270 THE PRESENT EVOLUTION OE MAN — PHYSICAL 



can be noted, is observable during all stages of develop- 

 ment, but which is most pronounced in the individual 

 when he reaches, and after he passes, that stage in his 

 development which corresponds to a stage in the evo- 

 lution of his race, when it already had had extended 

 experience of the disease, i. e. to the very last stages in 

 the evolution of the race. 



If the above chain of reasoning be correct, it leads 

 us to the further deduction that, since both the mature 

 and the immature individuals of races that have under- 

 gone evolution in relation to any disease exhibit in- 

 creased powers of resisting it, but the former the greater 

 powers, therefore there should be a greater difference in 

 this respect between the mature and the immature of 

 such races, than between the mature and the immature 

 of races that have had no experience of the disease ; as 

 in the latter case there can have been no evolution 

 against it in particular, though possibly there may have 

 been an evolution against zymotic disease in general. 

 For instance, the susceptibility of negro children to 

 malaria as compared to that of the adults of their race 

 should be greater than the susceptibility of English 

 children as compared to that of the adults of the latter 

 race. In fact, the mature and the immature individuals 

 of a race that has not undergone evolution in relation 

 to any disease, should be very much on a par as regards 

 their powers of resisting it. I am not aware that ob- 

 servations confirming this d priori conclusion have been 

 made, but I have little doubt that any observation that 

 may be made wiU confirm it. It must, however, be 

 borne in mind, that immature individuals are more 

 liable than adults to have their vitality reduced by 

 various disorders which adults, because they correspond 

 to a later stage in the phylogeny, less commonly suffer 

 from, and under these circumstances to fall an easier 

 prey to the prevalent zymotic disease. This source of 



