A more genial climate, however, is far from
necessary; the kidney-bean, which is often injured by our spring frosts,
and peaches, which require the protection of a wall, have varied much in
England, as has the orange-tree in northern Italy, where it is barely able
to exist.[609] Nor can we overlook the fact, though not immediately
connected with our present subject, that the plants and shells of the
arctic regions are eminently variable.[610] Moreover, it does not appear
that a change of climate, whether more or less genial, is one of the most
potent causes of variability; for in regard to plants Alph. De Candolle, in
his 'Geographie {257} Botanique,' repeatedly shows that the native country
of a plant, where in most cases it has been longest cultivated, is that
where it has yielded the greatest number of varieties.

It is doubtful whether a change in the nature of the food is a potent cause
of variability. Scarcely any domesticated animal has varied more than the
pigeon or the fowl, but their food, especially that of highly-bred pigeons,
is generally the same. Nor can our cattle and sheep have been subjected to
any great change in this respect. But in all these cases the food probably
is much less varied in kind than that which was consumed by the species in
its natural state.[611]

Of all the causes which induce variability, excess of food, whether or not
changed in nature, is probably the most powerful. This view was held with
regard to plants by Andrew Knight, and is now held by Schleiden, more
especially in reference to the inorganic elements of the food.[612] In
order to give a plant more food it suffices in most cases to grow it
separately, and thus prevent other plants robbing its roots. It is
surprising, as I have often seen, how vigorously our common wild plants
flourish when planted by themselves, though not in highly manured land.
Growing plants separately is, in fact, the first step in cultivation. We
see the converse of the belief that excess of food induces variability in
the following statement by a great raiser of seeds of all kinds.[613] "It
is a rule invariably with us, when we desire to keep a true stock of any
one kind of seed, to grow it on poor land without dung; but when we grow
for quantity, we act contrary, and sometimes have dearly to repent of it."

In the case of animals the want of a proper amount of exercise, as
Bechstein has remarked, has perhaps played, independently of the direct
effe