sexual repro- 
duction as designed to mingle hereditary tendencies and to create in- 
dividual differences whereby nal ural selection may form new species. 
It is evident that these combinations must be mainly fortuitous and 
productive of indefinite variation; but we have seen that evolution 
advances largely by the accumulation of definite variations, or those in 
which each successive generation exhibits the same tendencies to de- 
part from the typical ancestral form in certain parts of the body, and 
that these tendencies stand out in relief among the diffused kaleido- 
scopic or fortuitous anomalies. 
The fact moreover that variability and evolution by the accumilla- 
tion of certain variations in successive generations is also observed 
in organizisms which reproduce asexually, both among plants and 
animals, shows that we must look in another direction for the under- 
lying cause or purpose of sexual.reproduetion. Weismann rightly com- 
bats the old idea of “ vital ization” of the ovum by the spermatozoon, 
and it is perfectly evident from the researches of Maupas and Hertwig 
that the ovum may as accurately be said to vitalize the spermatozoon 
as the reverse. Fecundation is simply the approximation of two 
hereditary substances of distinct origin and their incorporation into a 
single nucleus. The action and re action of these substances may be 
considered equal and mutual, so far as we now know. 
The remarkably ingenious experiments of Hertwig and Boveri, above 
alluded to, strengthen this idea. Some years ago Weismann wrote: 
“If it were possible to introduce the female pronucleus of an egg into 
another egg of the same species, immediately after the transformation 
of the nucleus of the latter into the female pronucleus, it is very prob- 
able that the two nuclei would conjugate just as if a fertilizing sperm 
nucleus had penetrated. If this were so, the direct proof that egg 
nucleus and sperm nucleus are identical would be furnished.” Boveri 
succeeded in accomplishing a similar feat by depriving an ovum of its 
nucleus and subsequently causing it to develop by admitting a spenna- 
tozoan which fertilized the denucleated ovum and produced a complete 
individual. 
In opposing the vitalizing properties of the sperm, Weismann how- PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
367 
ever went further, and advocated the view that there is nothing in the 
nature of vitalization or “rejuvenescence” in conjugation—that, given 
proper environment, protoplasm is immortal, and runs upon a course 
of undiminished activity. This we have seen is not the case in the in- 
fusoria, and, as recently remarked by Hartog, there is only one class 
of organisms which, according to our present knowledge, are com- 
pletely agamous and immortal—namely, the Monadina. It may in 
future appear that even in the monads there is a cycle for the develop- 
ment in which conjugation plays its part. 
Maupas’ experiments seem to establish the primitive, and therefore 
the true, interpretation of the purpose of conjugation as well as of sex, 
the latter being a consequence of the former, namely, that after a long 
period of direct subdivision of hereditary material from a single in- 
dividual, a limit is reached beyond which the forces of heredity are 
not reproduced in their original intensity unless combined with another 
set of similar forces of different origin. This combination restores 
the original intensity. It is objected to this that two sets of feeble 
forces can not constitute one vigorous force, but this is met by the 
observed fact that such union does start a new life cycle, and is there- 
fore rejuvenescent. We may regard this as the fundamental meaning 
of conjugation, and the production of variations as entirely secondary. 
The distribution of the chromatin.—We have now reviewed some 
of the main phenomena of fertilization; there still remains the relation 
of the hereditary substance to the future development of the individ- 
ual. There is, first, the astonishing fact that, as the chromatin goes 
on dividing, its mass or volume remains apparently nndiminished; 
that is, there is apparently as much chromatin in one of the many 
million active cells of the body as in the original fertilized ovum, and 
there is still an enigma as to the nature of this chromatin and its 
functions. Secondly, there is the problem of the maternal and pater- 
nal elements in each cell; do they lie side by side or are they fused? 
1. In plants De Tries* and others believed that all or by far the 
greater number of cells in the plant body contain the total hereditary 
characters of the species in a latent condition. Kollikert has fully 
discussed this question and called attention to Muller’s early views that, 
in spite of the physiological division of labor producing the tissues, 
the properties of all the tissues can be derived from the nuclear sub- 
stance of a single tissue, as proved by experiments upon the lower 
animals. Weismann, on the other hand, has held that the course of 
development is marked by a constant qualitative distribution of his 
germ-plasm or hereditary substance, so that, so far as nuclear content 
is concerned, there are three forms of cells: (1) with nucleo-plasm; 
(2) with nucleoplasm and germ-plasm; (3) with germ-plasm only. 
