8o8 



NATURE 



[August 25, 192 1 



Letters to the Editor. 



(The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake to 

 return, or to correspond tiHth the writers of, rejected manu- 

 scripts inten.li-d for this or any other part of NATURE. No 

 notice is tal^i'n of anovvmous communications.] 



The Natural History of Man. 



In an article on "New Experiments on the 

 Inheritance of Somatogenic Modifications," in 

 Nature of February 3 (p. 742), Prof. Arthur 

 Dendy writes : "It has long been suspected that the 

 problem of the transmission from parent to offspring 

 of somatogenic modifications (' acquired characters ') 

 mig-ht be solved more readily by physiological experi- 

 ments directly involving the complex metabolism of 

 the body than by crude surgical operations, such as 

 the amputation of limbs." He proceeds to tell us of 

 experiments which are thought to demonstrate that 

 when certain toxic substances are injected into the 

 blood of pregnant rabbits a deterioration of the eyes 

 of the offspring sets in, which is transmitted and in- 

 creased generation after generation. 



Now examine the other side of the shield. 

 Let us use a little of the evidence from associated 

 sciences which zoologists and botanists have igr^ored. 

 It has been said by naturalists that inan is a domesti- 

 cated animal, meaning, probably, that man is a social 

 animali like ants and bees. Man is a typical wild 

 animal, living under an enormous variety of con- 

 ditions, which have become perfectly natural to him. 

 At any rate, he is not under artificial selection. Again, 

 it has been said by opponents of Darwin that no one 

 has seen natural selection in operation, and that, there- 

 fore, its existence is a pure guess ; and by yet others, 

 supporters of Darwin, that man has escaped from 

 selection. As a fact, every man, except the biologist 

 as such, has seen natural selection in full blast, and, 

 so far from having- escaped from selection, man is 

 everj'where stringently selected in a glaringly obvious 

 way. Indeed, since we are able to follow the career 

 of men with a completeness that is unique in the 

 animal world, man is the only animal in which natural 

 selection can be observed and its consequences traced 

 to the last little detail. Apart from each man's per- 

 sonal experience and a voluminous literature, it is the 

 principal function of all Departments of Public Health 

 to collect precise statistical information bearing on this 

 very subject. 



Man is the prey of a multitude of living microbic 

 species, which have become parasitic on him, and 

 attack him in all sorts of ways and with every degree 

 of stringency of selection. It is common knowledge 

 that men vary in their powers of resisting various 

 microbic diseases, and that these powers of resistance 

 tend to "run in families " (i.e. are inheritable), as is 

 conspicuously the case in tuberculosis — a fact which 

 is still better observable when we compare men of 

 different races ; for example, West African negroes and 

 Englishmen in respect to tuberculosis and malaria. 

 Again, it is common knowledge that flowers of re- 

 sisting any disease do not necessarily imply p>owers of 

 resisting another disease. 



Here, then, is natural selection indubitably manifest 

 in the only wild species in which observation of its 

 operations is possible. What is the effect on races? 

 Does any change result? If so, does it accord with 

 Lamarckian or Darwinian doctrine? It may be laid 

 down as a rule to which there is no exception that 

 every human race is resistant to every prevalent 

 and lethal human disease in proportion to the 

 length and severity of its past experience of that 

 disease. Here, then, is evolution indubitably manifest 

 as a consequence of natural selection. 



But this recital gives no conception of the fidelity 

 NO. 2704, VOL. 107] 



with which evolution follows selection. Not only does 

 selection by any disease cause evolution against itself 

 alone, but there are also two main types of diseases 

 which select in unlike ways and cause extraordinarily 

 unlike racial effects. In one group (e.g. measles and 

 smallpox}^ the microbes flood the victim with toxins, 

 soaking even his germ-cells. He dies; or, reacting 

 against these toxins, recovers within a definite period, 

 the duration of which, speaking generally, varies with 

 the abundance and virulence of the toxins. Recovery- 

 implies "acquired " immunity, which is often of life- 

 long duration, and is simply a "use-acquirement" — 

 a response to functional activity. The individual has 

 become "used to," or trained by, the toxins through 

 some physiological process, just' as he becomes used 

 to tobacco, or exertion, or the performance of difficult 

 and complex thinking. Practically everyone is sus- 

 ceptible to this class of disease. As a rule, therefore, 

 the survivors are not those who resist illness, but 

 those who recover from it. In the other group (e.g. 

 tuberculosis and leprosy) the microbes retain their 

 poisons within themselves, and the illnesses caused by 

 them are usually prolonged and of indefinite duration. 

 The survivors are, as a rule, not those who recover 

 from illness, but those who resist it, i.e. those who 

 are "innately " immune. In this class of disease in- 

 dividuals vary greatly in resisting power. Thus, in 

 tuberculosis, there are those who seem quite immune 

 under the worst conditions, those who fall ill under 

 bad conditions but recover when the conditions are 

 improved, those who die after lingerinit illness, and 

 those who perish swiftly and, as a rule, in early life. 

 Here there is no "acquired " immunity ,; whoever is 

 infected suffers nothing but injury. 



Plainly in disease we have on a vast scale just 

 those " physiological experiments directly involving the 

 complex metabolism of the body " concerning which 

 Prof. Dendy is so hopeful. If the Lamarckian doctrine 

 be true, diseases of the measles type should, by the 

 "transmission of acquired immunity," render the 

 race less and less susceptible to infection until it 

 acquires "innate" immunity; on the other hand, 

 diseases of the tuberculosis type should enfeeble the 

 race by the accumulation of injury until at last it 

 perished. But nothing of this has happened. On the 

 contrary, racial changes have followed precisely con- 

 trary lines, those of natural selection. Thus English- 

 men who have long been exposed to measles are fully 

 as susceptible to infection as Polynesians, but recover 

 from illness more easily and frequently ; whereas races 

 which have long been exposed to tuberculosisi (e.g. 

 Jews) resist infection much more stoutly than those 

 that have been less exposed (e.g. American Indians). 

 The diseases of animals and plants (e.e. in the fly 

 districts of Africa) tell the same story, but here 

 natural selection cannot be as closely studied as in 

 the case of man. We see only the consequent evolu- 

 tion. Now compare as to volume and duration the 

 minutely studied and easily observed physiological 

 experiments of Nature which Prof. Dendy ignores 

 with those to which he pins his faith. Obviously, if 

 anyone did establish that the injection of a toxin 

 caused hereditary degeneration, he would discover, not 

 a rule, but one of the rarest exceptions in Nature. 



In order to demonstrate the importance of disease 

 selection, it is worth while to pursue this subject a 

 little further. Doubtless there have been many 

 great human migrations, but two especially are re- 

 corded in history — that immense surge of Eastern . 

 people which established in their present sites many 

 of the modern races of Europe, and that still vaster 

 overflow which carried the inhabitants of modern 

 Europe to the Western hemisphere. If history teaches 

 any lesson with clearness, it teaches this — that unless 



