440 Darwin, and after Darwin. 
evidence have been exposed while advancing before, and 
retreating afier, the ice. Rather do I agree with Darwin 
that “they would not have been exposed during their long 
migrations to any great diversity of temperature; and as they 
all migrated in a body together, their mutual relations will 
not have been much disturbed; hence, in accordance with 
the principles indicated in this volume, these forms will not 
have been liable to much modification’.’ But, be this 
matter of opinion as it may, a much better test is afforded 
by those numerous cases all the world over, where arctic 
species have been left stianded on alpine areas by the retreat 
of glaciation ; because here there is no room for differences 
of opinion as to a “change of environment” having taken 
place. Not to speak of climatic differences between arctic 
and alpine stations, consider merely the changes which must 
have taken place in the relations of the thus isolated species 
to each other, as well as to those of all the foreign plants, 
insects, &c., with which they have long been thrown into 
close association. If in such cases no variation or transmu- 
tation had taken place since the glacial epoch, then indeed 
there would have been a difficulty of some magnitude. But, 
by parity of reasoning, whatever degree of difficulty would 
have been thus presented is not merely discharged, but 
converted into at least an equal degree of corroboration, 
when it is found that under such circumstances, in whatever 
part of the world they have occurred, some considerable 
amount of variation and transmutation has always taken 
place,—and this in the animals as well as in the plants. 
For instance, again to quote Darwin, “If we compare the 
present Alpine plants and animals of the several great Euro- 
pean mountain-ranges one with another, though many of 
the species remain identically the same, some exist as varie- 
ties, some as doubtful forms or sub-species, and some as 
distinct yet closely allied species representing each other on 
1 Origin of Species, p. 332. 
