of animals and plants is a much more difficult
question. In many cases natural selection can hardly have failed to have
come into play and complicated the result. It is notorious that mountain
sheep resist severe weather and storms of snow which would destroy lowland
breeds; but then mountain sheep have been thus exposed from time
immemorial, and all delicate individuals will have been destroyed, and the
hardiest preserved. So with the Arrindy silk-moths of China and India; who
can tell how far natural selection may have taken a share in the formation
of the two races, which are now fitted for such widely different climates?
It seems at first probable that the many fruit-trees, which are so well
fitted for the hot summers and cold winters of North America, in contrast
with their poor success under our climate, have become adapted through
habit; but when we reflect on the multitude of seedlings annually raised in
that country, and that none would succeed unless born with a fitting
constitution, it is possible that mere habit may have done nothing towards
their acclimatisation. On the other hand, when we {313} hear that Merino
sheep, bred during no great number of generations at the Cape of Good
Hope--that some European plants raised during only a few generations in the
cooler parts of India, withstand the hotter parts of that country much
better than the sheep or seeds imported directly from England, we must
attribute some influence to habit. We are led to the same conclusion when
we hear from Naudin[793] that the races of melons, squashes, and gourds,
which have long been cultivated in Northern Europe, are comparatively more
precocious, and need much less heat for maturing their fruit, than the
varieties of the same species recently brought from tropical regions. In
the reciprocal conversion of summer and winter wheat, barley, and vetches
into each other, habit produces a marked effect in the course of a very few
generations. The same thing apparently occurs with the varieties of maize,
which, when carried from the Southern to the Northern States of America, or
into Germany, soon become accustomed to their new homes. With vine-plants
taken to the West Indies from Madeira, which are said to succeed better
than plants brought directly from France, we have some degree of
acclimatisation in the individual, independently of the production of new
varieties by seed.

The common experience of agriculturists is of some value, and