Origin of Species Variorum

the nestling birds of these several breeds were placed in a row, though most of them could just be distinguished, yet 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 in all its proportions from the young of the wild rock-pigeon and of the other breeds, ... almost exactly as much as in the adult state.

The above two principles explain these facts. Fanciers select for breeding their dogs, horses, pigeons, &c., 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 pigeons, show that the characteristic differences which give value to the breeds and which have been accumulated by mans selection, have not generally appeared at a very early period of life, and have been 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 be left but little modified and will