38 Merriam Geographic Distribution of Life. 



gress of species from the south. No mountain range or arm of 

 the sea or other tangible obstacle marks the northern boundary 

 of the semi-tropical fauna of northeastern Mexico where it 

 ends abruptly near the Nueces River in Texas, or the semi-trop- 

 ical belt of Florida where it ends near Tampa Bay on the west 

 and Cape Malabar on the east. 



If the Tropical fauna and flora stopped at the narrow 

 Isthmus of Panama, or even in southern Nicaragua, where 

 the last union of the North and South American continents prob- 

 ably took place, the case would be very different ; but instead 

 of doing this it pushes northward 1,500-2,000 miles and ends ab- 

 ruptly where the most painstaking search fails to reveal any 

 barrier to further extension except an uncongenial decrease in 

 temperature and humidity (see also remarks under change of 

 climate following Pleistocene times p. 44.) 



No more striking illustration could be desired of the potency 

 of climate compared witli the inefficiency of physical barriers 

 than is presented by the almost total dissimilarity of the North 

 American Tropical and Sonoran Regions, though in direct con- 

 tact, contrasted with the great similarity of the Boreal Regions 

 of North America and Eurasia now separated by broad oceans, 

 though formerly united, doubtless, in the region of Bering Sea. 

 Of the thirty-one Boreal genera of North American mammals 

 all but eight, or three-fourths, occur also in Eurasia, and but a 

 single family is restricted to cold-temperate America. This 

 family (the Aplodontidas) is the sole representative of a group 

 approaching extinction, and the accident of its survival (in a 

 single genus and two closely related species) in a very limited 

 area along our west coast can hardly be construed as of much 

 faunal significance. Contrasted with this one family (which 

 ought not to be counted) and eight genera of Boreal North 

 American mammals not occurring in Eurasia, Tropical North 

 America (Central America and part of Mexico, exclusive of 

 the West Indies) has no less than eight families and fifty-three 

 genera not belonging to the immediately adjoining Sonoran 

 Region of the southern United States and the plateau of Mexico. 



THE SONORAN NOT A TRANSITION REGION. 



Before leaving this part of the subject reference should be made 

 to the view recently advanced by some naturalists, notably by 



