s nature, owing to the areas they occupy
being unconnected with the ice-bearing Antarctic continent. It follows,
that whereas the northern plants find in all these southern lands a milder
and more equable climate than that to which they have been accustomed, and
are thus often able to grow and flourish even more vigorously than in their
native land, the southern plants would find in almost every part of Europe,
North America or Northern Asia, a more severe and less equable climate,
with winters that usually prove fatal to them even under cultivation. These
causes, taken separately, are very powerful, but when combined they must, I
think, be held to be amply sufficient to explain why examples of the
typical southern vegetation are almost unknown in the north temperate zone,
while a very few of them have extended so far as the northern tropic.[144]

{529}

_Concluding Remarks on the Last Two Chapters._--Our inquiry into the
external relations and probable origin of the fauna and flora of New
Zealand, has thus led us on to a general theory as to the cause of the
peculiar biological relations between the northern and the southern
hemispheres; and no better or more typical example could be found of the
wide range and great interest of the study of the geographical distribution
of animals and plants.

The solution which has here been given of one of the most difficult of this
class of problems, has been rendered possible solely by the knowledge very
recently obtained of the form of the sea-bottom in the southern ocean, and
of the geological structure of the great Australian continent. Without this
knowledge we should have nothing but a series of guesses or probabilities
on which to found our hypothetical explanation, which we have now been able
to build up on a solid foundation of fact. The complete separation of East
from West Australia during a portion of the Cretaceous and Tertiary
periods, could never have been guessed till it was established by the
laborious explorations of the Australian geologists; while the hypothesis
of a comparatively shallow sea, uniting New Zealand by a long route with
tropical Australia, while a profoundly deep ocean always separated it from
temperate Australia, would have been rejected as too improbable a
supposition for the foundation of even the most enticing theory. Yet it is
mainly by means of these two facts, that we are enabled to give an adequate
explanation of the strange anomalies in t