440 Darwin, and after Darwin. 



evidence have been exposed while advancing before, and 

 retreating after, 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 stranded 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, 

 w hen 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 

 ^ Origin of Species, p. 332. 



