can hardly be considered as
living under strictly natural conditions; the seedlings are raised and
protected in nursery-grounds, and must often be transplanted into places
where wild trees of the kind would not naturally grow. It would be esteemed
a prodigy if a dog-rose growing in a hedge produced by bud-variation a
moss-rose, or a wild bullace or wild cherry-tree yielded a branch bearing
fruit of a different shape and colour from the ordinary fruit. The prodigy
would be enhanced if these varying branches were found capable of
propagation, not only by grafts, but sometimes by seed; yet analogous cases
have occurred with many of our highly cultivated trees and herbs.

These several considerations alone render it probable that variability of
every kind is directly or indirectly caused by changed conditions of life.
Or, to put the case under another point of view, if it were possible to
expose all the individuals of a species during many generations to
absolutely uniform conditions of life, there would be no variability.

_On the Nature of the Changes in the Conditions of Life which induce
Variability._

From a remote period to the present day, under climates and circumstances
as different as it is possible to conceive, organic beings of all kinds,
when domesticated or cultivated, have {256} varied. We see this with the
many domestic races of quadrupeds and birds belonging to different orders,
with gold-fish and silkworms, with plants of many kinds, raised in various
quarters of the world. In the deserts of northern Africa the date-palm has
yielded thirty-eight varieties; in the fertile plains of India it is
notorious how many varieties of rice and of a host of other plants exist;
in a single Polynesian island, twenty-four varieties of the bread-fruit,
the same number of the banana, and twenty-two varieties of the arum, are
cultivated by the natives; the mulberry-tree in India and Europe has
yielded many varieties serving as food for the silkworm; and in China
sixty-three varieties of the bamboo are used for various domestic
purposes.[607] These facts alone, and innumerable others could be added,
indicate that a change of almost any kind in the conditions of life
suffices to cause variability--different changes acting on different
organisms.

Andrew Knight[608] attributed the variation of both animals and plants to a
more abundant supply of nourishment, or to a more favourable climate, than
that natural to the species.