484 ORIGIN OF SPECIES 



only appear at corresponding ages ; for instance, peculiarities 

 in the caterpillar, cocoon, or imago states of the silk-moth: 

 or, again, in the full-grown horns of cattle. But variations, 

 which, for all that we can see might have first appeared 

 either earlier or later in life, likewise tend to reappear at a 

 corresponding age in the offspring and parent. I am far 

 from meaning that this is invariably the case, and I could 

 give several exceptional cases of variations (taking the word 

 in the largest sense) which have supervened at an earlier 

 age in the child than in the parent. 



These two principles, namely, that slight variations gen- 

 erally appear at a not very early period of life, and are in- 

 herited at a corresponding not early period, explain, as I 

 believe, 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, main- 

 tain that the greyhound and bulldog, though so different, 

 are really closely allied varieties, descended from the same 

 v^^ild 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 meas- 

 uring 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 made of the dams and of the 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 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 mature, differ 

 in so extraordinary a manner in the length and form of beakj 



