392 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 



These two principles, namely, that slight variations generally 

 appear at a not very early period of life, and are inherited at a cor- 

 responding not early period, explain, as I helieve, all the above 

 specified leading facts in embryology. But first let us look to a few 

 analogous cases in our domestic varieties. Some authors who have 

 written on Dogs, maintain that the greyhound and bulldog, though 

 so different, 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 domesti- 

 cation — differed as much as the full-grown animals ; but having had 

 careful measurements made of the dams and of threc-d ays-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 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 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 

 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 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 



