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.