502 



ISLAND LIFE. 



[part II. 



the immediate vicinit}^ of land. The general stability of con- 

 tinents 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 distribution 

 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 fierce controversies. The occurrence of a re- 

 cent 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 

 many 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 view^s as to the causes of the 

 "glacial epoch," I have introduced certain limitations and modi- 

 fications. I have pointed out with more precision than has, I 

 believe, hitherto been done, the very different effects on climate 

 of water in the liquid and in the solid state ; and I have shown, 

 by a variety of evidence, that without high land there can be 

 no permanent snow and ice. From these facts and principles 

 the very important conclusion is reached, that the alternate 

 phases of precession — causing the winter of each hemisphere 

 to be in ajphelion and perihelion each 10,500 years — would pro- 

 duce a complete change of climate only where a country was 

 partially snow-clad ; while, whenever a large area became 

 almost ivholly buried in snow and ice — as was certainly the 