“ Hugo de Vries: Pangenesis. Jena, 1889. 
t Die Bedeutung dev Zellkerne fiir die Yorgauge der Vererbung, Zeit. f. Wiss. 
Zool.; 1885. And, Das Karyoplasxna uud die Yererbung, op. cit., 1886. 368 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
Kolliker opposes this idea and maintains that the “idioplasma” passes 
into all cells, in which it divides in course of development. Step by 
step from the embryonic layers to the tissues, the constructive proc- 
esses are under the direction of the nuclei containing this hereditary 
substance. It remains in every nucleus for along period unaltered, in 
order to finally, here earlier, there later, impress its constructive forces. 
In certain elements, as in blood corpuscles, epidermal scales, etc., it 
disappears, as the last product of division. 
It. Hertwig' takes a similar view. Since embryonic and adult cell 
division is differential, there must be a form of differentiation in the 
nucleus; but this does not consist in the total elimination of some 
qualities and survival of others, nor of a reduction in mass. The 
mass and the properties remain the same in every cell; the differen- 
tiation consists in the activity of certain elements in certain tissues. 
Thus we may say with De Tries that different “pangene” may leave 
the nucleus and enter the cell in different tissues; or with Nageli, that 
special “micella)” come into activity at certain points; in other words, 
the potential of the nucleus is differently exerted. Here again we 
have the idea of patent and latent hereditary elements, such as appear 
in the entire individual upon a larger scale. 
This is one of the most interesting problems for future investigation, 
but the direction of research will, I imagine, cover a larger area of 
cell content than the nucleus, as we are now swinging back to regard 
the extra-nuclear archoplasin as an important factor in the process. 
In the following paragraph Hertwig expressed his view of nuclear 
control and cyto plasmic differentiation: 
“As I saw in the transformation of the nucleus during fertilization 
proof that it is the bearer of hereditary substance, 1 recognized a great 
advance in the fact that the nucleus leaves in the same form in every 
cell, and in its vesicular capsule is somewhat removed from the metamor- 
phoses of the cells. As Nageli spreads his idio-plasm as a net work 
throughout the whole body, so, according to my theory, every body- 
cell contained in its nucleus its quota of hereditary substance, while its 
specific histological peculiarities were to be regarded as its plasma 
products. ” 
2d. The next question is the fate of the maternal and paternal con- 
tributions to the embryo. Here there is a wide difference of opinion. 
On the one side Van Beneden is the leader of those who regard each 
cell of the body as in a sense hermaphrodite; as we have seen, his views 
of maturation and the significance of the extension of the polar bodies 
were colored by this theory, for he regarded the germ cells as hermaph- 
rodite until one sex was eliminated. But now that the researches 
of Hertwig have given the last blow to Van Beneden’s theory, and it 
follows that there can be no male and female cliromasomes, there still 
remains room for the analogous view that the maternal and paternal 
cliromasomes remain distinct throughout the course of development, 
not as sexual elements, but as substances with the same racial and 369 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
specific but different individual tendencies. Rabl, an eminent em- 
bryologist, shares this view, and it is supported by Boveri upon the 
observation that in each division the paternal and maternal elements 
are kept distinct, and in Ascaris, for example, two of the chromasomes 
of each division ligure are paternal and two are maternal. 
In favor of this hypothesis we may place the following facts: 1st, 
that there are an even number of chromasome rods in all cells; 2d, that 
the number is constant throughout all the subsequent changes in the 
tissues; 3d, that the number is fixed for each species or variety; 4th, 
that the number is the same in each sex. 
Against this replacement hypothesis we must consider the extreme 
complexity of the division process, and the long-resting, or thread 
stage, in which the chromatin lies in a confused coil. Further, Hert- 
wig argues that if the elements are distinct we should find some evidence 
that the maternal or paternal part is atrophied or replaced, or excluded 
from the nucleus, for both parts can not share alike in the control of 
the cell. These are Hertwig’s grounds for supporting the “verschmel- 
sungstheorie,” or fusion theory, also advocated by Waldeyer, to the 
effect that by the complete union of the maternal and paternal sub- 
stance a new product is formed; in this fusion the law of pre-potency 
may come into play, causing one or other of the parental tendencies to 
predominate, or there may be an even re-distribution, whereby, as ex- 
pressed by Hensen, “the hereditary substance of the son is not that 
of the father plus that of the mother, but is his own, with a new heredi- 
tary form resulting from the combination.” 
