er, sometimes
differ considerably from each other, though both the young and the parents,
as Mueller has remarked, have apparently been exposed to exactly the same
conditions of life; and this shows how unimportant the direct effects of
the conditions of life are in comparison with the laws of reproduction, of
growth, and of inheritance; for had the action of the conditions been
direct, if any of the young had varied, all would probably have varied in
the same manner. To judge how much, in the case of any variation, we should
attribute to the direct action of heat, moisture, light, food, &c., is most
difficult: my impression is, that with animals such agencies have produced
very little direct effect, though apparently more in the case of plants.
Under this point of view, Mr. Buckman's recent experiments on plants are
extremely valuable. When all or nearly all the individuals exposed to
certain conditions are affected in the same way, the change at first
appears to be directly due to such conditions; but in some cases it can be
shown that quite opposite conditions produce {11} similar changes of
structure. Nevertheless some slight amount of change may, I think, be
attributed to the direct action of the conditions of life--as, in some
cases, increased size from amount of food, colour from particular kinds of
food or from light, and perhaps the thickness of fur from climate.

Habit also has a decided influence, as in the period of flowering with
plants when transported from one climate to another. In animals it has a
more marked effect; for instance, I find in the domestic duck that the
bones of the wing weigh less and the bones of the leg more, in proportion
to the whole skeleton, than do the same bones in the wild-duck; and I
presume that this change may be safely attributed to the domestic duck
flying much less, and walking more, than its wild parent. The great and
inherited development of the udders in cows and goats in countries where
they are habitually milked, in comparison with the state of these organs in
other countries, is another instance of the effect of use. Not a single
domestic animal can be named which has not in some country drooping ears;
and the view suggested by some authors, that the drooping is due to the
disuse of the muscles of the ear, from the animals not being much alarmed
by danger, seems probable.

There are many laws regulating variation, some few of which can be dimly
seen, and will be herea