INTRODUCTORY 



have to trust to collateral evidence and more or less prob- 

 able hypothetical explanations. Hardly less valuable is 

 the evidence of stratigrajDhical geology ; for this often 

 shows us what parts of a country have been submerged 

 at certain epochs, and thus enables us to prove that 

 certain areas have been long isolated and the fauna and 

 tlora allowed time for special development. Here, too, 

 our knowledge is exceedingly imperfect, though the 

 blanks upon the geological map of the world are yearly 

 diminishing in extent. Lastly, as a most valuable supple- 

 ment to geology, we require to know approximately, the 

 depth and contour of the ocean-bed, since this affords an im- 

 portant clue to the former existence of now-submerged lands, 

 uniting islands to continents, or affording intermediate 

 stations which have aided the migTations of many organ- 

 isms. This kind of information has only been partially 

 obtained during the last few years ; and it will be seen in 

 the latter j)art 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 importance 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 o'laciation in the one and of luxuriant ves^etation in the 

 other; and, secondly, the theory of the permanence of exist- 

 ing continents and oceans. If glacial ejDOchs 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 

 geological changes of sea and land. It is therefore neces- 

 sary to consider the evidence for these climatal changes ; 



