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 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; for they
live under the cold climate of Faroe in the north and of the Falklands in
the south, and on many an island in the torrid zones. Hence adaptation to
any special climate may be looked at as a quality readily grafted on an
innate wide flexibility of constitution, common to most animals. On this
view, the capacity of enduring the most different climates by man himself
and by his domestic animals, and the fact of the extinct elephant and
rhinoceros having formerly endured 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 as examples of a very common flexibility of
constitution, brought, under peculiar circumstances, into action.
How much of the acclimatisation of species to any peculiar climate is due
to mere habit, and how much to the natural selection of varieties having
different innate constitutions, and how much to both means combined, is an
obscure question.
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