462 DEVELOPMENT AND EMBRTOLOar. 



tic varieties. Some authors who have written on dogs 

 maintain that the greyhound and bull-dog, though so dif- 

 ferent, are really closely allied varieties, descended from 

 the same wild stock, hence I was curious to see how far 

 their puppies differed from each other. I was told by 

 breeders that they differed Just as much as their parents, 

 and this, judging by the eye, seemed almost to be the case; 

 but on actually measuring" the old dogs and their six-days- 

 old puppies, I found that the puppies had not acquired 

 nearly their full amount of proportional difference. So, 

 again, I was told that the foals of cart and race horses — 

 breeds which have been almost wholly formed by selection 

 under domestication — differed as much as the full-grown 

 animals; but having had careful measurements rnade of 

 the dams and of three-days-old colts of 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 com- 

 pared 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 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 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 dif- 

 ferences 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 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 



