434 DEVELOPMENT AND EMBRYOLOGY. 



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 care- 

 fully measured the proportions (but will not here give 

 the details) of the beak, width of mouth, length of nostril 

 and of eyelid, size of feet and length of leg, in the wild 

 parent species, in pouters, fantails, runts, barbs, dragons, 

 carriers, and tumblers. Now, some of these birds, when 

 j nature, 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 differ- 

 ence — for instance, that of the width of mouth — could 

 hardly be detected in the young. But there was one re- 

 markable 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 breed- 

 ing, 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. 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, 



