tend to
vary in the same manner, as we see in long and short-legged, or in thick
and thin-legged races of the horse and dog. Isidore Geoffroy[807] has
remarked on the tendency of supernumerary digits in man to appear, not only
on the right and left sides, but on the upper and lower extremities. Meckel
has insisted[808] that, when the muscles of the arm depart in number or
arrangement from their proper type, they almost always imitate those of the
leg; and so conversely the varying muscles of the leg imitate the normal
muscles of the arm.

In several distinct breeds of the pigeon and fowl, the legs and the two
outer toes are heavily feathered, so that in the trumpeter pigeon they
appear like little wings. In the feather-legged bantam the "boots" or
feathers, which grow from the outside of the leg and generally from the two
outer toes, have, {323} according to the excellent authority of Mr.
Hewitt,[809] been seen to exceed the wing-feathers in length, and in one
case were actually nine and a half inches in length! As Mr. Blyth has
remarked to me, these leg-feathers resemble the primary wing-feathers, and
are totally unlike the fine down which naturally grows on the legs of some
birds, such as grouse and owls. Hence it may be suspected that excess of
food has first given redundancy to the plumage, and then that the law of
homologous variation has led to the development of feathers on the legs, in
a position corresponding with those on the wing, namely, on the outside of
the tarsi and toes. I am strengthened in this belief by the following
curious case of correlation, which for a long time seemed to me utterly
inexplicable, namely, that in pigeons of any breed, if the legs are
feathered, the two outer toes are partially connected by skin. These two
outer toes correspond with our third and fourth toes. Now, in the wing of
the pigeon or any other bird, the first and fifth digits are wholly
aborted; the second is rudimentary and carries the so-called
"bastard-wing;" whilst the third and fourth digits are completely united
and enclosed by skin, together forming the extremity of the wing. So that
in feather-footed pigeons, not only does the exterior surface support a row
of long feathers, like wing-feathers, but the very same digits which in the
wing are completely united by skin become partially united by skin in the
feet; and thus by the law of the correlated variation of homologous parts
we can understand the curious connectio