idth of
mouth, length of nostril and of eyelid, size of feet and length of leg,
in the wild parent species, in pouters, fantails, runts, barbs, dragons,
carriers, and tumblers. Now, some of these birds, when mature, differ in
so extraordinary a manner in the length and form of beak, and in other
characters, that they would certainly have been ranked as distinct
genera if found in a state of nature. But when the nestling birds of
these several breeds were placed in a row, though most of them could
just be distinguished, the proportional differences in the above
specified points were incomparably less than in the full-grown birds.
Some characteristic points of difference--for instance, that of the
width of mouth--could hardly be detected in the young. But there was
one remarkable exception to this rule, for the young of the short-faced
tumbler differed from the young of the wild rock-pigeon, and of the
other breeds, in almost exactly the same proportions as in the adult
stage.

These facts are explained by the above two principles. Fanciers select
their dogs, horses, pigeons, etc., for breeding, when nearly grown up.
They are indifferent whether the desired qualities are acquired earlier
or later in life, if the full-grown animal possesses them. And the
cases just given, more especially that of the pigeons, show that
the characteristic differences which have been accumulated by man's
selection, and which give value to his breeds, do not generally appear
at a very early period of life, and are inherited at a corresponding not
early period. But the case of the short-faced tumbler, which when twelve
hours old possessed its proper characters, proves that this is not the
universal rule; for here the characteristic differences must either have
appeared at an earlier period than usual, or, if not so, the differences
must have been inherited, not at a corresponding, but at an earlier age.

Now, let us apply these two principles to species in a state of nature.
Let us take a group of birds, descended from some ancient form and
modified through natural selection for different habits. Then, from
the many slight successive variations having supervened in the several
species at a not early age, and having been inherited at a corresponding
age, the young will have been but little modified, and they will still
resemble each other much more closely than do the adults, just as we
have seen with the breeds of the pigeon. We may extend thi