These remarkable facts clearly point to one conclusion—that the flora of tropical Australia is, comparatively, recent and derivative. If we imagine the greater part of North Australia to have been submerged beneath the ocean, from which it rose in the middle or latter part of the Tertiary period, offering an extensive area ready to be covered by such suitable forms of vegetation as could first reach it, something like the present condition of things would inevitably arise. From the north, widespread Indian and Malay plants would quickly enter, while from the south the most dominant forms of warm-temperate Australia, and such as were best adapted to the tropical climate and arid soil, would intermingle with them. Even if numerous islands had occupied the area of Northern Australia for long periods anterior to the final elevation, very much the same state of things would result.
The existence in North and North-east Australia of enormous areas covered with Cretaceous and other Secondary deposits, as well as extensive Tertiary formations, lends support to the view, that during very long epochs temperate Australia was cut off from all close connection with the tropical and northern lands by a wide extent of sea; and this isolation is exactly what was required, in order to bring about the wonderful amount of specialisation and the high development manifested by the typical Australian flora. Before proceeding further, however, let us examine this flora itself, so far as regards its component parts and probable past history.
The Floras of South-eastern and South-western Australia.—The peculiarities presented by the south-eastern and south-western subdivisions of the flora of temperate Australia are most interesting and suggestive, and are, perhaps, unparalleled in any other part of the world. South-west Australia is far less extensive than the south-eastern division—less varied in soil and climate, with no lofty mountains, and much sandy desert; yet, strange to say, it contains an equally rich flora and a far greater proportion of peculiar species and genera of plants. As Sir
Joseph Hooker remarks:—"What differences there are in conditions would, judging from analogy with other countries, favour the idea that South-eastern Australia, from its far greater area, many large rivers, extensive tracts of mountainous country and humid forests, would present much the most extensive flora, of which only the drier types could extend into South-western Australia. But such is not the case; for though the far greater area is much the best explored, presents more varied conditions, and is tenanted by a larger number of Natural Orders and genera, these contain fewer species by several hundreds."[[182]]
The fewer genera of South-western Australia are due almost wholly to the absence of the numerous European, Antarctic, and South-American types found in the south-eastern region, while in purely Australian types it is far the richer, for while it contains most of those found in the east it has a large number altogether peculiar to it; and Sir Joseph Hooker states that "there are about 180 genera, out of 600 in South-western Australia, that are either not found at all in South-eastern, or that are represented there by a very few species only, and these 180 genera include nearly 1,100 species."
Geological Explanation of the Differences of these Two Floras.—These facts again clearly point to the conclusion that South-western Australia is the remnant of the more extensive and more isolated portion of the continent in which the peculiar Australian flora was principally developed. The existence there of a very large area of granite—800 miles in length by nearly 500 in maximum width with detached masses 200 miles to the north and 500 miles to the east—indicates such an extension; for these
granitic masses were certainly once buried under piles of stratified rock, since denuded, and then formed the nucleus of the old Western Australian continent. If we take the 1000-fathom line around the southern part of Australia to represent the probable extension of this old land we shall see that it would give a wide additional area south of the Great Australian Bight, and form a continent which, even if the greater part of tropical Australia were submerged, would be sufficient for the development of a peculiar and abundant flora. We must also remember that an elevation of 6000 feet, added to the vast amount which has been taken away by denudation, would change the whole country, including what are now the deserts of the interior, into a mountainous and well-watered region.
But while this rich and peculiar flora was in process of formation, the eastern portion of the continent must either have been widely separated from the western or had perhaps not yet risen from the ocean. The whole of this part of the country consists of Palæozoic and Secondary formations with granite and metamorphic rocks, the Secondary deposits being largely developed on both sides of the central range, extending the whole length of the continent from Tasmania to Cape York, and constituting the greater part of the plateau of the Blue Mountains and other lofty ranges. During some portion of the Secondary and Tertiary periods therefore, this side of Australia must have been almost wholly submerged beneath the ocean; and if we suppose that during this time the western part of the continent was at nearly its maximum extent and elevation, we shall have a sufficient explanation of the great difference between the flora of Western and Eastern Australia, since the latter would only have been able to receive immigrants from the former, at a later period, and in a more or less fragmentary manner.
If we examine the geological map of Australia (given in Stanford's Compendium of Geography and Travel, volume Australasia), we shall see good reason to conclude that the eastern and the western divisions of the country first existed as separate islands, and only became united at a comparatively recent epoch. This is indicated by an
enormous stretch of Cretaceous and Tertiary formations extending from the Gulf of Carpentaria completely across the continent to the mouth of the Murray River. During the Cretaceous period, therefore, and probably throughout a considerable portion of the Tertiary epoch,[[183]] there must have been a wide arm of the sea occupying this area, dividing the great mass of land on the west—the true seat and origin of the typical Australian flora—from a long but narrow belt of land on the east, indicated by the continuous mass of Secondary and Palæozoic formations already referred to which extend uninterruptedly from Tasmania to Cape York. Whether this formed one continuous land, or was broken up into islands, cannot be positively determined; but the fact that no marine Tertiary beds occur in the whole of this area, renders it probable that it was almost, if not quite, continuous, and that it not improbably extended across to what is now New Guinea. At this epoch, then (as shown in the accompanying map), Australia may, not improbably, have consisted of a very large and fertile western island, almost or quite extratropical, and extending from the Silurian rocks of the Flinders range in South Australia, to about 150 miles west of the present west coast, and southward to about 350 miles south of the Great Australian Bight. To the east of this, at a distance of from 250 to 400 miles, extended in a north and south direction a long but comparatively narrow island, stretching from far south of Tasmania to New Guinea; while the crystalline and Secondary formations of central North Australia probably indicate the existence of one or more large islands in that direction.