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