Page:A sketch of the physical structure of Australia.djvu/106

94 coast, nearly up to Torres' Straits, though we then pass from the latitude of 34&deg; nearly to the neighbourhood of the equator.

Now our hypothetical geological history would account for this. We may suppose that previous to the depression that let in the tertiary sea over Australia, that country, (and perhaps a large portion of the adjacent surface of the earth), was covered by trees, plants, and animals, whether terrestrial or freshwater, and its coasts bordered by marine animals, all of a common type, that their species and genera varied only with the variation in latitude, depth, altitude, nature of the soil, &c., or all those circumstances that go to make up the "climate" of a place. When this country, however, was broken up into islands, it is probable that each island or group of islands would preserve some species or genera of animals or plants that would be lost in the others. Lapse of time would tend still farther to modify these different assemblages by the extinction of some of the old species, and the introduction of new ones, until at last the more distant groups exhibited a specific contrast while they had a generic resemblance to each other. We should thus have a reason for a certain peculiarity of the fauna and flora of each tract of country;—for the eastern coast which was once one long island or group of islands, having plants and animals of the