750 



GENETICS AND EVOLUTION 



Figure 36.10. Restorations by Dr. J. H. McGregor of what prehistoric men prob- 

 ably looked like. From left to right, the Java ape-man. Neanderthal man and Cro- 

 Magnon man. (Courtesy of Dr. J. H. McGregor and the American Museum of Natural 

 History, New York.) 



The Negroid races spread south on both sides of the Indian Ocean 

 to Africa and Melanesia. It appears that they, too, displaced more prim- 

 itive races and pushed the Bushmen to the tip of South Africa and the 

 Australoids into Australia. 



The Mongoloids spread east and north, occupying Siberia and 

 China. About 20,000 years ago they crossed the Bering Straits to occupy 

 North and South America. 



There are four basic stocks of modern man, all of which belong to 

 the species Homo sapiens. The Australian aborigines appear to be the 

 most primitive and perhaps have a slightly different line of descent 

 from the others. The other three, the whites, the negroids, and the mon- 

 goloids, are each subdivided into a number of races. A race, whether 

 of human beings, or some other animal or plant, may be defined genet- 

 ically as a population which differs significantly from other populations 

 with respect to the frequency of one or more of the genes it possesses. 

 Or it may be defined phenotypically as a population whose members, 

 though varying individually, are distinguished as a group by a certain 

 combination of morphologic and physiologic characteristics which they 

 share because of their common descent. 



In the course of his evolution from the ape men, man has increased 

 slightly in height but his frame has become much less massive. He now 

 stands completely erect and his head is balanced on a relatively slender 

 neck, instead of jutting forward from the shoulders and being held in 

 place by massive neck muscles. His cranial capacity has increased, the 

 frontal lobes of the brain have enlarged and the skull is more rounded, 

 the forehead is more vertical and the bony ridges over the eyes have 

 become smaller. The face and jaws have become smaller and the re- 



