CuAP. XIII. AND EMBRYOLOGY. 399 



tion is not, at what period of life each vaiiation may have been 

 caused, but at wliat period the effects arc disphiyed. The 

 cause may have acted, and I beheve generally has acted, on 

 one or both parents before reproduction. It deserves notice 

 that it is of no importance to a very yoimi^ animal, as long as 

 it remains in its mother's womb or in the egg, or as long as it 

 is nourished and protected by its parent, wlicther most of its 

 characters are acquired a little earlier or later in life. It 

 "would not signify, for instance, to a bird Avhich obtained its 

 food by having a much-curved beak, Avhether or not "while 

 young it possessed a beak of this shape, as long as it was fed 

 by its parents. 



I have stated, in tlu^ first chapter, that at whatever age a 

 variation first appears in the parent, it tends to reappear at a 

 corresponding age ni tlie offsjiring. 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 appeared either ear- 

 lier or later in life, likewise tend to appear 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 excep- 

 tional 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 gener- 

 ally appear at a not very early period of life, and are inher- 

 ited at a corresponding not early period, explain, as I believe, 

 all the aljove specilieil 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 bull-dog, though a})pearing so different, are 

 really closely-allied varieties, descended from the same wild 

 stock; hence I was curious to see how far their puppies dif- 

 fered from each other: I was told by breeders that they dif- 

 fered just as much as their parents, and this, judging by the 

 eye, seemed almost to l)e the case ; but on actually measuring 

 the old dogs and their six-days-old jnijipies, I found that the 

 ]>iipI)i('S had not acquired nearly their full amount of propor- 

 tional difference. So, again, I was t(jld that the foals of cart 

 and race lujrses — breeds which have been almost wholly formed 

 by selection imder domestication — differed as mmh as the full- 

 grown animals ; but having had careful measurements made 



