stralian
plant has entered any part of the north temperate zone, and the same may be
said of the typical southern vegetation in general, whether developed in
the Antarctic lands, New Zealand, South America, or South Africa. The
furthest northern outliers of the southern flora are a few genera of
Antarctic type on the Bornean Alps; the genus Acaena which has a species in
California; two representatives of the Australian flora--Casuarina and
Stylidium, in the peninsula of India; while China and the Philippines have
two strictly Australian genera of Orchideae--Microtis and Thelymitra, as
well as a Restiaceous genus. Several distinct causes appear to have
combined to produce this curious inability of the southern flora to make
its way into the northern hemisphere. The primary cause is, no doubt, the
totally different distribution of land in the two hemispheres, so that in
the south there is the minimum of land in the colder parts of the temperate
zone and in the north the maximum. This is well shown by the fact that on
the parallel of Lat. 50deg N. we pass over 240deg of land or shallow sea,
while on the same parallel of south latitude we have only 4deg, where we
cross the southern part of Patagonia. Again the three most important south
temperate land-areas--South Temperate America, South Africa, and
Australia--are widely separated from each other, and have in all
probability always been so; whereas the whole of the north temperate lands
are practically continuous. It follows that, instead of the enormous
northern area, in which highly organised and dominant groups of plants have
been developed gifted with great colonising and aggressive powers, we have
in the south three comparatively small and detached areas, in which rich
floras have been developed with _special_ {528} adaptations to soil,
climate, and organic environment, but comparatively impotent and inferior
beyond their own domain.

Another circumstance which makes the contest between the northern and
southern forms still more unequal, is the much greater hardiness of the
former, from having been developed in a colder region, and one where alpine
and arctic conditions extensively prevail; whereas the southern floras have
been mainly developed in mild regions to which they have been altogether
confined. While the northern plants have been driven north or south by each
succeeding change of climate, the southern species have undergone
comparatively slight changes of thi