EVOLUTION OF MAN 251 



productive isolation and (2) smaller amount of genetic difference. Both 

 these attributes are complex variables, since there are degrees of reproduc- 

 tive isolation as well as degrees of genetic difference. As noted above, it is 

 largely because of the absence of reproductive isolation that we conclude 

 that the races of man are races and not species, despite the genetic dif- 

 ferences between them. 



The next point is of such great importance for the understanding of what 

 races are, and what they are not, that we regret lack of space to discuss it 

 in greater detail than will be possible here. The differences between races 

 are of the same kind as the differences between groups of people within 

 races. Much of our deplorable race prejudice would disappear if people 

 generally could come to understand that fact, with all its implications. In 

 recent years a great volume of evidence has been amassed on this point 

 (see Boyd, 1950; Cold Spring Harbor Symposia on Quantitative Biol- 

 ogy, Vol. 15, 1950; Dobzhansky, 1950; Dunn, 195 I ). A few brief examples 

 must suffice us here. 



The blood groups are the human characteristics most thoroughly ana- 

 lyzed as to genetic basis. As we saw earlier (pp. 121-122), racial groups 

 differ in the proportions in which these blood groups occur (i.e., in the 

 gene frequencies involved). So do populations within racial groups. Most 

 American Indians, for example, exhibit a high percentage of individuals 

 belonging to group O. Yet the Blackfoot and Blood tribes in Montana have 

 an unusually high proportion of members belonging to group A. Contrari- 

 wise, groups of people usually considered to belong to different races may 

 be quite similar in their blood group distributions. Thus while a high pro- 

 portion of group B characterizes Asiatic peoples it is also characteristic of 

 Abyssinians, and of Pygmies in the Congo. Eskimos, Portuguese, and Aus- 

 tralian aborigines resemble one another in blood group distributions. We 

 have picked a few examples at random; many others will be found in the 

 references cited in the preceding paragraph. Similar differences and diver- 

 sity exist in the distributions of the other blood ceU substances (M, N, 

 Rh, etc.). A point of great importance for us is the fact that there is no cor- 

 relation between the distributions of these various substances — they vary 

 in frequency independently of each other, and of such characteristics as 

 skin color. 



Another genetically determined trait concerns ability to taste the organic 

 compound phenylthiocarbamide (PTC). Populations in different parts of 

 the world differ in the proportion of persons able to taste this substance. 

 Variability in this regard is independent of the variability in distribution of 

 the several blood group substances. 



