Page:Darwinism by Alfred Wallace 1889.djvu/416

 any kind of special creation, or by sudden advances of organisation in the offspring of preceding types, such close relationship would not be found; and facts of this kind become, therefore, to some extent a test of evolution under natural selection or some other law of gradual change. Of course the relationship will not appear when extensive migration has occurred, by which the inhabitants of one region have been able to take possession of another region, and destroy or drive out its original inhabitants, as has sometimes happened. But such cases are comparatively rare, except where great changes of climate are known to have occurred; and we usually do find a remarkable continuity between the existing fauna and flora of a country and those of the immediately preceding age. A few of the more remarkable of these cases will now be briefly noticed.

The mammalian fauna of Australia consists, as is well known, wholly of the lowest forms—the Marsupials and Monotremata—except only a few species of mice. This is accounted for by the complete isolation of the country from the Asiatic continent during the whole period of the development of the higher animals. At some earlier epoch the ancestral marsupials, which abounded both in Europe and North America in the middle of the Secondary period, entered the country, and have since remained there, free from the competition of higher forms, and have undergone a special development in accordance with the peculiar conditions of a limited area. While in the large continents higher forms of mammalia have been developed, which have almost or wholly exterminated the less perfect marsupials, in Australia these latter have become modified into such varied forms as the leaping kangaroos, the burrowing wombats, the arboreal phalangers, the insectivorous bandicoots, and the carnivorous Dasyuridae or native cats, culminating in the Thylacinus or "tiger-wolf" of Tasmania—animals as unlike each other as our sheep, rabbits, squirrels, and dogs, but all retaining the characteristic features of the marsupial type.

Now in the caves and late Tertiary or Post-Tertiary deposits of Australia the remains of many extinct mammalia have been found, but all are marsupials. There are many kangaroos, some larger than any living species, and others more allied to