It is interesting to note that almost every part of the
bird, whose variations can be noted and selected, has led to variations
of a considerable extent, and many of these have necessitated changes in
the plumage and in the skeleton quite as great as any that occur in the
numerous distinct species of large genera. The form of the skull and
beak varies enormously, so that the skulls of the Short-faced tumbler
and some of the Carriers differ more than any wild pigeons, even those
classed in distinct genera. The breadth and number of the ribs vary, as
well as the processes on them; the number of the vertebrae and the
length of the sternum also vary; and the perforations in the sternum
vary in size and shape. The oil gland varies in development, and is
sometimes absent. The number of the wing-feathers varies, and those of
the tail to an enormous extent. The proportions of the leg and feet and
the number of the scutellae also vary. The eggs also vary somewhat in
size and shape; and the amount of downy clothing on the young bird, when
first hatched, differs very considerably.
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