120 
ISLAND LIFE. 
[part I. 
the Alps, and then by the Mediterranean ; whereas in eastern 
America and Asia the mountain chains run in a north and 
south direction, and there is nothing to prevent the flora from 
having been preserved by a southward migration into a milder 
region. 
Our next two chapters will be devoted to a discussion of the 
causes which brought about the glacial epoch, and that still 
more extraordinary climatic phenomenon — the mild climate and 
luxuriant vegetation of the Arctic zone. If my readers will 
follow me with the care and attention so difficult and interest- 
ing a problem requires and deserves, they will find that I have 
grappled with all the facts which have to be accounted for, and 
offered what I believe is the first complete and sufficient ex- 
planation of them. The important influence of climatal changes 
on the dispersal of animals and plants is a sufficient justification 
for introducing such a discussion into the present volume. 
