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.