increased by selection during many
generations. Now, with the increased size and changed direction of the
ears, not only has the bony auditory meatus become changed in outline,
direction, and greatly in size, but the whole skull has been slightly
modified. This could be clearly seen in "half-lops"--that is, in rabbits
with one ear alone lopping forward--for the opposite sides of their skulls
were not strictly symmetrical. This seems to me a curious instance of
correlation, between hard {325} bones and organs so soft and flexible, as
well as so unimportant under a physiological point of view, as the external
ears. The result no doubt is largely due to mere mechanical action, that
is, to the weight of the ears, on the same principle that the skull of a
human infant is easily modified by pressure.

The skin and the appendages of hair, feathers, hoofs, horns, and teeth, are
homologous over the whole body. Every one knows that the colour of the skin
and that of the hair usually vary together; so that Virgil advises the
shepherd to look whether the mouth and tongue of the ram are black, lest
the lambs should not be purely white. With poultry and certain ducks we
have seen that the colour of the plumage stands in some connexion with the
colour of the shell of the egg,--that is, with the mucous membrane which
secretes the shell. The colour of the skin and hair, and the odour emitted
by the glands of the skin, are said[812] to be connected, even in the same
race of men. Generally the hair varies in the same way all over the body in
length, fineness, and curliness. The same rule holds good with feathers, as
we see with the laced and frizzled breeds both of fowls and pigeons. In the
common cock the feathers on the neck and loins are always of a particular
shape, called hackles: now in the Polish breed, both sexes are
characterised by a tuft of feathers on the head; but through correlation
these feathers in the male always assume the form of hackles. The wing and
tail-feathers, though arising from parts not homologous, vary in length
together; so that long or short winged pigeons generally have long or short
tails. The case of the Jacobin-pigeon is more curious, for the wing and
tail feathers are remarkably long; and this apparently has arisen in
correlation with the elongated and reversed feathers on the back of the
neck, which form the hood.

The hoofs and hair are homologous appendages; and a careful observer,
namely Azara,[813]