366 THE HISTORY OF CREATION. 
It is unnecessary here to enter into detail as to the ice 
period itself, and into investigations about its limits, and 
I may omit this all the more reasonably since the whole 
of our recent geological literature is full of it. It will be 
found discussed in detail in the works of Cotta,?! Lyell, 
Vogt,” Zittel,” ete. Its great importance to us here is 
that it helps us to explain the most difficult chorological 
problems, as Darwin has correctly perceived. 
For there can be no doubt that this glaciation of the 
present temperate zones must have exercised an exceedingly 
important influence on the geographical and topographical 
distribution of organisms, and that it must have entirely 
changed it. While the cold slowly advanced from the poles 
towards the equator, and covered land and sea with a con- 
nected sheet of ice, it must of course have driven the whole 
living world before it. Animals and plants had to migrate 
if they wished to escape being frozen. But as at that time 
the temperate and tropical zones were probably no less 
densely peopled with animals and plants than at present, 
there must have arisen a fearful struggle for life between 
the latter and the intruders coming from the poles. During 
this struggle, which certainly lasted many thousands of 
years, many species must have perished and many become 
modified and been transformed into new species. ~ The 
hitherto existing tracts of distribution of species must have 
become completely changed, and the struggle have been 
continued, nay, indeed, must have broken out anew and 
been carried on in new forms, when the ice period had 
reached and gone beyond its furthest point, and when in 
the post-glacial period the temperature again increased, and 
organisms began to migrate back again towards the poles. 
