ds. But
it is expressly stated that they are not constant; they appear
"frequently," or "occasionally," they are "not strictly inherited, for
they occur or fail in animals of the same litter;" and they are not
always symmetrical, sometimes appearing on one side of the face alone.
Now whatever may be the cause or explanation of these anomalous
appendages they cannot be classed with "specific characters," the most
essential features of which are, that they _are_ symmetrical, that they
_are_ inherited, and that they _are_ constant. Admitting that this
peculiar appendage is (as Mr. Romanes says rather confidently, "we
happen to know it to be") wholly useless and meaningless, the fact would
be rather an argument against specific characters being also
meaningless, because the latter never have the characteristics which
this particular variation possesses.

These useless or non-adaptive characters are, apparently, of the same
nature as the "sports" that arise in our domestic productions, but
which, as Mr. Darwin says, without the aid of selection would soon
disappear; while some of them may be correlations with other characters
which are or have been useful. Some of these correlations are very
curious. Mr. Tegetmeier informed Mr. Darwin that the young of white,
yellow, or dun-coloured pigeons are born almost naked, whereas other
coloured pigeons are born well clothed with down. Now, if this
difference occurred between wild species of different colours, it might
be said that the nakedness of the young could not be of any use. But the
colour with which it is correlated might, as has been shown, be useful
in many ways. The skin and its various appendages, as horns, hoofs,
hair, feathers, and teeth, are homologous parts, and are subject to very
strange correlations of growth. In Paraguay, horses with curled hair
occur, and these always have hoofs exactly like those of a mule, while
the hair of the mane and tail is much shorter than usual. Now, if any
one of these characters were useful, the others correlated with it might
be themselves useless, but would still be tolerably constant because
dependent on a useful organ. So the tusks and the bristles of the boar
are correlated and vary in development together, and the former only may
be useful, or both may be useful in unequal degrees.

The difficulty as to how individual differences or sports can become
fixed and perpetuated, if altogether useless, is evaded by those who
hold that