can be noted, is observable during all stages of development, but which is most pronounced in the individual when lie reaches, and after lie passes, that stage in his development which corresponds to a stage in the evolution 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 undergone evolution in relation to any disease exhibit increased 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 observations confirming this à priori conclusion have been made, but I have little doubt that any observation that may be made will 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
