ALPINE AND ARCTIC SPECIES. 367 



In any case this great change of climate, whether a 

 greater or less importance be ascribed to it, is one of 

 those occurrences in the history of the earth which have 

 most powerfully influenced the distribution of organic 

 forms. But more especially one important and difficult 

 chorological circumstance is explained by it in the simplest 

 manner, namely, the specific agTcement of many of our 

 Alpine inhabitants with some of those living in polar 

 regions. There is a great number of remarkable animal 

 and vegetable forms which are common to these two far 

 distant parts of the earth, and which are found nowhere 

 in the wide plains lying between them. Their migration 

 from the polar lands to the Alpine heights, or vice versa, 

 would be inconceivable under the present climatic circum- 

 stances, or could be assumed at least only in a few rare 

 instances. But such a migration could take place, nay, 

 was obliged to take place, during the gradual advance and 

 retreat of the ice-sheet. As the glaciation encroached from 

 Northern Europe towards our Alpine chains, the polar in- 

 habitants retreating before it — gentian, saxifrage, polar 

 foxes, and polar hares — must have peopled Germany, in 

 fact all Central Europe. When the temperature again in- 

 creased, only a portion of these Arctic inhabitants returned 

 with the retreating ice to the Arctic zones. Another portion 

 of them climbed up the mountains of the Alpine chain 

 instead, and there found the cold climate suited to them. 

 The problem is thus solved in a most simple manner. 



We have hitherto principally considered the theory of the 

 tnigrations of organisms in so far as it explains the radiation 

 of every animal and vegetable species from a single pri- 

 maeval home, from a " central point of creation," and the 



