It would be tedious even to
enumerate the multitude of points which still go on varying or have
recently varied. Many variations would occur in correlation, as the length
of the wing and tail feathers--the number of the primary wing-feathers, as
well as the number and breadth of the ribs, in correlation with the size
and form of the body--the number of the scutellae, with the size of the
feet--the length of the tongue, with the length of the beak--the size of
the nostrils and eyelids and the form of lower jaw in correlation with the
development of wattle--the nakedness of the young with the future colour of
the plumage--the size of the feet and beak, and other such points. Lastly,
as our birds are supposed to be confined in an aviary, they would use their
wings and legs but little, and certain parts of the skeleton, such as the
sternum and scapulae and the feet, would in consequence become slightly
reduced in size.

As in our assumed case many birds have to be indiscriminately killed every
year, the chances are against any new variety surviving long enough to
breed. And as the variations which arise are of an extremely diversified
nature, the chances are very great against two birds pairing which have
varied in the same manner; nevertheless, a varying bird even when not thus
paired would occasionally transmit its character to its young; and these
would not only be exposed to the same conditions which first caused the
variation in question to appear, but would in addition inherit from their
one modified parent a tendency again to vary in the same manner. So that,
if the conditions decidedly tended to induce some particular variation, all
the birds might {422} in the course of time become similarly modified. But
a far commoner result would be, that one bird would vary in one way and
another bird in another way; one would be born with a little longer beak,
and another with a shorter beak; one would gain some black feathers,
another some white or red feathers. And as these birds would be continually
intercrossing, the final result would be a body of individuals differing
from each other slightly in many ways, yet far more than did the original
rock-pigeons. But there would not be the least tendency to the formation of
distinct breeds.

If two separate lots of pigeons were to be treated in the manner just
described, one in England and the other in a tropical country, the two lots
being supplied with different food, would th