506 THE CHANGING GENERATIONS 



of change was the same in all. By this hypothesis Java-Peking, the 

 Neanderthaloids, and Homo sapiens are merely three evolutionary levels 

 in a universal human stock that was continuously evolving in the direc- 

 tion of modern man. 



This is an attractive, neat hypothesis, but it seems to have serious 

 objections. It requires acceptance of one or the other of two postulates 

 rejected by most biologists — the existence of some orthogenetic directing 

 factor in human evolution or of straight-line selection leading in the same 

 direction in all branches of the human stock. Modern genetics lends no 

 support to the concept of orthogenesis. As for the second alternative, 

 selection is not observed to result in independent parallel changes in 

 isolated populations living in different environments — conditions which 

 were almost certainly characteristic of human populations through much 

 of the Pleistocene. On the contrary, mutation, isolation, selection, and 

 drift operate to produce divergence. 



There is an alternative interpretation of the facts that seems in better 

 accord with the evolutionary views of most biologists. This assumes that 

 until very recently human evolution followed the general pattern seen 

 in other mammals. Spreading accompanied by isolation and divergence 

 led to the formation of various species and races of man. Most of the 

 earlier ones eventually died out, but two more successful lines persisted. 

 One, the more conservative, became Neanderthal man; the other, more 

 progressive, gave rise to Homo sapiens. The latter prevailed in the end, 

 and the Neanderthalers disappeared, though perhaps not without a 

 certain amount of hybridization having occurred between the two. We 

 call these two groups of people distinct species, but possibly it would be 

 more correct to regard them as strongly differentiated subspecies. 



Neither of these interpretations of man's evolutionary history is yet 

 established or disproven; perhaps each contains an element of truth, as 

 may be inferred from the discussions in Chaps. XXIV and XXXI. We 

 have chosen the second upon which to base our diagrams of the relation- 

 ships of fossil and living men. 



