The reader will observe that all these extinct quadrupeds of Australia belong to the marsupial family, or, in other words, that they are referable to the same peculiar type of organization which now distinguishes the Australian mammalia from those of other parts of the globe. This fact is one of many pointing to a general law deducible from the fossil vertebrate and invertebrate animals of the eras immediately antecedent to the human, namely, that the present geographical distribution of organic _forms_ dates back to a period anterior to the creation of existing _species_; in other words, the limitation of particular genera or families of quadrupeds, mollusca, &c., to certain existing provinces of land and sea, began before the species now contemporary with man had been introduced into the earth. Mr. Owen, in his excellent "History of British Fossil Mammals," has called attention to this law, remarking that the fossil quadrupeds of Europe and Asia differ from those of Australia or South America. We do not find, for example, in the Europaeo-Asiatic province fossil kangaroos or armadillos, but the elephant, rhinoceros, horse, bear, hyaena, beaver, hare, mole, and others, which still characterize the same continent. In like manner in the Pampas of South America the skeletons of Megatherium, Megalonyx, Glyptodon, Mylodon, Toxodon, Macrauchenia, and other extinct forms, are analogous to the living sloth, armadillo, cavy, capybara, and llama. The fossil quadrumana, also associated with some of these forms in the Brazilian caves, belong to the Platyrrhine family of monkeys, now peculiar to South America. That the extinct fauna of Buenos Ayres and Brazil was very modern has been shown by its relation to deposits of marine shells, agreeing with those now inhabiting the Atlantic; and when in Georgia in 1845, I ascertained that the Megatherium, Mylodon, _Harlanus americanus_ (Owen), _Equus curvidens_, and other quadrupeds allied to the Pampean type were posterior in date to beds containing marine shells belonging to forty-five recent species of the neighbouring sea. There are indeed some cosmopolite genera, such as the Mastodon (a genus of the elephant family), and the horse, which were simultaneously represented by different fossil species in Europe, North America, and South America; but these few exceptions can by no means invalidate the rule which has been thus expressed by Professor Owen, "that in the highest organized class of animals the same forms were restricted to the same great provinces at the Pliocene periods as they are at the present day."