364 THE HISTOEY OF CREATION. 



the poles remained so warm that the whole surface of the 

 earth could be inhabited by organisms. It was only at a 

 comparatively very recent period of the earth's history, 

 namely, at the beginning of the tertiary period, that ther-j 

 occurred, as it seems, the first perceptible cooling of the 

 earth's crust at the poles, and through this the first differen- 

 tiation or separation of the different zones of temperature 

 or climatic zones. But the slow and gradual decrease or 

 temperature continued to extend more and more within the 

 tertiary period, until at last, at both poles of the earth, the 

 fir?5t permanent ice caps were formed. 



I need scarcely point out in detail how very much this 

 change of climate must have affected the geographical dis- 

 tribution of organisms, and the origin of numerous new 

 species. The animal and vegetable species, which, down 

 to the tertiary period, had found an agreeable tropical 

 climate all over the earth, even as far as the poles, 

 were now forced either to adapt themselves to the in- 

 truding cold, or to flee from it. Those species which 

 adapted and accustomed themselves to the decreasing 

 temperature became new species simply by this very accli- 

 matization, under the influence of natural selection. The 

 other species, which fled from the cold, had to emigrate and 

 seek a milder climate in lower latitude-s. The tracts of dis- 

 tribution which had hitherto existed must by this have 

 been vastly changed. 



However, during the last great period of the earth's 

 history, during the quaternary period (or diluvial period) 

 succeeding the tertiary one, the decrease of the heat 

 of the earth from the poles did not by any means remain 

 stationary. The temperature fell lower and lower, nay, even 