While suspending judgment between these two views as to the sep- 
aration or fusion of the chromatin, we may appeal to the external 
phenomena of heredity for light upon the probabilities in the question 
First, I refer to the very decided opinion of Francis Gfalton in regard to 
particulate inheritance; he is so impressed with the fact that we are 
made up bit by bit of separate structures derived from different ances- 
tors that he has even suggested that the skin of the mulatto may 
represent not a fusion of white and black, but an excessively line 
mosaic in which the colors are so distributed as to give the appearance 
of blending. We do sometimes observe patches of color as evidence 
of uneven distribution. As Galton distinguishes two types of structures 
with reference to inheritance, viz, those which blend and those which 
do not blend, we might correlate these types with pre-potency, replace- 
ment, and fusion. Where characteristics do not blend, as in eye-color, 
it is evident that, while the offspring must receive from both parents 
the material basis for the formation of the complete color of the eye, 
either the maternal or paternal material must be prepotent and ex- 
clude the development of the other; the logical inference is that the 
former activity replaces the latter; but it is not necessary that exclusion 
from the cell chromatin should follow. Aow, while some blends seem 
to support the theory of fusion, the sum total of facts of heredity are 
H. Mis. 114 24 370 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
strongly against this as a universal principle, for many maternal and 
paternal structures are preserved in their absolute integrity for gener- 
ations without the least indication of mixture. 
Cell forces and heredity.—We have thus far been considering only 
the chromatin as the heredity substance par excellence, and have dis- 
regarded for the time the archo-plasm or dynamic material of the cell. 
If we advance upon the hypothesis that a typical cell contains the 
more or less passive chromatin, and the archoplasm playing upon this 
chromatin in course of every phase of re-distribution, it seems d priori 
improbable that elements which are associated with every vital change 
should be dissociated in the phenomena of heredity. We might suppose 
that the mechanics of karyokinesis are exactly similar in every cell 
of one individual, but it is highly improbable that they should be ex- 
actly similar in two individuals. We should therefore anticipate the 
joint transmission of the chromatin and archoplasm, implying by the 
latter the dynamic centers especially connected with hereditary function 
as distinguished from the general functions of metabolism. 
This leads us to look for evidence from the life of the cell in its 
totality. We owe to Dr. Max Verworn* a fresh treatment of this 
subject, based upon experimental researches among the Infusoria, 
mainly by the extirpation method. As his experiments included only 
the phenomena of living cells in which the chromatin substance was 
of course undifferentiated to the eye, he treats of the nucleus as a 
whole without distinction as to chromatin and aeliromatin. He con- 
cludes that the physiological importance of the nucleus is exhibited 
in its constant interchange of materials with the remainder of the cell 
body; only through this interchange does it influence the cell and 
control its life processes. The interchange is in triple currents, «, 
from outside of cell to cyto-plasm; b, from cyto plasm to nucleus; c, 
from nucleus to cyto plasm. These movements of interchange are the 
expression of life phenomena. He compares the role of the nucleus to 
that of a cell organoid, like chlorophyll, as not constantly present but 
as invaribly necessary to activity. Thus he believes even the most 
lowly organized cells have nuclear centers, and that even bacteria are 
differentiated into nuclear and extra-nuclear areas. Coupled with this 
idea of nuclear control is the somewhat paradoxical statement that 
the nucleus is not a dynamic center, either automatic or regulating, 
and the conclusion that the nucleus alone can not be the seat of fer- 
tilization and heredity, but both the nucleus and extra-nuclear pro- 
toplasm must constitute the material basis of heredity. This conclusion 
is in the direction of the general reaction of opinion which is now 
taking place against the centralization of cell-government in the nucleus. 
Vague as they must necessarily be, our ideas of cell forces are some- 
what further defined by the brilliant experiments of the Hertwig 
* “Die Physiologische Bedeutung des Zellkerns,” Arcliiv fur Physiologic, 1891, pp. 
