THE NEARCTIC REGION 175 



Toxodon, a member of a peculiar extinct family of 

 Ungulates. 



The occurrence of all these animals indubitably proves 

 that now for the first time a connection had been formed 

 between the continents of North and South America. 

 Before this epoch, no trace of a Neotropical admixture 

 can be anywhere detected in the Nearctic mammal-fauna. 



Thus the evidence of palseontology in every way sup- 

 ports the deductions drawn from a study of the distribu- 

 tion of recent forms, namely, that the bulk of the present 

 Nearctic fauna has been mainly derived from the Old 

 World, although at times the Region has been sufficiently 

 isolated and sufficiently extensive for the independent 

 evolution of its own characteristic forms. In accordance 

 with these deductions, the present remaining inhabitants 

 of the Nearctic Region may be divided into three cate- 

 gories, as follows: (1) The Endemic Fauna, the bulk of 

 which has had, at some considerably remote geological 

 period, a common origin with that of the Palsearctic 

 Region, although it has enjoyed ample time to develop 

 and differentiate itself on its own lines. (2) A Neotropical 

 constituent, which first appeared in the Nearctic Region in 

 Pliocene times. (3) A comparatively modern Palsearctic 

 fragment, in which not only the genera, but frequently the 

 species, are identical in both Regions. This portion of the 

 fauna has probably reached the Nearctic Region by the 

 passage which must have existed in comparatively modern 

 times across Behring Straits. Consequently, while the 

 Neotropical element is the stronger in the south, this last, 

 the Palrearctic element, is far more prevalent in the 

 extreme north. 



