tion
of the various points in which their organization differs. In the first
place, the back of the skull may differ a good deal, and the development
of the bones of the face may vary a great deal; the back varies a good
deal; the shape of the lower jaw varies; the tongue varies very greatly,
not only in correlation to the length and size of the beak, but it seems
also to have a kind of independent variation of its own. Then the amount
of naked skin round the eyes, and at the base of the beak, may vary
enormously; so may the length of the eyelids, the shape of the nostrils,
and the length of the neck. I have already noticed the habit of blowing
out the gullet, so remarkable in the Pouter, and comparatively so in the
others. There are great differences, too, in the size of the female and
the male, the shape of the body, the number and width of the processes
of the ribs, the development of the ribs, and the size, shape, and
development of the breastbone. We may notice, too,--and I mention
the fact because it has been disputed by what is assumed to be high
authority,--the variation in the number of the sacral vertebrae. The
number of these varies from eleven to fourteen, and that without any
diminution in the number of the vertebrae of the back or of the tail.
Then the number and position of the tail-feathers may vary enormously,
and so may the number of the primary and secondary feathers of the
wings. Again, the length of the feet and of the beak,--although they
have no relation to each other, yet appear to go together,--that is, you
have a long beak wherever you have long feet. There are differences also
in the periods of the acquirement of the perfect plumage,--the size
and shape of the eggs,--the nature of flight, and the powers of
flight,--so-called "homing" birds having enormous flying powers; [1]
while, on the other hand, the little Tumbler is so called because of its
extraordinary faculty of turning head over heels in the air, instead of
pursuing a direct course. And, lastly, the dispositions and voices of
the birds may vary. Thus the case of the pigeons shows you that there
is hardly a single particular,--whether of instinct, or habit, or
bony structure, or of plumage,--of either the internal economy or the
external shape, in which some variation or change may not take place,
which, by selective breeding, may become perpetuated, and form the
foundation of, and give rise to, a new race.

[Footnote 1: The "Carrier,"