Lastly, the fact of the flora of the glacial epoch not having exhibited any modifications during the long residence of some of its specific types in Great Britain and elsewhere, is a fact of some importance to the general theory of evolution, since it shows a higher degree of stability on the part of these specific types than might perhaps have been expected, supposing the theory to be true. But I do not see that this constitutes a difficulty against the theory, when we have so many other cases of proved transmutation to set against it. For instance, not to go further afield than this very glacial flora itself, it will be remembered that in an earlier chapter I selected it as furnishing specially cogent proof of the transmutation of species. What, then, is the explanation of so extraordinary a difference between Mr. Carruthers' views and my own upon this point? I believe the explanation to be that he does not take a sufficiently wide survey of the facts. To begin with, it seems to me that he exaggerates the vicissitudes to which the species of plants that he calls into 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[60]." 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 transmutation 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 European mountain-ranges one with another, though many of the species remain identically the same, some exist as varieties, some as