?62 



NATURE 



[August 20, 1908 



" biological term," but when we speak of filial 

 regression it is. 



He gives full weight to the question of the trans- 

 mission of acquirements, but declares that " some 

 subtle minds have found satisfaction in maintaining 

 that the distinction between an acquired modification 

 and an inborn variation is a distinction without a 

 difference." He is mistaken. It has been maintained 

 merely that the erroneous terms " innate " and 

 " acquired " obscure rather than reveal the very real 

 and immensely important difference between the two 

 classes of characters. 



He discusses the Mendelian experiments which 

 demonstrate that in certain cases descendants tend 

 to reproduce the unlike characteristics of ancestors 

 in the well-known proportion, and alludes to the 

 "increased subtlety of Mendelian interpretation." 

 The facts are compatible with a theory of segregation 

 or with one of latency of the recessive in the pure 

 dominant, and vice versa. He does not mention, how- 

 ever, the crucial instance of the reappearance of 

 latent ancestral characters in pure-bred varieties in 

 which can have occurred no re-union of previously 

 separated colour (or other) factors. 



One of the principal topics discussed is the question 

 of the causation of variations. The evidence is that 

 some are due to the direct action of the environment 

 (nutriment, toxins and the like) on the germ-plasm, 

 while others are spontaneous in the sense that they 

 result from a tendency to vary as much inherent in 

 the germ-plasm as its tendency to grow and divide. 

 But what is the origin of the great mass of 

 variations, those on which racial change is founded ? 

 If variations are usually caused by direct action, then 

 a human race, constantly exposed to a virulent toxin 

 {e.g. that of malaria) or to such a complex of ill- 

 conditions as that found in the slums of great cities, 

 should deteriorate steadily. Natural selection could 

 have no scope, for every generation would be inferior 

 to its predecessor. The race would drift helplessly. 

 If, on the contrary, variations are, with rare excep- 

 tions, spontaneous, and occur all round the specific 

 mean, natural selection has scope, and every race, or 

 section of a race, tends to become resistant to the 

 ill-conditions to which it is exposed. Prof. Thomson 

 holds the first opinion, and draws his arguments 

 mainly from medical sources. From time immemorial 

 doctors have attributed all sorts of filial and racial de- 

 generacy to all sorts of parental mishap. Lately, how- 

 ever, a rapid change of opinion has occurred, as mav 

 be seen by examining the report (just published) of the 

 Royal Commission on the Care and Control of the 

 Feeble-minded. The Commission follow-s Sir E. Ray 

 Lankester, who declares that " no facts are known 

 which support these imaginative teachings." Allud- 

 ing to the rather widely known fact that every race 

 is resistant to ever)' ill-condition precisely in proportion 

 to the length and severity of its past experience of 

 it, it declares that " It is not to be conceived that a 

 race which deteriorates in every generation can emerge 

 from the struggle not weakened, but strengthened." 

 In truth, the hypothesis that variations are usually due 

 to direct action is wholly incompatible with the theory 

 NO. 2025, VOL. 78] 



of natural selection, which, nevertheless, Prof. Thom- 

 son upholds. If additional evidence be needed it is fur- 

 nished by plants, which, when propagated asexually 

 and taken to all climates of the world, hardly vary 

 until the first seminal generation, and then not more 

 apparently than if no such long and diverse exposure 

 of the germ-plasm had occurred. Obviously varia- 

 tions occur normally precisely when they are useful — 

 at the genesis of a new individual when they furnish 

 materials for natural selection. It seems reasonable 

 to conclude, therefore, that they are under the control 

 of natural selection, a superior or inferior tendency to 

 vary being in itself a variation liable to selection. This 

 hypothesis is strongly confirmed by the fact that retro- 

 gressive variations tend to predominate over progres- 

 sive variations — an immensely useful tendency, for, ■■ 

 while useful variations and structures are preserved 

 by natural selection, useless variations and structures 

 are planed away without elimination of individuals. 



When cultivated in non-living media, the parasitic 

 microbes of disease gradually lose their virulence, 

 which is nothing other than the means by which they 

 protect themselves from the cells of the body. Non- 

 virulent saprophytic micro-organisms, introduced 

 under fit conditions into the living body, gradually 

 acquire virulence. In the one case, apparently, retro- 

 gression follows cessation of selection, in the other 

 progression follows selection. The widely accepted 

 hypothesis that microbes " acquire " and transmit 

 virulence in the Lamarckian sense is demonstrably 

 untenable. How could the direct action of the en- 

 vironment on the bodies of the microbes cause them 

 to " acquire " the mechanism necessary for the 

 production of such adaptive and elaborate chemical 

 compounds as toxins? Presumably all parasitic 

 microbes have evolved from saprophytic types. 

 Men have made the microbes of human diseases 

 virulent, and each human disease has made the race 

 exposed to it resistant to itself. \MiiIe races {e.g. 

 British and Negro) which have evolved in conjunction 

 with their familiar diseases {e.g. tuberculosis and 

 malaria) are able to persist when exposed to them, 

 other races (e.g. Polynesians and Red Indians) tend 

 to perish. Disease supplies the only instance in 

 nature in which wc are able to see natural selection 

 actually at work, and the study of diseases reveals a 

 multitude of very beautifully adjusted and unmistak- 

 able adaptations. The facts are not disputed ; the 

 inferences, I believe, are indisputable. Prof. Thomson 

 thinks, however, 



" It would be a subtler and more convincing line 

 of argument to say that, throughout the ages, man 

 has been selecting the microbes, lessening the 

 viiulence, in a sense taming them — sometimes to 

 death — as his phagocytes were strengthened by more 

 suitable food, or as his ' opsonic ' index improved, 

 again also in relation to food." 



He means that man has somehow selected the 

 weaker, the less protected, of his persecutors for sur- 

 vival, that his present food would have been more 

 suited to his ancestors than that which evolution fitted 

 them to consume, and that negroes are more resistant 

 than Englishmen to malaria because they are better 