113-115. PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
371 
brothers upon germ cell physiology and pathology, which are full of 
suggestion as to the causation of abnormalities in inheritance. These 
were begun in 1884, and were first directed to the influence of gravita- 
tion upon the planes of embryonic cell division, following up the ex- 
periments of Pfltiger and Eauber. In 1885 the conditions of bastard 
fertilization were studied ; in 1887 the causes of polyspermy or mul- 
tiple fertilization; and in 1890 the effects of extreme heat and cold 
upon germ-cell functions.* In general the conclusions reached were 
that in the normal state there exist regulating forces in the ovum which 
prevent multiple fertilization or bastard fertilizations (i. e., by sperma- 
tozoa of other varieties), but these forces are neutralized where the 
life energy of the cell is diminished by reagents or by extremes of 
temperature. 
For example, in the normal state the entrance of a single sperma- 
tozoon produces a reaction in the ovum wall preventing the entrance 
of other spermatozoa, but when the ovum is weakened by chloroform 
solution two or more spermatozoa enter before the reaction appears; in 
fact that degree of polyspermy is directly proportional to the intensity 
of the chemical, thermic, or mechanical disturbance of the ovum. 
Double fertilization or over-fertilization has not in a single case resulted 
in the production of twins, so that Fobs supposition is negatived, 
although other forms may behave differently. The cell function may 
be arrested at any stage by thermic influences; thus two pronuclei, 
paternal and maternal, about to unite, can be held apart by lowering 
the temperature. Polyspermy also results from a lowered tempera- 
ture. It is noteworthy that the conditions of bastard fertilization and 
polyspermy are different; chloroform produces the latter but not the 
former. Kupffer has, I believe, succeeded in producing twins, or 
rather two headed monsters, by abnormal fertilization in fishes. 
These researches, although made with a different object, re-establish 
the older views as to the inter dependence of nuclear and extra-nuclear 
activities, and show that no sharp line of demarcation of function can 
be drawn between the nucleus as a center of reproduction and heredity 
and the cyto plasm, as the seat of tissue building and nutrition. In 
Boveri’s discovery of the areho-plasmie centers, or centrosomes, we find 
positive ground for this broader view. It is connected with the cell 
phenomena of heredity in the following manner: 
While the union of the nuclei in fertilization is the most obvious 
feature, this union is dependent upon the archoplasm, which re-arranges 
the nuclear elements. If the spermatozoon contains no archoplasm, 
this power can not come from the parental side; but Boveri shows that 
this is probably not the case and that the spermatozoon brings its cen- 
trosome with it, thus entering the ovum with both the parental chro- 
matin substance and dynamic material. It is certain from this and 
*Experimentelle Untersnclnmgeu iiber die Bedingungen der Bastardbefruclitung. 
Jena, 1885. See series of papers in Jenaische Zeitschrift. 372 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
from the observations of Roux that the sperm cell is now to be regarded 
as more than a mere nucleus, that it contains both nuclein and para- 
nuclei n. 
Intercellular forces.—The forces within the different portions of the 
cell lead us to consider those which must exist between different cells. 
This is an obscure question at present ; but, as I have observed in the 
close of the second lecture, it is an extremely important one in connec- 
tion with the problem of heredity. 
As Prof. Wilson writes: “My own conviction steadily grows that the 
cell is not a self-regulating mechanism in itself, that no cell is isolated, 
and that Weisman’s fundamental proposition is false.” 
it is a long step between an a priori conviction and the demonstra- 
tion by experiment of a correlation of forces between the cells. This 
seems to me a most important field of experiment. We have seen in 
Maupas’s work that the contact of two infusoria initiates a rapid 
series of internal changes; we have only to conceive of analogous 
changes taking place when two cells are not in actual contact, as in 
the phenomena of previous fertilization referred to in my second lec- 
ture. Hertwig and others have shown how gravitation is related to 
cell activity. Roux has destroyed half an embryo with a hot needle in 
the first stages of segmentation and followed the other half through 
the stages of subsequent development. Another clever experimenter 
has turned fertilized ova upside down during the early stages of 
development, and shown how the protoplasmic pole and yolk pole 
forcibly change places. Driesch has traced the connection and mean- 
ing of the first plane of cleavage in the embryos of echinoderms, and 
has succeeded in raising a small adult from half an embryo artificially 
separated during the first cleavage stage. Wilson, in the larva of 
Nereis, has shown how a certain stage of division in one group of cells 
affects all the other groups. All these experiments are in the line of 
determining the relations which exist between internal cell forces and 
other natural forces. What we must now seek to determine is the 
relation of cell to cell throughout the body, in connection with the 
phenomena of heredity. 
