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 all its proportions, almost
exactly as much as in the adult state.

The two principles above given seem to me to explain these facts in regard
to the later embryonic stages of our domestic varieties. Fanciers select
their horses, dogs, and pigeons, for breeding, when they are nearly grown
up: they are indifferent whether the desired qualities and structures have
been acquired earlier or {446} later in life, if the full-grown animal
possesses them. And the cases just given, more especially that of pigeons,
seem to show that the characteristic differences which give value to each
breed, and which have been accumulated by man's selection, have not
generally first appeared at an early period of life, and have been
inherited by the offspring at a corresponding not early period. But the
case of the short-faced tumbler, which when twelve hours old had acquired
its proper proportions, 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 the corresponding, but at an earlier age.

Now let us apply these facts and the above two principles--which latter,
though not proved true, can be shown to be in some degree probable--to
species in a state of nature. Let us take a genus of birds, descended on my
theory from some one parent-species, and of which the several new species
have become modified through natural selection in accordance with their
diverse habits. Then, from the many slight successive steps of variation
having supervened at a rather late age, and having been inherited at a
corresponding age, the young of the new species of our supposed genus will
manifestly tend to resemble each other much more closely than do the
adults, just as we have seen in the case of pigeons. We may extend this
view to whole families or even classes. The fore-limbs, for instance, which
served as legs in the parent-species, may have become, by a long course of
modification, adapted in one descendant to act as hands, in another as
paddles, in another as wings; and on the above two principles--namely of
each successiv