Chap. I. 
UNDEK DOMESTICATION. 
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 par¬ 
ticular 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 hereafter briefly 
mentioned. I will here only allude to what may be 
called correlation of growth. Any change in the embryo 
or larva will almost certainly entail changes in the 
mature animal. In monstrosities, the correlations be¬ 
tween quite distinct parts are very curious; and many 
instances are given in Isidore Geoffrey St. Hilaire's great 
work on this subject. Breeders believe that long limbs 
are almost always accompanied by an elongated head. 
Some instances of correlation are quite whimsical: thus 
