364 THE HISTORY 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 there 
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 of 
temperature continued to extend more and more within the 
tertiary period, until at last, at both poles of the earth, the 
first 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 latitudes. 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 
