the flora
is fundamentally Australian both as regards orders and genera, for it was
due either to a direct land connection or a somewhat close approximation
between the two countries. It shows also why the great mass of typical
Australian forms are unrepresented, for the Australian flora is typically
_western_ and _temperate_, and New Zealand received its immigrants from the
_eastern_ island which had itself received only a fragment of this flora,
and from the _tropical_ end of this island, and thus could only receive
such forms as were not exclusively temperate in character. It shows,
further, why New Zealand contains such a very large proportion of tropical
forms, for we see that it derived the main portion of its flora directly
from the tropics. Again, this hypothesis shows us why, though {507} the
specially Australian _genera_ in New Zealand are largely tropical or
sub-tropical, the specially Australian _species_ are wholly temperate or
alpine; for these are comparatively recent arrivals, they must have
migrated across the sea in the temperate zone, and these temperate and
alpine forms are exactly such as would be best able to establish themselves
in a country already stocked mainly by tropical forms and their modified
descendants. This hypothesis further fulfils the conditions implied in Sir
Joseph Hooker's anticipation that--"these great differences (of the floras)
will present the least difficulties to whatever theory may explain the
whole case,"--for it shows that these differences are directly due to the
history and development of the Australian flora itself, while the
resemblances depend upon the most certain cause of all such broad
resemblances--close proximity or actual land connection.

One objection will undoubtedly be made to the above theory,--that it does
not explain why some species of the prominent Australian genera Acacia,
Eucalyptus, Melaleuca, Grevillea, &c., have not reached New Zealand in
recent times along with the other temperate forms that have established
themselves. But it is doubtful whether any detailed explanation of such a
negative fact is possible, while general explanations sufficient to cover
it are not wanting. Nothing is more certain than that numerous plants never
run wild and establish themselves in countries where they nevertheless grow
freely if cultivated; and the explanation of this fact given by Mr.
Darwin--that they are prevented doing so by the competition of better
adapted