142 LAND MAMMALS IN THE WESTERN HEMISPHERE 



less evident, upon animal life. Many animals . . . are so 

 fitted for a forest life, as regards both food and shelter, that 

 their very existence depends upon such surroundings. . . . 

 Thus moisture alone may determine the character of life over 

 extensive regions." 



While climate is thus the most important of the barriers 

 which determine distribution in continuous land-areas, the 

 absence of any particular species from a given region is no 

 proof that the climate is unsuitable to that species. This is 

 sufficiently shown by the manner in which animals introduced 

 into a new country often run wild and multiply to an incredible 

 extent, as the rabbits have done in Australia, the Mongoose 

 in Jamaica, horses on our western plains, horses and cattle 

 on the Pampas of Argentina, etc. 



Topographical features, such as great mountain-ranges and 

 plateaus, also limit many species, not only by the difficulty of 

 crossing them, but also by the effect which they have upon 

 temperature and moisture. For this reason long ranges of 

 mountains and table-lands may carry a northern fauna very 

 far to the south of its ordinary range, as do the mountain- 

 systems of North America in a very conspicuous manner. 

 The great Mexican plateau is zoologically a part of North 

 America, while the low coastal lands as far as southeastern 

 Texas have Central American affinities. 



A different kind of obstacle to the spread of a species into 

 a new area may be the pre-occupation of that area by another 

 species. The pre-occupier may be one that plays so similar 

 a part in the economy of nature as to leave no opportunity 

 for the newcomer to estabhsh itself. On the other hand, 

 the obstructing form may be an active enemy and of a totally 

 different character from the intruder, as in the case of the 

 Tse-tse Fly in parts of Africa. The bite of the fly is fatal to 

 horses and oxen, so that these mammals are unable to enter 

 the fly-infested regions. Many times in the course of the 

 Tertiary period various mammals reached North America 