Conclusions.—Perhaps the most impressive result of our review of 
recent researches in evolution and heredity is the uniformity of life 
processes throughout the whole scale of life from the infusoria to man. 
The most striking analogy is that seen in the laws of fertilization and 
conjugation, which are shown by Maupas’s researches to have been 
established substantially in their present form at a very early period in 
the evolution of living organisms. Such uniformity furnishes a powerful 
argument for the advocates of the study of biology as an introduction to 
the applied science of medicine. Much that is now entirely omitted 
from medical education, because it is considered too remote, is in reality 
at the very roots of the scieuee, To understand the disorders of life PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
373 
we should first thoroughly understand the essential phenomena of 
normal life. Of course we shall never see life as it really is, because 
there is always something beyond our highest magnifying powers; but 
we come nearest to this invisible form of energy when, with such 
investigators as Hertwig and Maupas, we strip the life processes of all 
their accessories and view them in their simplest external form. 
The problems of evolution are found to be inseparably connected with 
those of heredity. No theory is at all adequate which does not explain 
both classes of facts, and we have seen that the explanations offered 
by the two opposed schools—those who believe in the transmission of 
acquired characters and those who do not—are directly exclusive of 
each other. We should suspend judgment entirely rather than cease 
to gather from every quarter facts which bear upon the most important 
and central problem of the transmission of acquired characters. I 
have endeavored to point out the opportunities which medical practi- 
tioners enjoy of contributing evidence upon this mooted question. It 
must not be forgotten that while the inheritance of individual adapta- 
tion to environment is the simplest method of explaining race 
adaptation such as we observe in the evolution of man, we know abso- 
lutely nothing of how such inheritance can be effected through the 
germ cells. We can not at present construct even any form of working 
hypothesis for such a process. On the other hand, we have found how 
untenable is the alternative theory offered to us by Weismann, that it 
is solely natural selection or the survival of the fittest which 
“ - - - .shapes our ends, 
Rough hew them as we will.” 
At the same time Weismann’s conception of a continuity of germinal 
protoplasm, which we have found to consist in chromatin plus archo- 
plasm, helps us over many of the phenomena of heredity, especially on 
the retrogressive side, and if it were not that we must also account 
for progressive and definite transformation in heredity, we might credit 
the distinguished Freiburg naturalist with having loosened the Gor- 
dian knot. 
In summing up, the order of treatment followed in the lectures may 
be reversed, and we can begin with the germ cells, and condense the 
more or less ascertained facts. 
The germ cells: 
(1) The material substance of hereditary transmission is the highly 
coloring protoplasm, or chromatin, in the nucleus of the germ cells, 
probably connected with a certain form of arclioplasm, or dynamic 
protoplasm outside of the nucleus. 
(2) Before conjugation and fertilization the hereditary substance 
of both the male and female cells is reduced to one-half that found in a 
typical cell. This substance is however first doubled and then quar- 
tered, the meaning of which process is not understood. 374 
PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 
(3) There is a difference of opinion as to whether the paternal and 
maternal hereditary substances, during fertilization, are fused or lie 
side by side; also as to how the substance is distributed through the 
tissues, whether en masse or by qualitative distribution. 
Heredity: 
(4) No connection between the germ cells and body cells is known, 
but the facts of heredity seem to render such a connection theoreti- 
cally necessary. Several classes of facts connected with reproduction 
seem to support this theory. 
(5) The facts of heredity support the theory of a continuous and 
specific form of protoplasm as the basis of repetition of type. 
Evolution : 
(6) The facts of evolution, both in present and past time, point to 
transformism by definite progression toward new types of structure 
in succeeding generations, opposing the retrogressive forces of heredity. 
(7) The theory (natural selection) of definite progression by the 
accumulation of fortuitous favorable variations is found to be not only 
theoretically improbable, but not to correspond with the observed laws 
of variation. . 
(8) The laws of variation (anomalies) lend support to the theory of 
hereditary transmission of individual acquired variations, but even this 
(Lamarckian) theory encounters many difficulties. 
I think this is as fair a statement as can be made at the present 
time, and it rests upon a general survey of the whole field.