Acquired Immunity from Cer- 
tain Infectious Diseases a 
Result of Heredity and 
Natural Selection. 
BY 
J. WEST ROOSEVELT, M. D., 
Visiting Physician to Bellevue Hospital, Attending 
Physician to the Roosevelt Hospital, 
New York. 
REPRINTED FROM 
®be Neto ¥ortt JfHeUical journal 
for March 18, 1893. Reprinted from the New York Medical Journal 
for March 18, 1893. 
ACQUIRED IMMUNITY 
FROM CERTAIN INFECTIOUS DISEASES 
A RESULT OF HEREDITY AND NATURAL SELECTION. 
By J. WEST ROOSEVELT, M. D., 
VISITING PHYSICIAN TO BELLEVUE HOSPITAL, 
ATTENDING PHYSICIAN TO THE ROOSEVELT HOSPITAL, NEW YOBK. 
The various theories which have been advanced in ex- 
planation of the protection afforded by one attack of cer- 
tain infectious diseases from subsequent attacks may be 
summarized as follows : 
1. The pathogenic micro-organism is assumed to ex- 
haust the supply of some substance present in the bodies 
of unprotected people, which substance is necessary for the 
nourishment of the parasite, and which, once removed by it, 
is not reproduced by the body. This is called " the ex- 
haustion theory." 
2. The micro-organism is thought to produce within 
the body some substance inimical to its own existence, and 
this antidotal substance, once formed, is not destroyed or 
excreted by the body. This is called " the antidote theory." 
3. It is assumed that in a successful struggle with the 
invading bacteria the body cells acquire an increased 
Copyright, 1893, by D. Appleton and Company. 2 
ACQUIRED IMMUNITY FROM 
strength-become educated, one might say-and thus are 
able to destroy the enemy. This new strength they trans- 
mit to their descendants, so that the body is better able 
than before to repel subsequent invaders. Thus is estab- 
lished " tolerance " to the poison, as it is maintained. 
The first theory may be regarded as untenable for sev- 
eral reasons, among which are the following, which seems 
to me to be fatal: If we accept it we must believe that 
Nature has provided man with a number of different sub- 
stances which can have no conceivable use other than that 
of providing food and accommodation for pathogenic 
micro-organisms. These hypothetical substances are evi- 
dently not needed by the human body, since, ex hypothesis 
bacteria remove them and they are not reproduced; yet 
the patient recovers entirely, and is not at all the worse for 
his illness. It would be hard to believe that this could be 
the case if provision were only made for the growth and 
nutrition of some one species of germ ; but when we are 
ailed upon to believe that the majority of mankind comec 
into the world with a separate and distinct " substance " 
suited to the needs of the micro-organisms of small-pox, 
measles, scarlatina, chicken-pox, vaccinia, yellow fever, and 
a number of other diseases, the imagination is staggered 
and the reason revolts against such a preposterous idea. 
In a sense which will be indicated in another part of this 
paper, and which was not in the minds of its inventors, it 
may be said that there is some truth in the theory. 
A certain degree of plausibility is lent to the antidotal 
hypothesis by the fact that, like all living things, bacteria 
produce by their own vital processes substances which, if 
sufficiently concentrated, are fatal to the producers. The 
great objection to it is that we know of no organic com- 
pound which is not excreted or destroyed by the body 
within a short time after its introduction into the system. CERTAIN INFECTIOUS DISEASES. 
3 
This makes it hard to conceive that any permanent protec- 
tion can be afforded by bacteria] action. 
Of the three explanations suggested-the third, that of 
acquired tolerance-is nearer to the truth than the others, 
yet it does not altogether satisfy the mind. While it is free 
from the objections which apply to the others, it seems to 
me weak in one important respect. It is perfectly logical to 
assume that the power of resistance existing in a body-cell 
should be transmitted to its descendants, in accordance with 
the laws of heredity ; but it is an assumption hardly warranted 
by experience or observation of other biological phenomena 
which abscribes to cells the power of acquiring and trans- 
mitting peculiar resisting powers during a period of stress 
such as must exist under conditions which obtain in the in- 
fectious diseases. It seems to be improbable that such 
should be the result of their tight. 
To me it seems that the objections to the theory vanish 
if we apply the law of the survival of the fittest to the 
problem as well as the law of heredity. Let it be supposed 
that the feebler cells concerned in the struggle are for the 
most part killed. When recovery takes place the body 
will resume its original functional activity, but it will con- 
tain descendants of the cells originally strong enough to 
destroy the poison of the particular disease through which 
it has passed. Naturally the qualities of the parent cells 
are transmitted to their offspring. It is not so much by 
reason of new powers acquired by the stronger as by rea- 
son of the destruction of the weaker cells that immunity 
is afforded. This is much more in accord with Nature's 
methods as actually observed in the whole domain of biol- 
ogy. She seems to prefer to slay the weak rather than to 
leave them to transmit their weakness to others. The 
strong survive because of their strength, and in the end 
this is a benefit, for it tends to perpetuate and improve the ACQUIRED IMMUNITY FROM 
4 
species and elevate the type which composes the majority 
of such species. In the long run it is far better that, rela- 
tively speaking, a few individuals should suffer and perish 
than that the whole number should do so at a later day. 
The survival of the fittest cells in the human body pre- 
serves the whole body from danger, if the theory of im- 
munity set forth in this paper be true. Living cells then 
form the " substance" assumed in the " exhaustion theory " 
to be destroyed and never reproduced. 
