rly in size, colour,
and habits, but in the form of the beak and of the skull: in the
proportions of the beak to the skull; in the number of tail-feathers; in
the absolute and relative size of the feet; in the presence or absence
of the uropygial gland; in the number of vertebrae in the back; in
short, in precisely those characters in which the genera and species of
birds differ from one another.

And it is most remarkable and instructive to observe, that none of these
races can be shown to have been originated by the action of changes
in what are commonly called external circumstances, upon the wild
rock-pigeon. On the contrary, from time immemorial, pigeon-fanciers have
had essentially similar methods of treating their pets, which have
been housed, fed, protected and cared for in much the same way in all
pigeonries. In fact, there is no case better adapted than that of
the pigeons to refute the doctrine which one sees put forth on
high authority, that "no other characters than those founded on the
development of bone for the attachment of muscles" are capable of
variation. In precise contradiction of this hasty assertion, Mr.
Darwin's researches prove that the skeleton of the wings in domestic
pigeons has hardly varied at all from that of the wild type; while, on
the other hand, it is in exactly those respects, such as the relative
length of the beak and skull, the number of the vertebrae, and the
number of the tail-feathers, in which muscular exertion can have no
important influence, that the utmost amount of variation has taken
place.

We have said that the following out of the properties exhibited by
physiological species would lead us into difficulties, and at this point
they begin to be obvious; for if, as the result of spontaneous variation
and of selective breeding, the progeny of a common stock may become
separated into groups distinguished from one another by constant, not
sexual, morphological characters, it is clear that the physiological
definition of species is likely to clash with the morphological
definition. No one would hesitate to describe the pouter and the tumbler
as distinct species, if they were found fossil, or if their skins and
skeletons were imported, as those of exotic wild birds commonly
are--and without doubt, if considered alone, they are good and distinct
morphological species. On the other hand, they are not physiological
species, for they are descended from a common stock, the rock-p