with the law of analogous variation. This law implies, as stated in a
previous chapter, that the varieties of one species frequently mock
distinct but allied species; and this fact is explained, according to the
views which I maintain, on the principle of allied species having descended
from one primitive form. The white Silk fowl with black skin and bones
degenerates, as has been observed by Mr. Hewitt and Mr. R. Orton, in our
climate; that is, it reverts to the ordinary colour of the common fowl in
its skin and bones, due care having been taken to prevent any cross. In
Germany[390] a distinct breed with black bones, and with black, not silky
plumage, has likewise been observed to degenerate.

Mr. Tegetmeier informs me that, when distinct breeds are crossed, fowls are
frequently produced with their feathers marked or pencilled by narrow
transverse lines of a darker colour. This may be in part explained by
direct reversion to the parent-form, the Bankiva hen; for this bird has all
its upper plumage finely mottled with dark and rufous brown, with the
mottling partially and obscurely arranged in transverse lines. But the
tendency to pencilling is probably much strengthened by the law of
analogous variation, for the hens of some other species of Gallus are more
plainly pencilled, and the hens of many gallinaceous birds belonging to
other genera, as the partridge, have pencilled feathers. Mr. Tegetmeier has
{244} also remarked to me, that, although with domestic pigeons we have so
great a diversity of colouring, we never see either pencilled or spangled
feathers; and this fact is intelligible on the law of analogous variation,
as neither the wild rock-pigeon nor any closely-allied species has such
feathers. The frequent appearance of pencilling in crossed birds probably
accounts for the existence of "cuckoo" sub-breeds in the Game, Polish,
Dorking, Cochin, Andalusian, and Bantam breeds. The plumage of these birds
is slaty-blue or grey, with each feather transversely barred with darker
lines, so as to resemble in some degree the plumage of the cuckoo. It is a
singular fact, considering that the male of no species of Gallus is in the
least barred, that the cuckoo-like plumage has often been transferred to
the male, more especially in the cuckoo Dorking; and the fact is all the
more singular, as in gold and silver pencilled Hamburghs, in which
pencilling is characteristic of the breed, the male is hardly at all
pencilled, this