642 HISTORY OF GEOLOGY. 



to be extirpated. If, for instance, that reduction of 

 the climate of the earth which appears, from geo- 

 logical evidence, to have taken place already, be 

 supposed to go on much further, the advancing 

 snow and cold of the polar regions may destroy the 

 greater part of our plants and animals, and drive 

 the remainder, or those of them which possess the 

 requisite faculties of migration and accommodation, 

 to seek an asylum near the equator. And if we 

 suppose the temperature of the earth to be still fur- 

 ther reduced, this zone of now-existing life, having 

 no further place of refuge, will perish, and the 

 whole earth will be tenanted, if at all, by a new 

 creation. Other causes might produce the same 

 effect as a change of climate ; and, without sup- 

 posing such causes to affect the whole globe, it is 

 easy to imagine circumstances such as might en- 

 tirely disturb the equilibrium which the powers of 

 diffusion of different species have produced; might 

 give to some the opportunity of invading and con- 

 quering the domain of others ; and in the end, the 

 means of entirely suppressing them, and establish- 

 ing themselves in their place. 



That this extirpation of certain species, which, 

 as we have seen, happens in a few cases under com- 

 mon circumstances, might happen upon a greater 

 scale, if the range of external changes were to be 

 much enlarged, cannot be doubted. The extent', 

 therefore, to which natural causes may account for 

 the extinction of species, will depend upon the 



