136 THE THEORY OF EVOLUTION 



There is a remarkable contrast between the fates 

 that befell the northern invaders of the Neotropical 

 region, and the southern invaders of the Sonoran 

 region. The former, in most cases, conquered a 

 permanent footing in South America and their de- 

 scendants, more or less modified, are living there 

 now, while the latter could not maintain themselves 

 and only one, or at most two, has survived to the 

 present time. The reason for this very striking 

 difference is probably to be found in the fact that 

 the northern animals were of higher type and more 

 advanced grade than the southern. North America 

 was a part of the vast land-area of Arctogsea, a con- 

 dition much more favourable to the development of 

 the higher forms than was the insular state of South 

 America. Such a conclusion might seem to be con- 

 tradicted by Central America, which is Neotropical 

 not Sonoran, but here the dominant factor is ob- 

 viously climatic, as is shown by the conditions in 

 Mexico. The native and immigrant mammals of 

 North America are Arctic and Temperate and most 

 of them are not adapted to life in the Tropics, yet 

 in spite of this, many have permanently established 

 themselves in South America, while the southern 

 immigrants failed to keep their footing in the north. 



The zoological realms and regions which were estab- 

 lished only to express the present order of things, 

 thus prove themselves to be an expression of past 

 changes. Even were nothing known of the geology 

 and paleontology of the western hemisphere, but 



