502 
ISLAND LIFE. 
[part II. 
the immediate vicinity 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 views 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 aphelion 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 wholly buried in snow and ice — as was certainly the 
