STRUCTURE OF THE VERTEBRATES 329 



the higher group. It is one of the finest examples of adaptive 

 radiations known among the mammals. 



The opposite conditions are found among the marsupials which 

 exist in America. INIost of them failed to meet the competition 

 with animals with better brains, and placentae which better 

 equipped the young for the struggle of life. Those which remained 

 were the highly generalized, nocturnal opossums which survived 

 by hiding from their enemies. They eked out a precarious ex- 

 istence while the placentals conquered the earth. The American 

 opossum is skeletally almost identical with the Cretaceous mar- 

 supials; more generalized than any mammal known to Australia. 



Human migrations have been less controlled by natural bar- 

 riers, but human enemies have been a telling influence. The 

 Eskimos have been a thorn in the flesh of anatomists who contend 

 that pigmentation and climate are definitely correlated. These 

 arctic dwellers are a yellow-skinned race with apparently no 

 physical adaptation for their environment. They are a Mon- 

 golia race, but not of the warlike Tartar type. When they mi- 

 grated across the Alaskan chain of islands and settled in the 

 north, they evidently attempted to find a southern home, for 

 relics of their civilization are found along the Pacific coast. War- 

 like Indians drove them back and they remained in the frozen 

 north because they could not fight their way out, not because 

 they particularly liked the neighborhood. 



The same evidence could be adduced as to the location of 

 the pigmies in the valleys of the Amazon and the Congo. Small, 

 low thyroid, unwarlike, they were never a match for the taller 

 tribes of the plains. And when forced into an unfit environment, 

 climate and disease soon selected a race which was adapted 

 for survival. In this way barriers, both natural and human, 

 have largely dictated the course of racial and cultural history. 



