flowers are sterile; on the other hand, plants raised from sweet-peas
grown near Darjeeling in Upper India, but originally derived from
England, can be successfully cultivated on the plains of India; for
they flower and seed profusely, and their stems are lax and scandent.
In some of the foregoing cases, as Dr. Hooker has remarked to me, the
greater success may perhaps be attributed to the seeds having been more
fully ripened under a more favourable climate; but this view can hardly
be extended to so many cases, including plants, which, from being
cultivated under a climate hotter than their native one, become fitted
for a still hotter climate. We may therefore safely conclude that
plants can to a certain extent become accustomed to a climate either
hotter or colder than their own; although these latter cases have been
more frequently observed.

We will now consider the means by which acclimatisation may be effected,
namely, through the spontaneous appearance of varieties having a different
constitution, and through the effects of use or habit. In regard to the
first process, there is no evidence that a change in the constitution of
the {312} offspring necessarily stands in any direct relation with the
nature of the climate inhabited by the parents. On the contrary, it is
certain that hardy and tender varieties of the same species appear in the
same country. New varieties thus spontaneously arising become fitted to
slightly different climates in two different ways; firstly, they may have
the power, either as seedlings or when full-grown, of resisting intense
cold, as with the Moscow pear, or of resisting intense heat, as with some
kinds of Pelargonium, or the flowers may withstand severe frost, as with
the Forelle pear. Secondly, plants may become adapted to climates widely
different from their own, from flowering and fruiting either earlier or
later in the season. In both these cases the power of acclimatisation by
man consists simply in the selection and preservation of new varieties. But
without any direct intention on his part of securing a hardier variety,
acclimatisation may be unconsciously effected by merely raising tender
plants from seed, and by occasionally attempting their cultivation further
and further northwards, as in the case of maize, the orange, and the peach.

How much influence ought to be attributed to inherited habit or custom in
the acclimatisation