n to believe that species in a state of nature are closely
limited in their ranges by the competition of other organic beings quite
as much as, or more than, by adaptation to particular climates. But
whether or not this adaptation is in most cases very close, we have
evidence with some few plants, of their becoming, to a certain extent,
naturally habituated to different temperatures; that is, they become
acclimatised: thus the pines and rhododendrons, raised from seed
collected by Dr. Hooker from the same species growing at different
heights on the Himalayas, were found to possess in this country
different constitutional powers of resisting cold. Mr. Thwaites informs
me that he has observed similar facts in Ceylon; analogous observations
have been made by Mr. H.C. Watson on European species of plants brought
from the Azores to England; and I could give other cases. In regard to
animals, several authentic instances could be adduced of species having
largely extended, within historical times, their range from warmer to
colder latitudes, and conversely; but we do not positively know that
these animals were strictly adapted to their native climate, though in
all ordinary cases we assume such to be the case; nor do we know that
they have subsequently become specially acclimatised to their new homes,
so as to be better fitted for them than they were at first.

As we may infer that our domestic animals were originally chosen by
uncivilised man because they were useful, and because they bred readily
under confinement, and not because they were subsequently found capable
of far-extended transportation, 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 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 Falkland