CHAPTER I. VARIATION UNDER DOMESTICATION.
2. EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS; CORRELATED VARIATION; INHERITANCE.
Changed habits produce an inherited effect as in the period of the
flowering of plants when transported from one climate to another. With
animals the increased use or disuse of parts has had a more marked
influence; thus 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 this change may be
safely attributed to the domestic duck flying much less, and walking more,
than its wild parents. The great and inherited development of the udders
in cows and goats in countries where they are habitually milked, in
comparison with these organs in other countries, is probably another
instance of the effects of use. Not one of our domestic animals can be
named which has not in some country drooping ears; and the view which has
been suggested that the drooping is due to disuse of the muscles of the
ear, from the animals being seldom much alarmed, seems probable.
Many laws regulate variation, some few of which can be dimly seen, and will
hereafter be briefly discussed. I will here only allude to what may be
called correlated variation. Important changes in the embryo or larva will
probably entail changes in the mature animal. In monstrosities, the
correlations between quite distinct parts are very curious; and many
instances are given in Isidore Geoffroy 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
cats which are entirely white and have blue eyes are generally deaf; but it
has been lately stated by Mr. Tait that this is confined to the males.
Colour and constitutional peculiarities go together, of which many
remarkable cases could be given among animals and plants. From facts
collected by Heusinger, it appears that white sheep and pigs are injured by
certain plants, while dark-coloured individuals escape: Professor Wyman
has recently communicated to me a good illustration of this fact; on asking
some farmers in Virginia how it was that all their pigs were black, they
informed him that the pigs ate the paint-root (Lachnanthes), which coloured
their bones pink, and which caused the hoofs of all but the black varieties
to drop off; and one of the "crackers" (i.e. Virginia squatters) added, "we
select the black members of a litter for raising, as they alone have a good
chance of living." Hairless dogs have imperfect teeth; long-haired and
coarse-haired animals are apt to have, as is asserted, long or many horns;
pigeons with feathered feet have skin between their outer toes; pigeons
with short beaks have small feet, and those with long beaks large feet.
Hence if man goes on selecting, and thus augmenting, any peculiarity, he
will almost certainly modify unintentionally other parts of the structure,
owing to the mysterious laws of correlation.