CHAP. I.] 



INTBODUCTORY. 



9 



are yearly diminisliing 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 imjoortance perhaps than even the geo- 

 logical changes of sea and land. It is therefore necessary to 

 consider the evidence for these climatal 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 v/hich 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 



