HISTORY FROM A BIOLOGICAL POINT OF VIEW 499 



have long experienced it, but severe to those who first 

 come in contact with it. Among ourselves, for ex- 

 ample, all those strains or groups which could not en- 

 dure measles have long ago perished, leaving those to 

 whom it is a light matter. To the Indians of Alaska it 

 is a different case ; they have undergone no such selec- 

 tive process. To the negroes of West Africa the 

 pernicious forms of malaria, which kill so many Euro- 

 peans, are not pernicious at all. The negro children 

 run about with the parasites in their blood, and are a 

 menace to white people, to whom these parasites are 

 conveyed by mosquitoes. 



II. Great changes in the character of a population Results of 

 will result from differences in the birth or mortality 

 rates, and yet these may be so gradual as to escape 

 observation. Thus if one part of a population produces 

 two children for every pair of parents, and another three, 

 and each starts with 1000 members, the first group at 

 the seventh generation will have 1000 descendants, the 

 second, 11,391. This assumes that the children grow 

 up and become parents ; it takes no account of those 

 who fail to do this, so that the total number of children 

 born is no exact criterion of the vitality of a race. It is 

 easy to see from considerations of this sort that, quite 

 apart from conquests and migrations, changes are going 

 on within the nations themselves. We may pride our- 

 selves on belonging to an ancient people, without realiz- 

 ing that perhaps little is left of that people in those who 

 now bear the name. The biologist will therefore not 

 conclude too hastily that differences in the course of 

 history, in the reactions to environment, are wholly due 

 to external circumstances or the effects of education; 

 they may be due in part to actual changes in the heredi- 

 tary make-up of the group concerned. 



