-CHAP. I.] 
INTRODUCTORY. 
9 
are yearly diminishing in extent. Lastly, as a most valuable 
-supplement to geology, we require to know the exact depth 
and contour of the ocean-bed, since this affords an important 
clue to the former existence of now-submerged lands, uniting 
islands to continents, or affording intermediate stations which 
have aided the migrations of many organisms. This kind 
of information has only begun to be obtained during the last 
few years ; and it will be seen in the latter part of this volume, 
that some of the most recent deep-sea soundings have afforded 
a basis for an explanation of one of the most difficult and 
interesting questions in geographical biology — the origin of the 
fauna and flora of New Zealand. 
Such are the various classes of evidence that bear directly on 
the question of the distribution of organisms ; but there are 
■others of even a more fundamental character, and the impor- 
tance of which is only now beginning to be recognised by 
students of nature. These are, firstly, the wonderful alterations 
of climate which have occurred in the temperate and polar 
zones, as proved by the evidences of glaciation in the one and 
of luxuriant vegetation in the other ; and, secondly, the theory of 
the permanence of existing continents and oceans. If glacial 
epochs in temperate lands and mild climates near the poles 
have, as now believed by men of eminence, occurred several 
times over in the past history of the earth, the effects of such 
great and repeated changes, both on the migration, modification, 
and extinction, of species, must have been of overwhelming 
importance — of more importance perhaps than even the geo- 
logical changes of sea and land. It is therefore necessary to 
consider the evidence for these dim atal changes; and then, by 
a critical examination of their possible causes, to ascertain 
whether they were isolated phenomena, were due to recurrent 
cosmical actions, or were the result of a great system of terres- 
trial development. The latter is the conclusion we arrive at ; 
and this conclusion brings with it the conviction, that in the 
theory which accounts for both glacial epochs and warm polar 
climates, we have the key to explain and harmonize many of 
the most anomalous biological and geological phenomena, and 
one which is especially valuable for the light it throws on the 
