THE DESTR UCTIVE A GEXCIES OF NATURE. 1 6 1. 



the Fauna and Flora presented but slight dif- 

 ferences. Possibly the earth was then vivified 

 rather by its own internal heat, than by the 

 heat of the sun. But this condition of things 

 was only temporary. More or less gradually 

 the earth cooled down to the intense cold of 

 the Glacial Period. Then the surface of the 

 globe became divided into climates, and the 

 cold drove everything before it towards the 

 tropics. Whole continents appear to have been 

 depopulated, great numbers of species were 

 destroyed, arid nearly all the remainder must 

 have been profoundly modified.* The original 

 forms of many of our existing animals and 

 plants are supposed to be those which now 

 inhabit the Arctic Eegions, or the mountains 

 of the Northern Hemisphere, to which they 

 retreated on the diminution of the cold. In 

 North America, it appears that the remains 

 of pre-glacial life escaped into California, and 

 that continent was subsequently re-stocked from 

 Asia-Europe. It is generally supposed that 

 there has been a succession of glacial epochs ; 

 and there is at least good reason to believe 



Cf . Murray, " Geographical Distribution of Mammals," ch. iv. v. 



M 



