GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 393 
intrusive genera is only nine,* as has been shown, and three of these 
are bats. These genera are: Didelphis Tatusia, Dicotyles, Felis, Pro- 
cyon, Nasua, Molossus, Nyctinomus, and Otopterus. Tatusia and Nasua 
barely reach our southern boundary; Dicotyles extends only part way 
through Texas; Molossus a short distance into southern California; 
Nyctinomus and Otopterus do not pass beyond 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, Sciurus, Sciuropterus, Spermophilus, 
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 atavery 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 America is the 
more remarkable in view of the absence of barriers of any kind, save 
climatic conditions alone, to impede the free ingress 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-tropical 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 probably took place, the case . 
would be very different, but instead of doing this it pushes northward 
1,500 to 2,000 miles and ends abruptly where the most painstaking 
search fails to reveal any barrier to further extension, except an uncon- 
genial decrease in temperature and humidity. (See also remarks under 
change of climate following Pleistocene times, p. 398.) 
No more striking illustration could be desired of the potency of cli- 
mate compared with the inefficiency of physical barriers than is pre- 
sented by the almost total dissimilarity of the North American Tropical 
and Sonoran regions, though in direct contact, 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 
*Among birds the number of intrusive forms is greater, as would be expected from 
their superior powers of locomotion and dispersion. ° 
