CENOZOIC ERA: AGE OF MAMMALS 641 



comprised of (1) marsupials resembling, in a marked degree, those of 

 Australia to-day, and (2) true mammals differing greatly from those 

 that had been developing in North America. The explanation of 

 this peculiar fauna is probably to be found (1) in the absence of all 

 true carnivores (the cat and dog family having, so far as is known, 

 failed to send any Eocene representatives there), and (2) in the small 

 variety of ancestral forms from which the fauna developed. This 

 small number of ancestral true mammals indicates that the Eocene 

 Central American connection had been of brief duration. These 

 South American ancestral forms came from North America, or from 

 Australia by way of the Antarctic Continent, or from both. 



During the period of separation, several families of strange hoofed 

 mammals were evolved to take advantage of the varied physical 

 conditions, some of which (Litopterna) were odd-toed, with bodily 

 proportions resembling those of the horse and llama. In this 

 family the third toe was always the largest, and in some species the 

 evolution of the foot had been carried to the one-toed stage, producing 

 a foot similar to that of the horse. They were, however, inferior in 

 brain and teeth to the even and odd-toed herbivores of North America. 



The most remarkable development occurred in the sloth tribe 

 (edentates) (p. 670), in which huge forms, elephantine in size but of 

 different proportions, some of which were covered with armor, were 

 numerous and conspicuous. 



Rodents of the porcupine type and monkeys were abundant during 

 the Tertiary and are living in South America to-day. 



With the joining of North America and South America, an inter- 

 migration of animals from the two continents began. Horses, 

 llamas, deer, mastodons, tapirs, members of the cat and dog family, 

 and others invaded South America ; and at the same time the giant 

 sloths and other South American forms moved north. The result 

 was to be expected. Not all of the families of North American mam- 

 mals found a home in South America, either because they did not 

 migrate there or because the conditions were unfavorable for their 

 existence; but, as a rule, these immigrants from the north in which 

 brain and limb had been highly developed as a result of the severe 

 struggle with highly specialized carnivorous enemies, as well as be- 

 cause of the competition with other herbivorous forms, soon crowded 

 out the more conspicuous but less highly developed indigenous 

 animals. To-day the most conspicuous South American animals 

 are those whose ancestors reached that continent in the Pliocene, 



