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 agreement 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 porticn 
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 
migrations of organisms in so far as it explains the radiation 
of every animal and vegetable species from a single pri- 
meval home, from a “central point of creation,” and the 
