248 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 



race and heavy cart-horses, I find that this is by no 

 means the case. 



As we have conclusive evidence that the breeds of 

 the Pigeon are descended from a single wild species, I 

 compared the young within twelve hours after being 

 hatched; I carefully measured the proportions (but 

 will not here give the details) of the beak, width of 

 mouth, length of nostril and of evelid, size of feet and 

 length of leg, in the wild parent-species, in pouters, 

 fantails, runts, barbs, dragons, carriers, and tumblers. 

 Xow some of these birds, when mature, differ in so 

 extraordinary a manner in the length and form of beak, 

 and in other characters, that they would certainly have 

 been ranked 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 tacts are explained by the above two principles. 

 Fanciers select their dogs, horses, pigeons, &c, 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 



