Seedlings from the same fruit, and the young of the same litter,
sometimes differ considerably from each other, though both the young
and the parents, as Muller 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, and 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, etc., 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 seem 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
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 and 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 hereafter briefly mentioned. I will here only
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