cts of the disuse of any particular organ, an important part in causing
variability. We can see in a vague manner that, when the organised and
nutrient fluids of the body are not used during growth, or by the wear and
tear of the tissues, {258} they will be in excess; and as growth,
nutrition, and reproduction are intimately allied processes, this
superfluity might disturb the due and proper action of the reproductive
organs, and consequently affect the character of the future offspring. But
it may be argued that neither an excess of food nor a superfluity in the
organised fluids of the body necessarily induces variability. The goose and
the turkey have been well fed for many generations, yet have varied very
little. Our fruit-trees and culinary plants, which are so variable, have
been cultivated from an ancient period, and, though they probably still
receive more nutriment than in their natural state, yet they must have
received during many generations nearly the same amount; and it might be
thought that they would have become habituated to the excess. Nevertheless,
on the whole, Knight's view, that excess of food is one of the most potent
causes of variability, appears, as far as I can judge, probable.

Whether or not our various cultivated plants have received nutriment in
excess, all have been exposed to changes of various kinds. Fruit-trees are
grafted on different stocks, and grown in various soils. The seeds of
culinary and agricultural plants are carried from place to place; and
during the last century the rotation of our crops and the manures used have
been greatly changed.

Slight changes of treatment often suffice to induce variability. The simple
fact of almost all our cultivated plants and domesticated animals having
varied in all places and at all times, leads to this conclusion. Seeds
taken from common English forest-trees, grown under their native climate,
not highly manured or otherwise artificially treated, yield seedlings which
vary much, as may be seen in every extensive seed-bed. I have shown in a
former chapter what a number of well marked and singular varieties the
thorn (_Crataegus oxyacantha_) has produced; yet this tree has been
subjected to hardly any cultivation. In Staffordshire I carefully examined
a large number of two British plants, namely, _Geranium phaeum_ and
_Pyrenaicum_, which have never been highly cultivated. These plants had
spread spontaneously by seed from a common garden into a