Page:George Archdall Reid 1896 The present evolution of man.djvu/281

is plainly proved by the circumstance that the young of various animals are less resistant to zymotic disease, artificially induced by means of an attenuated virus, than adult animals of the same species; for instance, young guinea-pigs are less resistant to the attenuated virus of anthrax than are older animals. The popular belief that the immature are more susceptible than the mature appears, therefore, to be well grounded. It is difficult at first sight to perceive the reason for this difference. In the young, as in the old, phagocytes are present, and presumably are as capable of performing their functions. The explanation probably lies in the fact that immature individuals correspond to an earlier stage in the phylogeny than more mature individuals, to a stage, that is, when any particular zymotic disease, and indeed zymotic diseases in general, did not afflict, or had not so long afflicted, the race. We know that characters that appeared late in the phylogeny appear late in the ontogeny also, and therefore, on à priori grounds, it is to be expected that the mature are more resistant than the immature to the action of zymotic disease; but, on the other hand, it does not follow, because children in their ontogeny correspond, generally speaking, to a stage in the phylogeny when the race had not encountered zymotic disease, that therefore they should have no powers of resistance at all. Natural Selection, as we know, acts not only at end of the ontogeny, but during the whole course of it, in consequence of which the ontogeny is but a blurred recapitulation of the phylogeny; and therefore evolution is to be observed not only in the adults during any stage of the phylogeny, but also in the immature during all stages of the ontogeny, thus causing them to differ more or less from their prototypes in the phylogeny. Malaria, for instance, by attacking the immature as well as the mature, causes an evolution, which, so far as it