ence 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 {496} 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,[131] 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 Palaeozoic 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.

{497}

The white portions represent land; the shaded parts sea.
The existing land of Australia is shown in outline.]

The eastern and the western islands--with which we are now chiefly
concerned--would then differ considerably i