om their identity in the caterpillar, cocoon, and mature
states;[763] yet they differ much in constitution: the Indian form "will
flourish only in warm latitudes," the other is quite hardy and withstands
cold and rain.

Plants are more strictly adapted to climate than are animals. The
latter when domesticated withstand such great diversities of climate,
that we find nearly the same species in tropical and temperate
countries; whilst the cultivated plants are widely dissimilar. Hence a
larger field is open for inquiry in regard to the acclimatisation of
plants than of animals. It is no exaggeration to say that with almost
every plant which has long been cultivated varieties exist, which are
endowed with constitutions fitted for very different climates; I will
select only a few of the more striking cases, as it would be tedious to
give all. In North America numerous fruit-trees have been raised, and
in horticultural publications,--for instance, in Downing,--lists are
given of the varieties which are best able to withstand the severe
climate of the northern States and Canada. Many American varieties of
the pear, plum, and peach are excellent in their own country, but until
recently hardly one was known that succeeded in England; and with
apples,[764] not one succeeds. Though the American varieties can
withstand a severer winter than ours, the summer here is not hot
enough. Fruit-trees have originated in Europe as in America with
different constitutions, but they are not here much noticed, as the
same nurserymen do not supply a wide area. The Forelle pear flowers
early, and when the flowers have just set, and this is the critical
period, they have been observed, both in France and England, to
withstand with complete impunity a frost of 18deg and even 14deg Fahr.,
which killed the flowers, whether fully expanded or in bud, of all
other kinds of pears.[765] This power in the flower of resisting cold
and afterwards producing fruit does not invariably depend, as we know
on good authority,[766] on general constitutional vigour.

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In proceeding northward, the number of varieties which are enabled to
resist the climate rapidly decreases, as may be seen in the list of the
varieties of the cherry, apple, and pear, which can be cultivated in
the neighbourhood of Stockholm.[767] Near Moscow, Prince Tro