134 THE THEORY OF EVOLUTION 



and marmosets are but remotely related to the mon- 

 keys and apes of the Old World; the horde of most 

 curious creatures which we call ant-eaters, sloths 

 and armadillos, are altogether peculiar to the Neo- 

 tropical region; the extremely large and varied assem- 

 blage of porcupine-like rodents are either peculiar 

 or, in a few cases, related to those of Africa. The 

 many species of opossums are related, though not 

 very closely, to the marsupials of Australia. Aus- 

 tralia excepted, no other continent has so many 

 different kinds of mammals peculiar to itself as 

 South America, and the birds are almost equally 

 characteristic. The second category contains those 

 mammals which are nearly related, often belonging 

 to the same genera, to those which now live, or 

 formerly lived, as shown by the fossils, in North 

 America. This section includes all the beasts of 

 prey, the wolves, large and small cats, weasels, 

 otters, skunks, raccoons; all the hoofed animals, the 

 tapirs, peccaries (native swine) llamas, deer (all of 

 which belong to the Sonoran group), the rats and 

 mice, squirrels and rabbits. 



This twofold division immediately suggests that 

 the mammals belonging to the second category are 

 immigrants from the North and the suggestion is 

 very strongly confirmed by the testimony of the 

 fossils. The geology of the Isthmian region proves 

 that South America was formerly cut off from the 

 northern continent and if we examine the fossil 

 remains of the mammals which lived there during 



