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 {495}
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 Palaeozoic 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 differ