[Footnote: Compare Weismann's _Essays Upon Heredity_, p. 310, _et
seq_. 1893.]

But a race, once produced, is no more a fixed and immutable entity than the
stock whence it sprang; variations arise among its members, and as these
variations are transmitted like any others, new races may be developed out
of the pre-existing one _ad infinitum_, or, at least, within any limit
at present determined. Given sufficient time and sufficiently careful
selection, and the multitude of races which may arise from a common stock
is as astonishing as are the extreme structural differences which they may
present. A remarkable example of this is to be found in the rock-pigeon,
which Mr. Darwin has, in our opinion, satisfactorily demonstrated to be the
progenitor of all our domestic pigeons, of which there are certainly more
than a hundred well-marked races. The most noteworthy of these races are,
the four great stocks known to the "fancy" as tumblers, pouters, carriers,
and fantails; birds which not only differ most singularly 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