GEOGKAPHICAL DISTRIBUTION OF ORGANIC BEINGS. 121 



The former influence of the glacial climate on the dis- 

 tribution of the inhabitants of Europe, as explained by 

 Edward Forbes, is substantially as follows. But we shall 

 follow the changes more readily by supposing a new gla- 

 cial period slowly to come on, and then pass away, as 

 formerly occurred. As the cold came on, and as each 

 more southern zone became fitted for the inhabitants of 

 the north, these would take the places of the former in- 

 habitants of the temperate regions. The latter, at the 

 same time, would travel farther and farther southward, 

 unless they were stopped by barriers, in which case they 

 would perish. The mountains would become covered 

 with snow and ice, and their former Alpine inhabitants 

 would descend to the plains. By the time that the cold 

 had reached its maximum, we should have an Arctic fauna 

 and flora, covering the central parts of Europe, as far 

 south as the Alps and Pyrenees, and even stretching into 

 Spain. The now temperate regions of the United States 

 would likewise be covered by Arctic plants and animals, 

 and these would be nearly the same with those of Europe ; 

 for the present circumpolar inhabitants, which we suppose 

 to have everywhere traveled southward, are remarkably 

 uniform round the world. 



As the warmth returned, the Arctic forms would re- 

 treat northward, closely followed up in their retreat by the 

 productions of the more temperate regions. And, as the 

 snow melted from the bases of the mountains, the Arctic 

 forms would seize on the cleared and thawed ground, 

 always ascending, as the warmth increased and the snow 

 still further disappeared, higher and higher, while their 

 brethren were pursuing their northern journey. Hence, 

 when the warmth had fully returned, the same species, 

 which had lately lived together on the European and 

 Korth American lowlands, would again be found in the 



