EVIDENCE FROM DISTRIBUTION 133 



deer are all different from the northern genera of the 

 family, which came in during the Pleistocene; they 

 too were ultimately derived from the Old World, but 

 at a very much earlier date, probably as far back as 

 the lower Miocene epoch. This Sonoran fauna was 

 greatly impoverished by the Pleistocene extinctions, 

 which put an end to so many of the ancient American 

 groups, but it retains a large number of characteristic 

 forms, chiefly rodents and carnivores, or beasts of prey. 



Besides the immigration from the eastern hemi- 

 sphere, there were successive waves of invaders from 

 the Neotropical region; the earliest one of which we 

 have found a trace arrived in the Miocene and the 

 latest in the Pleistocene, when North America had 

 several characteristic Neotropical forms, the most 

 striking of which were the giant ground sloths, and 

 the huge, armoured, armadillo-like glyptodonts, both 

 now extinct. Of this southern invasion little now 

 remains; the Canada porcupine is certainly one 

 survivor from the invasion and probably, though not 

 certainly, the opossum is another. The influence of 

 climate upon present distribution is very well shown 

 in Mexico; the high table-land, with its moderate and 

 relatively dry climate, is Sonoran, while the hot, low- 

 lying coast-lands are Neotropical. 



When the existing mammals of South America are 

 reviewed, they are plainly separable into two cat- 

 egories. In the first are included those which are 

 evidently indigenous and are, for the most part, very 

 unlike those of any other continent. The monkeys 



