EVOLUTION OF MAN 255 



ments having sunlight of low intensity, mutations decreasing pigmentation 

 of the skin might have been favored by natural selection, since mainte- 

 nance of a certain level of ultraviolet irradiation of the skin is important in 

 providing the body with vitamin D. Consequently in this region a lightly 

 pigmented population might have evolved. 



Many of the differences between races are adaptive in this manner, or 

 were adaptive under the conditions in which the races originated. The 

 exact nature of the adaptation has, however, not been analyzed in most 

 instances. Such racial characteristics as are not adaptive may have become 

 established in these isolated populations by chance, as noted above. The net 

 result of these processes was development of diversity among geographi- 

 cally isolated peoples — the production of geographic races. 



When these diverse peoples came into contact with each other (migra- 

 tion, conquest) they exchanged genes, as discussed previously (pp. 

 244-248) and diagramed in Fig. 11.16. Some of the combinations of char- 

 acteristics thus arising may have proved superior to the characteristics of 

 one or both of the parental populations. If so, natural selection would have 

 favored the new combinations at the expense of the old. Particularly, natu- 

 ral selection seems to have favored the development of bigger and better 

 brains, as we have noted. Such development was of first importance in en- 

 abling man to devise tools and to improve them — in other words, in 

 enabling him to become increasingly a civilized man. 



If our interpretation is correct, then, modern races are descendants of 

 ancient races, but probably no one modern race is the descendant of any 

 one ancient race alone. Our inability to draw any clear-cut lines between 

 races gives added confidence that such is the case. The genes have been 

 continually "reshuffled" as time, in geologic copiousness, has gone by. 



We have sketched in broadest outline the probable course of race forma- 

 tion and racial change, stressing (1) geographic isolation; (2) mutations, 

 and their fate as determined by chance and by natural selection; (3) ex- 

 change of genes between populations. These are among the important fac- 

 tors operative in animal, as well as human, evolution. They receive further 

 amplification and discussion in Chapters 15-21. 



References and Suggested Readings 



Boule, M., and H. V. Vallois. Fossil Men. 4th ed. New York: Dryden Press, 



1957. 

 Boyd, W. C. Genetics and the Races of Man. Boston: Little, Brown «& Co., 1950. 

 Broom, R., and J. T. Robinson. "Further evidence of the structure of the Sterk- 



