Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 247 



Certain variations can 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 

 generally appear at a not very early period of life, and 

 are inherited 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, 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 domestication — differed as much as the 

 full-grown animals; but having had careful measure- 

 ments made of the dams and of three-days-old colts of 

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