490 ZOOLOGY 



uals who leave no offspring are the true dead from the 

 evolutionary standpoint, the selective agencies must be 

 various. Yet it is evident that the most important 

 factor is disease, particularly the group of diseases due 

 to bacteria. Throughout the centuries, consumption, 

 smallpox, measles, and the rest have attacked mankind 

 and carried off those unable to resist. It is well known 

 to all that the susceptibility to particular forms of 

 disease varies greatly, and while this is partly due to 

 physical condition resulting from environment, it is 

 largely a matter of inherited constitution. This is so 

 true that it used to be supposed that consumption was 

 inherited, whereas we now know that what is inherited 

 is susceptibility to that disease. Even among plants 

 the same rule holds ; thus a particular strain of wheat 

 is readily attacked by the rust fungus, while another 

 is practically immune. The consequence of these 

 selective processes is the survival of those individuals 

 whose heredity is favorable for resistance, and the pro- 

 duction of a relatively immune type. If there are no 

 resistant individuals to be selected, the species may of 

 course become extinct. 



3. The races of mankind afford abundant evidence 

 of adaptation, which we can suppose to have arisen 

 only in the manner described. Thus in tropical Africa 

 the negroes suffer very much less from malaria than the 

 white man, while in the north the white man is more 

 resistant to consumption than the black. There are 

 two different types of adaptation: a race may be im- 

 mune or tolerant. If immune, it does not take the 

 disease, does not harbor the parasite. If tolerant, it 

 may readily become infected but suffers little in conse- 

 quence. In West Africa the negro children carry in 

 their blood the parasite of pernicious malaria, and thus 



