THE PRESENT EVOLUTION OF MAN — PHYSICAL 269 



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 corresjDond 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 a '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, gener- 

 ally speaking, to a stage in the phylogeny Avhen the 

 race had not encountered zymotic disease, that there- 

 fore 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 



