3(^^ THE HISTOKY OF CEEATION. 



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 hterature is full of it. It will be 

 found discussed in detail in the works of Cotta/^ Lyell,^^ 

 Vogt,^^ Zittel,^^ etc. 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 geogTaphical 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. 



