THE GEOGRAPHICAL DISTRIBUTION OF MAMMALS 141 



Dr. Merriam thus sums up the effects of climatic factors 

 upon distribution : ' ' Humidity and other secondary causes 

 determine the presence or absence of particular species in 

 particular localities within their appropriate zones, but tem- 

 perature pre-determines the possibilities of distribution ; it 

 fixes the limits beyond which species cannot pass." ''Con- 

 currently with these changes in vegetation from the south 

 northward occur equally marked differences in the mammals, 

 birds, reptiles, and insects. Among mammals the tapirs, 

 monkeys, armadillos, nasuas, peccaries, and opossums of 

 Central America and Mexico are replaced to the northward 

 by wood-rats, marmots, chipmunks, foxes, rabbits, short- 

 tailed field-mice of several genera, shrews, wild-cats, lynxes, 

 short-tailed porcupines, elk, moose, reindeer, sables, fishers, 

 wolverines, lemmings, musk-oxen, and polar bears." 



Dr. J. A. Allen has reached closely similar conclusions. 

 ''Of strictly climatic influences, temperature is by far the most 

 important, although moisture plays an influential part. Where 

 a low temperature prevails life, both animal and vegetable, is 

 represented by comparatively few forms ; under a high tem- 

 perature it is characterized by great diversity and luxuriance. 

 Within the Arctic Circle the species of both animals and plants 

 are not only few, but they are widely distributed, being for 

 the most part everywhere the same. Under the tropics they 

 are a hundred fold more numerous and of comparatively re- 

 stricted distribution." "The influence of temperature is 

 perhaps most strikingly displayed in the distribution of life 

 upon the slopes of a high mountain, especially if situated near 

 the tropics. While its base may be clothed with palms and 

 luxuriant tropical vegetation, its summit may be snow-capped 

 and barren. . . . The animal life becomes likewise corre- 

 spondingly changed, tropical forms of mammals, birds, and 

 insects of the lower slopes gradually giving place to such as 

 are characteristic of arctic latitudes." " The effect of humidity 

 upon plant life is thus obvious, but it is equally potent, though 



