Chap. XIV.] DEVELOPMENT ANB EMBRYOLOGY. 217 



tain 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 corre- 

 sponding age in the offspring and parent. I am 

 far from meaning that this is invariahly the case and 

 I could give several exceptional cases of variations 

 (taking the word in the largest sense) which have su- 

 pervened 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, ex- 

 plain, as I believe, all the ahove specified leading facts 

 in embryology. But first let us look to a few analo- 

 gous cases in our domestic varieties. Some authors 

 who have written on Dogs, maintain that the greyhound 

 and bulldog, though so different, are really closely al- 

 lied 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 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 three-days-old colts of race and 



