[24]. Placental mammals are those having a placenta to nourish the fœtus, as is the case with all mammals except the marsupials and monotremes.
Among the six zoological regions into which Wallace and Sclater divide the terra firma of the globe, one of the best marked and certainly the most peculiar one is the Australian. Australia and New Guinea are the largest countries in this region, which, in addition to New Zealand and the islands of the Pacific, includes the Indian Archipelago east of Borneo, Java, and Bali. The latter islands, all of which belong to the Indian-Malay region, are separated from the Australian by a belt of very deep water, where Wallace’s well-known line is found on the map. The water is shallow between all the islands south-east of this belt—Celebes, Timor, Amboina, Banda, and New Guinea—which evidently all lie on a submarine bank, and have at one time been united with Australia. There are the most striking differences between the fauna on each side of the belt. Apes, rhinoceroses, tapirs, tigers, leopards, and similar Indian and Malay animals disappear, and we enter an entirely new region, the Australian, the chief characteristic of which is that it lacks nearly all the groups of mammals found elsewhere in the world. Instead we either find the peculiar marsupials, or the mammals are entirely wanting, as is the case on most of the South Sea Islands. In ornithology the honey-eaters are especially remarkable, then we have the birds of paradise, the cassowary, and finally the kiwi-kiwi of New Zealand.
The zoological character of the region is most marked in Australia, which is rich in peculiar animal forms. As an island-continent extending from 39° to 11° S. lat., and which consequently is several times as large as the islands of the other regions added together, the country naturally has very various climates. In the southern part there is a climate like that of the countries along the Mediterranean; in the northern there is a regular season of rain; while the centre is more hot and more arid than any other part of the earth. Still, strange to say, the climatic differences are not attended by corresponding variations of the fauna, which is strikingly uniform throughout the country. Many important species are found everywhere in the continent. Generally speaking, Australia is a hot and dry country, and its flora and fauna have been developed in harmony with its physico-geographical conditions. This explains, for instance, why the tropical North Australia has not so luxurious and varied vegetations as the adjacent New Guinea, with its more humid climate. Many of the Australian mammals can subsist without water for a long time. Gould is even of opinion that the large kingfishers, whose food consists mainly of lizards and insects, never drink.
The fauna of Australia has many special forms, and occupies a peculiar, isolated position. This is most apparent among the mammals, which give to the Australian fauna its most marked feature. Imagine a continent about the size of Europe with no other mammals than marsupials, a few bats, rats, and mice. There are no apes, no beasts of prey, no hoofed animals. None of those groups are found from which our domestic animals have been developed. The only exception is the dingo, the Australian dog, but although fossil specimens have been found, it is generally supposed that the dingo was introduced by man; it does not differ much from the wild dogs of other lands. The fact that Australia at present has so many large land animals, which at one time were represented by kindred forms in Europe, shows that the country in some way or other has been united with Asia, just as Great Britain must at some time have been connected with the European continent. But the present remarkable isolation of the Australian mammals from the land fauna of the rest of the world is, as Wallace remarks, the best evidence that Australia and Asia were not united throughout the tertiary period, and it is a most characteristic fact that the only mammals which Australia has in common with the rest of the world are the flying-bats and such small mammals as could most easily be carried on floating logs, roots, and similar objects to foreign coasts. Marsupials are also found in America; but, with this exception, they now exist only in Australia and in the adjacent islands New Guinea and Tasmania, which is evidence that the latter islands were at one time united with Australia.
The marsupials are so called from their having a pouch (marsupium) for carrying the immature young. The young are born without much development, and they are at once transferred to the pouch, where they continue to grow until they are able to take care of themselves. The pouch is supported by the marsupial bones, which are equally developed in both sexes. There are also many other peculiarities in the structure of these animals, distinguishing them from the higher mammals, e.g. their teeth being quite different from those of other animals.
The large kangaroo bears a young “no larger than the little finger of a human baby, and not unlike it in form.” This helpless, naked, blind, and deaf being the mother puts in an almost inexplicable manner into the pouch with her mouth, and places it on one of the long, slender, milk-giving strings found in the pouch. Here the young remains hanging for weeks, and grows very rapidly. The mother possesses a peculiar muscle with which it is able to press milk into the mouth of the helpless little one, and the larynx of the young has a peculiar structure, so that it can breathe while it sucks, and consequently is not choked. Gradually it assumes the form of its parents, and when big enough it begins to make excursions from the pouch, which continues to enlarge with the growth of the young. These excursions become longer as the young grows larger, and thus this pouch serves both as a second womb and as a nest and home. All marsupials are propagated in this manner, but the number of young may vary from one to fourteen.
The brain of the marsupial is small and has but few convolutions, indicative of small mental development. They are the most stupid of all mammals, and indifferent in regard to all things save the wants of their stomachs. Brehm calls attention to the fact that no marsupial mother plays with her young or makes any effort to teach them.
The marsupials may differ widely in appearance, structure, and habits; they may be as large as a stag and as small as a mouse. Some move on the hind-feet alone, others on all fours; some live on the ground, others in trees, others again are able to fly. Most of them feed on grass, but some of them live on fruits, roots, and leaves; others again on meat and insects; while there are also marsupials that eat honey.
Ever since Captain Cook’s sailors in 1770 came and told him that they had seen the very devil hopping away on his hind legs in the form of an animal, the kangaroo has been inseparably associated with our ideas of Australia, the land of the kangaroo. The kangaroo (Macropus) is also the largest and most remarkable of all marsupials, and is represented by many species throughout Australia. The largest one is reddish (Macropus rufus) and is found in the interior. Of the smaller kinds we may mention the wallabies, kangaroo-rats, which are about the size of a rabbit, and the pademelon, which is easily recognised by the fact that when it runs it lets one arm drop as if it were broken. During recent years kangaroos have greatly increased in number, one of the causes being the systematic extermination of the dingoes and the decrease of the number of natives. Thus kangaroos, like their smaller relatives the wallabies and the kangaroo-rats, have become noxious animals that destroy the pastures, and the colonists are making great efforts to exterminate them. In Queensland the Government pays a premium for every such animal killed, and in this way the number of marsupials was reduced in the years 1880–1885 by six millions.
The tree-kangaroos (Dendrolagus), living in the dense scrubs of Northern Queensland, are very remarkable and very different from the other members of the family.