cordance 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
doubtful forms or sub-species, and some as distinct yet closely allied
species representing each other on the several ranges[61]." Lastly, if
instead of considering the case of alpine floras, we take the much
larger case of the Old and New World as a whole, we meet with much
larger proofs of the same general facts. For, "during the slowly
decreasing warmth of the Pliocene period, as soon as the species in
common, which inhabited the New and Old Worlds, migrated south of the
Polar Circle, they will have been completely cut off from each other.
This separation, as far as the more temperate productions are concerned,
must have taken place long ages ago. As the plants and animals migrated
southward, they will have become mingled in one great region with the
native American productions, and would have had to compete with them;
and, in the other great region, with those of the Old World.
Consequently