254 INTRODUCTION TO EVOLUTION 



than the other. As a matter of fact, of the genetic characteristics we have 

 mentioned the most "important" is Rh, since in a small proportion of 

 marriages incompatibility in this regard reduces the number of living off- 

 spring producible. There might be some justification for Rh-negative 

 women to develop "race prejudice" against Rh-positive men, and vice 

 versa; in a certain proportion (frequently exaggerated in the public press) 

 of marriages between such individuals difficulty in producing normal chil- 

 dren arises. But there is no justification for race prejudice based on skin 

 color differences. 



The idea that there once existed certain so-called "pure races" was for- 

 merly widely prevalent. A corollary was that the great human diversity 

 observed today arose through intermarriage of these "pure races." From 

 our discussion it will be evident that increasing knowledge of early and pre- 

 historic men affords no evidence of "pure races." In fact, quite the opposite 

 is the case: The more we learn of our predecessors on this planet the more 

 we understand that they were always a highly diversified lot of people. We 

 have seen that "racial" differences in the Pleistocene period were at least 

 as great as they are today. Furthermore, we have emphasized the fact 

 that the genes possessed by modern races were undoubtedly derived from 

 a mixed ancestry of Pleistocene peoples (pp. 246-248 and Fig. 11.16). 



An attempt to visualize the processes at work may help to unify points 

 included in the preceding discussions. When the ancestors of man first 

 descended from the trees, assumed upright posture, and began to use tools 

 they were probably few in number. As they succeeded in their new en- 

 vironmental niche they increased in numbers and migrated out into new 

 territories. These first men were hunters and their social organization was 

 undoubtedly that of small, roving bands and nomadic tribes. As groups of 

 people became separated from each other opportunity was presented for 

 gradual development of genetic diversity. Certain mutations would have 

 occurred in some isolated groups but not in others. By chance some muta- 

 tions would have been lost in some of the groups in which they occurred 

 and would have become established in some other isolated groups (genetic 

 drift, pp. 349 and 439). Mutations which conferred some advantage on 

 their possessors would have been favored by natural selection (pp. 10, 

 351, 450). For example, if some groups entered regions characterized by 

 high intensity of sunlight, mutations increasing the amount of pigment in 

 the skin might have been of advantage (protection from harmful concen- 

 trations of ultraviolet rays) and hence might have been favored by natural 

 selection. Thus in such a region the population might eventually have be- 

 come quite dark in color. If, conversely, other groups entered environ- 



