The diprotodont marsupials are peculiar to Notogaea. The diprotodonts as a group are herbivorous. The most primitive family, that of the phalangers, contains many animals all essen tially arboreal in habit, varying in size from that of a mouse to that of a cat. They take in Australia much the same place that is occupied by the squirrels and monkeys in other parts of the world, and they often present, as in the development of a flying or rather gliding habit, a remarkable parallel to certain squirrels and to the flying "lemur" Galeopithecus. One interesting derivative of the phalangers is the koala or native bear, a large animal which lives entirely on the leaves of the eucalyptus trees. From the same group has arisen the wombat, a large, very powerfully-built, bur rowing animal, strictly herbivorous in habit, with a remarkable dentition paralleling that of the larger rodents such as the capy bara, in its adaptations. Finally the kangaroos and their allies form a very distinct group of terrestrial animals fitted by their elongated hind-legs and long tail for a progression by a series of jumps. Their dentition is so arranged as to allow them to crop grass as do the artiodactyls and as they occur in large numbers in relatively open country, they are similar to this group in their general diet and habits.
Thus the Australian marsupials have experienced an adaptive radiation which has fitted them for most of the modes of life known amongst eutherian mammals and the fact that no diproto dont, either living or extinct, is known from any other part of the world, is a clear indication that the evolution of the group took place in Australia and that it was made possible by the absence of the competition of higher mammals.
It should be possible to establish the truth of the above view by studies' of the Australian fossil marsupials. At present, how ever, except for a single specimen, which is clearly a diprotodont marsupial, and probably a phalanger, from the Upper Miocene of Wynyard, Tasmania, no pre-Pleistocene mammals are known. The Pleistocene mammals include an interesting human skull presenting the characters of the living Australian aborigines, the dingo, some rodents of Australian type and a very large series of marsupials, including representatives of extinct animals. It is an interesting feature that in Pleistocene time the large carnivorous Tasmanian wolf and Tasmanian devil, at present restricted to Tasmania, occurred widely spread in Australia, and it is customary to attribute their disappearance on that continent to competition with the dingo, that animal having never reached Tasmania.
The large carnivorous marsupial, Thylacoleo found in Pleisto cene deposits all over Australia is remarkable in that it is re lated not to the carnivorous polyprotodonts but to the otherwise exclusively herbivorous diprotodonts. Of this order several gi gantic forms are found, a kangaroo, Macropus titan, perhaps 1 2 feet in height, Diprotodon and its allies, not closely related to any living forms, but as large and heavily built as a rhinoceros, and the giant wombat Phascolonus, which attained a similar size. Thus what little is known of their fossil history emphasizes still more the peculiarity of the Australian marsupials.
It is clear that man did not arise in Australia and that he is as certainly an immigrant into that continent as he is in those Pacific islands, where the date and manner of his coming are re corded. From the occurrence of a fossil human skull in the
Pleistocene of Queensland, we know that the human immigration took place as early as Pleistocene times, and from the fact that this skull belongs to the species Homo sapiens, which is not known in the early or indeed the middle Pleistocene, it follows that the human immigration was in late Pleistocene times.
With the exception of a rat, of the genus Mus, the Australian rodents belong to genera not found elsewhere, but all are mem bers of the family Muridae, which is not geologically ancient. It is certain that they represent the result of the evolution in situ of some immigrant form, but it is impossible to determine the time at which their introduction took place: it is, however, ex ceedingly unlikely that it was earlier than the Pliocene. The ab sence of larger eutherian mammals shows that they cannot have entered over a continuous land-bridge but must have crossed by some other mode not available to larger forms. On the other hand, one at any rate, of the diprotodont marsupials of Australian origin has migrated out of the region, Phalanger itself being found in Timor and even as far as Celebes. It also must have crossed by some accidental mode of transport, for which, being arboreal it is peculiarly well fitted.
Neogaea.—The mammal fauna of Neogaea, that is of South America, is less peculiar than that of Notogaea. It contains no monotremes but there are many living marsupials, the most abundant being the opossums of the family Didelphidae; the other forms, belonging to two genera of which Coenolestes is the more important, have ancestors in the Miocene deposits of South America and were certainly evolved in that region and have never spread beyond it. Except for certain shrews in Central America, no insectivores are found on the South American continent, but one peculiar family, the Solenodontidae, is restricted to certain islands of the West Indies. It belongs to the sub-order Zalambdodonta, which has living representatives in Africa and Madagascar and has been found fossil in a perfectly typical form in the Basal Eocene of North America. The most characteristic South Ameri can mammals are, however, the "edentates" belonging to the group Xenarthra. These fall into three groups, the sloths, the ant-eaters and the armadillos, none of which, except for the oc currence of an armadillo in Texas, is now living outside the region. The rodents of South America, which are numerous, belong in the main to peculiar families; the cavis, chinchillas and agoutis are not found elsewhere, whilst another important group, the octodonts are represented by forms both in South America and in Africa. There are also representatives of the Myomorpha and tree-porcupines. The Carnivora include dogs, bears and cats, to gether with many raccoons peculiar to the area. The perissodactyls are represented only by the tapir, whilst the artiodactyls include the peccaries and llamas which are peculiar to South America, together with certain deer related to North American forms. The Primates are represented by the group Platyrrhina which is re stricted to South America, and by man.