DEVELOPMENT AND EMBRYOLOGY. 433 



believe often has acted, on one or both parents before the 

 act of generation. It deserves notice that it is of no 

 importance to a very young 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, whether most of its characters 

 are acquired a little earlier or later in life. It would not 

 signify, for instance, to a bird which obtained its food by 

 having a much-curved beak, whether or not while young it 

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

 parents. 



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

 variation first appears in the parent, it tends to reappear 

 at a corresponding age in the offspring. Certain variations 

 can only appear at corresponding ages ; for instance, pecul- 

 iarities 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 

 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 domes- 

 tic varieties. Some authors who have written on dogs main- 

 tain that the greyhound and bull-dog, 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 whollv formed bv 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 heavy cart jiorses ; J fjRd thajj this 



is by m means tb§ case, 



