Distinctness of Tropical from Sonoran. 37 



DISTINCTNESS OF THE TROPICAL RP:GION FROM THE SONORAN. 



It has been shown that the fauna and flora of Tropical America 

 reach the United States, though in a somewhat dilute condition, 

 along the lower Rio Grande in Texas, and in southern Florida, 

 and that in the vast majority of cases their genera and species 

 differ widely from those of other parts of America. Except for 

 the presence, chiefly in the southern United States, of a com- 

 iniratively few forms derived from the Tropical region, the fauna 

 and flora of North America are as distinctive and independent of 

 the existence of this area as if separated from it by the broad 

 ocean. Among the eighty-one genera of non-pelagic Mammalia 

 inhabiting North America north of Mexico the number of these 

 intrusive genera is only nine,* as has been shown, and three of 

 these are bats. . These genera are : Didelphis, Tatusia, Dwotyles, 

 Fdi*, Procyon. Nusua, Molossus, Nyctinomus, and Otopterus. Tatusia 

 and Nasua barely reach our southern boundary ; Dicotylcs extends 

 only part way through Texas ; Molossus a short distance into 

 southern California ; Nyctirwmus and Otopterus do not pass be- 

 yond the Lower Sonoran Zone, and Didelphis is restricted to the 

 humid division of the Sonoran. Out of the nine intrusive 

 genera, therefore, but two (Felis and Procyon) reach the southern 

 edge of the Boreal. 



On the other hand, a few groups, such as the wolves, otters, 

 squirrels, and rabbits (genera Canis, Lutra, SciuruSj Sciuropterus, 

 SpermophiliM, and Lepus) occur over large parts of both North and 

 South America, presenting a seeming obstacle to the acceptance of 

 the view that the faunas in question are so wholly dissimilar. But 

 investigation shows that these animals are almost world-wide in 

 distribution, implying great antiquity of origin, and remains of 

 most of them have been found as low down at least as the 

 Miocene strata in both America and Eurasia. Hence it is clear 

 that these types became diffused over North and South America 

 at a very distant period, and their peculiar habits of life, though 

 wholly dissimilar, enabled them to survive the great mutations 

 these land areas have undergone since Miocene times. 



The paucity of species of tropical derivation in North Amer- 

 ica is the more remarkable in view of the absence of barriers of 

 any kind, save climatic conditions alone, to impede the free in- 



*Among birds the number of intrusive forms is greater, as would be 

 expected from their superior powers of locomotion and dispersion. 



