CHAr. XXIII.] ARCTIC PLANTS IN NEW ZEALAND. 
497 
theory as to the cause of the peculiar biological relations be- 
tween 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 the Cretaceous period, 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 the flora of Australia and its relation to 
that of New Zealand. 
In the more general explanation of the relations of the 
various northern and southern floras, I have shown what an 
important aid to any such explanation is the theory of repeated 
changes of climate, not necessarily of great amount, given in 
our eighth Chapter ; while the whole discussion justifies the 
importance attached to the theory of the general permanence 
of continents and oceans, as demonstrated in Chapter VI., 
since any rational explanation based upon facts (as opposed to 
mere unsupported conjecture) must take such general perma- 
nence as a starting-point. The whole inquiry into the pheno- 
mena presented by islands, which forms the main subject of the 
present volume has, I think, shown that this theory does afford 
K K 
