EVIDENCE FROM DISTRIBUTION 131 



North America is a zoological patchwork, consisting 

 of one entire region, the Sonoran, a large area of a 

 second, the Holarctic, and small portions of the 

 third, the Neotropical, while, on the other hand, the 

 southern continent is all included in the Neotropical 

 region. This contrast is explained by the different 

 geological history of the two continents. During the 

 earlier and longer moiety of the Tertiary period 

 North and South America were separated by a broad 

 sea which swept over the Isthmus of Panama and 

 most of Central America and throughout this time of 

 separation the northern and southern continents 

 developed faunas which were most radically different. 

 The student who is familiar with the Tertiary mam- 

 mals of the northern hemisphere, American, Euro- 

 pean and Asiatic, finds himself in a new and strange 

 world, when he takes up those of South America. 

 On the other hand, North America was repeatedly 

 joined to the Old World and as often severed from it, 

 chiefly by way of Alaska and northeastern Asia, but 

 probably also by a land-bridge from Greenland to 

 Scandinavia. It is almost always easy to distinguish 

 the times of separation from those of union, for the 

 former are marked by a rapid divergence from the 

 Old World forms, while the latter, because of the 

 intermigration of mammals, always possess a certain 

 number of families and genera which are common to 

 both hemispheres. The latest of these invasions 

 from Asia took place in the Pleistocene and brought 

 in a crowd of Old World mammals, bears and wol- 



