As I believe that our domestic animals were originally chosen by
uncivilised man because they were useful and bred readily under
confinement, and not because they were subsequently found capable of
far-extended transportation, I think the common and extraordinary
capacity in our domestic animals of not only withstanding the most
different climates but of being perfectly fertile (a far severer test)
under them, may be used as an argument that a large proportion of
other animals, now in a state of nature, could easily be brought to
bear widely different climates. We must not, however, push the
foregoing argument too far, on account of the probable origin of some
of our domestic animals from several wild stocks: the blood, for
instance, of a tropical and arctic wolf or wild dog may perhaps be
mingled in our domestic breeds. The rat and mouse cannot be considered
as domestic animals, but they have been transported by man to many
parts of the world, and now have a far wider range than any other
rodent, living free under the cold climate of Faroe in the north and
of the Falklands in the south, and on many islands in the torrid
zones. Hence I am inclined to look at adaptation to any special
climate as a quality readily grafted on an innate wide flexibility of
constitution, which is common to most animals. On this view, the
capacity of enduring the most different climates by man himself and by
his domestic animals, and such facts as that former species of the
elephant and rhinoceros were capable of enduring a glacial climate,
whereas the living species are now all tropical or sub-tropical in
their habits, ought not to be looked at as anomalies, but merely as
examples of a very common flexibility of constitution, brought, under
peculiar circumstances, into play.
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