and it
now remains to consider in what does this power of domestication
consist{210}, a subject of considerable difficulty. Observing that
organic beings of almost every class, in all climates, countries, and
times, have varied when long bred under domestication, we must conclude
that the influence is of some very general nature{211}. Mr Knight alone,
as far as I know, has tried to define it; he believes it consists of an
excess of food, together with transport to a more genial climate, or
protection from its severities. I think we cannot admit this latter
proposition, for we know how many vegetable products, aborigines of this
country, here vary, when cultivated without any protection from the
weather; and some of our variable trees, as apricots, peaches, have
undoubtedly been derived from a more genial climate. There appears to be
much more truth in the doctrine of excess of food being the cause,
though I much doubt whether this is the sole cause, although it may well
be requisite for the kind of variation desired by man, namely increase
of size and vigour. No doubt horticulturalists, when they wish to raise
new seedlings, often pluck off all the flower-buds, except a few, or
remove the whole during one season, so that a great stock of nutriment
may be thrown into the flowers which are to seed. When plants are
transported from high-lands, forests, marshes, heaths, into our gardens
and greenhouses, there must be a considerable change of food, but it
would be hard to prove that there was in every case an excess of the
kind proper to the plant. If it be an excess of food, compared with that
which the being obtained in its natural state{212}, the effects continue
for an improbably long time; during how many ages has wheat been
cultivated, and cattle and sheep reclaimed, and we cannot suppose their
_amount_ of food has gone on increasing, nevertheless these are amongst
the most variable of our domestic productions. It has been remarked
(Marshall) that some of the most highly kept breeds of sheep and cattle
are truer or less variable than the straggling animals of the poor,
which subsist on commons, and pick up a bare subsistence{213}. In the
case of forest-trees raised in nurseries, which vary more than the same
trees do in their aboriginal forests, the cause would seem simply to lie
in their not having to struggle against other trees and weeds, which in
their natural state doubtless would limit the conditions of their
exis