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-pigeon.

Under these circumstances, as it is admitted on all sides that races occur
in Nature, how are we to know whether any apparently distinct animals are
really of different physiological species, or not, seeing that the amount
of morphological difference is no safe guide? Is there any test of a
physiological species? The usual answer of physiologists is in the
affirmative. It is said that such a test is to be found in the phaenomena of