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.
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