DEVELOPMENT AND EMBRYOLOGY 485 



and in other characters, that they would certainly have been 

 Banked as distinct genera if found in a state of nature. 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 state. 



These facts are explained by the above two principles. 

 Fanciers select their dogs, horses, pigeons, &c., for breeding, 

 when nearly grown up: they are indifferent wdiether the de- 

 sired 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 charac- 

 teristic differences which have been accumulated by man's 

 selection, and which give value to his breeds, do not gen- 

 erally 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. We may 

 extend this view to widely distinct structures and to whole 

 classes. The fore-limbs, for instance, which once served 

 as legs to a remote progenitor, may have become, through 



