It is clear from this account that the South American mammal fauna is sharply marked off from all others by the possession of groups of animals peculiar to it, but that it is linked with the North American fauna by the presence of deer, bears, cats and dogs. The condition of South America differs from that of Australia in the increased number and variety of those animals which may be regarded from the existence of their relatives else where, as immigrants into an area which has long been separated. The abundance and varied nature of those mammal groups peculiar to South America, such as the Xenarthra and Platyrrhina, would suggest that these animals had undergone their evolution in isola tion as had the Australian marsupials. It is possible to test the truth of this idea by an investigation of the fossil history of these and other forms, and in South America, unlike Australia, it is possible to do this not only for one horizon but for the whole Tertiary period.
In Pleistocene times South America was inhabited by a fauna far more varied in character and including much larger animals than those now living there. Of the groups which appear to be of South American origin, the Xenarthra were represented not only by armadillos of modern type, but also by the giant armoured glyptodonts and the enormous ground-sloths; each of these groups being sufficiently varied to be divided into three families. The rodents are of the same types as those now living and the Platyrrhina are present. Extinct groups of hoofed mammals are found, falling within the Notoungulata and including most varied animals. There are the following sub-orders: Typotheria, the Pleistocene form being an animal about as large as a pig with a gnawing dentition; and Toxodontia, with a skull two feet in length and a dentition not unlike that of the typotheres ; and the order Litopterna with one Pleistocene form about as large as a camel but possibly of aquatic habits.
Living alongside these autochthonus animals were carnivores including not only the ancestors of those forms which still exist but also a sabre-toothed tiger, Smilodon; and a bear, Arctotherium, now extinct. The perissodactyls include horses, in part belonging to the modern genus Equus, and in part to four extinct genera. Proboscidea are represented by several forms less advanced in structure than true elephants and commonly referred to as masto dons.
Thus except that it is richer both in the number of groups rep resented and in the size of the individuals, the Pleistocene fauna of South America resembles the recent one in that it contains a mixture of mammals which had originated in situ with immigrants from the north. When we pass backwards to Miocene times, as
represented in the Santa Cruz beds, we find an even fuller and more varied representation of the true South American groups, and a complete absence of those which we had assumed to be immigrants. It is natural to believe that the continents of North and South America became connected by a land-ridge sometime between the Miocene and the Pleistocene, and the truth of this conclusion is established by the fact that giant ground sloths and glyptodonts first appear in the North American fauna in Pliocene times.
The mammal fauna of the Miocene of South America contains the following elements. The marsupials include not only opossums essentially of modern type, but also a variety of carnivorous animals closely similar to the Tasmanian wolf, but some like Borhyaena, greatly exceeding that animal in size. Relatives of the living Coenolestes occur. There is an animal Necrolestes, which is supposed to be an insectivore related to the Zalambdo dont golden moles of South Africa. All the rodents, which are extremely numerous and varied, belong to the South American families of tree-porcupines, cavies, chinchillas and agouties. There are a few unmistakable platyrrhine monkeys. Amongst the eden tates, no representatives of the living sloths and ant-eaters have been found, probably because these animals are restricted to tropical forests and the known mammal fauna is that of the more open, less well watered and considerably colder plain. The armadillos are very varied; the glyptodonts, small compared with their descendants of Pleistocene times, to some extent bridge the gap between the normal armadillos from which they sprang and the later glyptodonts. The ground-sloths, of relatively small size, include the ancestors of the later forms and the three families into which these creatures are divided can already be recognized.
The Notoungulata include an immense range of animals adapted to very varied habits. The Typotheria, then small forms which filled the place occupied at present by rabbits, fall into three dis tinct families, whilst the Toxodontia include an abundance of forms smaller than their Pleistocene successors. The Entelonychia include a remarkable animal, Homalodontotherium, with stilted fore-legs and digging claws. The Litopterna, another extinct family represented by a single type in Pleistocene times included forms which parallel the horses in the reduction of the toes in foot and hand to one. Yet another extinct order, Astrapotheria, includes gigantic creatures unlike any other forms.