534 ISLAND LIFE part ii 



and the chief land-areas — have remained, on the whole, 

 unchanged throughout geological time. Our continents 

 are shown to be built up mainly of " shore-deposits " ; and 

 oven the chalk, which is so often said to be the exact 

 equivalent of the " globigerina ooze" now forming in 

 mid- Atlantic, is shown to be a comparatively shallow- 

 water deposit formed in inland seas, or in the immediate 

 vicinity of land. The general stabihty of continents has, 

 however, been accompanied by constant changes of form, 

 and insular conditions have prevailed over every part in 

 succession ; and the effect of such changes on the distribu- 

 tion of organisms is pointed out. 



We then approach the consideration of another set of 

 changes — those of climate — which have probably been 

 agents of the first importance in modifying the specific 

 forms as well as the distribution of animals. Here again 

 we find ourselves in the midst of tierce controversies. 

 The occurrence of a recent glacial epoch of great severity 

 in the northern hemisphere is now universally admitted, 

 but the causes which brought it on are matter of dispute. 

 But unless we can arrive at these causes, as well as at 

 those which produced the equally well demonstrated mild 

 climate in the Arctic regions, we shall be quite unable to 

 determine the nature and amount of the changes of 

 climate which have occurred throughout past ages, and 

 shall thus be left without a most important clue to the 

 explanation of manj^ of the anomalies in the distribution 

 of animals and plants. 



I have therefore devoted three chapters to a full 

 investigation of this question. I have first given such a 

 sketch of the most salient facts as to render the phenomena 

 of the glacial epoch clear and intelligible. I then review 

 the various suggested explanations, and, taking up the 

 two which alone seem tenable, I endeavour to determine 

 the true principles of each. While adopting generally 

 Mr. Croll's views as to the causes of the " glacial epoch," 

 I have introduced certain limitations and modifications. 

 I have pointed out, I believe, more clearly than has 

 hitherto been done, the very different effects on climate of 

 water in the liquid and in the solid state ; and I have 



