Origin of Species Variorum

Comparison with 1859

the nestling birds of these several breeds were placed in a row, though most of them could be distinguished from each other, yet their proportional differences in the above specified several 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 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 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. And the cases 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 which give value to each breed, and which have been inherited, not at the corresponding, but at an early period of life, and have been inherited by the offspring at a corresponding not early period. ↑

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

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