The evidence of its truth is largely derived from a con- 
sideration of the action of certain laws of Nature upon liv- 
ing organisms of complex structure-viz., animals and 
plants. It is therefore not to be regarded as entirely 
trustworthy. Reasoning by analogy is not true inductive 
reasoning ; it is not based upon observed facts. Analogi- 
cal evidence should be regarded by the scientist very nearly 
as circumstantial evidence is regarded by the lawyer. The 
latter can not be considered as approaching in value the 
testimony of trustworthy eye-witnesses, but its importance 
increases with each additional demonstrated fact which 
tends to show that a certain allegation is probably correct. 
When a sufficient number of facts have been presented in 
court, which, although they do not directly prove a case, 
make it extremely probable that certain events have oc- 
curred (as the lawyers express it, " show its probability be- 
yond reasonable doubt "), circumstantial evidence amounts 
almost to proof, and has been regarded by the courts as actu- 
ally proof. This should be the position of the scientist in 
respect to any theory which does not rest upon indisputa- 
ble demonstration. The theory must explain all the known 
phenomena, and it must conflict with none of them. If 
based upon analogy, the closer and more evident the anal- 
ogy, the more probable the theory. As time goes on and 
new observations are made in the light of more extended CERTAIN INFECTIOUS DISEASES. 
5 
knowledge, each one which accords with the theory in- 
creases the probability of its truth. When all known phe- 
nomena confirm it and no single one fails to agree with it, it 
may be considered as proved. It matters not whether the 
hypothesis was based upon fact or fancy. 
It seems to me that this hypothesis is a rational expla- 
nation of the immunity conferred by attacks of the diseases 
under discussion. The fact that attacks of some acute in- 
fectious disorders do not diminish the susceptibility to in- 
fection does not, as I think, conflict with it. The reasons 
which justify the last statement will be given in another 
paper. For the present it is desired only to give very 
briefly an outline of this theory in its relation to certain 
others. 
If the doctrine of phagocytosis is correct-if the disease 
process, is a direct conflict between the phagocytes and the 
invading germs-then surely the weaker cells must perish, 
and, when the struggle is over, the body must find in the 
descendants of the stronger a safeguard against subsequent 
invasions. If it is by chemical substances produced within 
the body that the bacteria are overcome, the theory is 
perfectly reasonable. The bactericide necessarily is a 
product of cell activity; it is the direct or indirect ac- 
tion of the living tissue elements which determines the 
constitution of all the secretions and excretions. The 
blood plasma is no exception. If the latter contains as one 
of its normal constituents something toxic to certain forms 
of bacteria when present in sufficient amount, it is because 
some living cells produce it, as part of their life work. If 
the presence of the germ determines the production of such 
a substance, not previously existing in the plasma, it is 
still by cell activity that it is made. If recovery depends 
upon speedy elimination of the bacteria or some product of 
their growth, or something necessary for their nourishment, 6 
ACQUIRED IMMUNITY FROM 
again it must be accomplished by body cells. It might be 
that the micro-organisms of some diseases can only flour- 
ish by killing certain cells, directly or indirectly. What- 
ever be the method of attack or defense, the struggle is 
between germ and cell. The living tissue elements which 
are least able to withstand the stress of the conflict must 
succumb sooner than their stronger fellows; it matters not 
whether the weakness results from lack of sufficient de- 
structive power as a phagocyte, or of sufficient power to 
produce an unusual quantity of some bactericide, or of 
adaptability to changed environment sufficient to manu- 
facture some offensive or defensive substance different 
from that previously produced, or to withstand the delete- 
rious effects of some product of bacterial life, or to remove 
some substance necessary for bacterial nourishment, or to 
do any work in the fight of any sort. 
It may be objected that there is no proof that cells in- 
herit qualities possessed by their ancestors. There is no 
direct proof; but to deny the fact would be equivalent to 
asserting that the whole is not equal to the sum of all its 
parts, for the hereditary peculiarities of animals are admit- 
ted, and animals are composed of multitudes of cells. If 
the latter do not inherit and transmit certain peculiarities, 
how can the former ? If spermatozoon and ovum are able 
(as they unquestionably are) to influence so powerfully the 
development of the entire body as to cause physical or 
mental characteristics to recur generation after generation, 
it is impossible to conceive that this result can be produced 
unless every generation of their descendants (the body cells) 
receives and transmits hereditary traits. 
It seems equally inconceivable that the law of the sur- 
vival of the fittest, which is of universal application 
throughout the whole animal and vegetable world to each 
individual animal or vegetable of every species, should fail CERTAIN INFECTIOUS DISEASES. 
7 
to apply to every cell forming part of these individuals. 
No reasonable explanation can be (or, at all events, has 
been) adduced of the existence of such an anomaly in 
Nature. 
It is a much more plausible supposition that qualities 
already possessed congenitally by the cells should be 
transmitted to descendants than that those acquired in a 
short, fierce struggle should be transmitted. Indeed, the 
possibility of the transmission of acquired traits is denied 
by some biologists. Of the congenital traits there is no 
doubt. 
' Of this theory it may be said, at least, that it is very 
fascinating. Is it not also suggestive and plausible ? Is 
it not worthy of consideration as a working hypothesis, if 
nothing more ? In another paper I shall discuss in detail 
the application of it to a number of diseases, and also to 
the protective effects of inoculations with attenuated virus 
and with vaccine. ft REASONS WHY 
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