444 EMBRYOLOGY. Chap. XIII. 



acquired a little earlier or later in life. It would 

 not signify, for instance, to a bird which obtained its 

 food best by having a long beak, whether or not it 

 assumed a beak of this particular length, as long as it 

 was fed by its parents. Hence, I conclude, that it is 

 quite possible, that each of the many successive modi- 

 fications, by which each species has acquired its pre- 

 sent structure, may have supervened at a not very 

 early period of life ; and some direct evidence from our 

 domestic animals supports this view. But in other cases 

 it is quite possible that each successive modification, or 

 most of them, may have appeared at an extremely early 

 period. 



I have stated in the first chapter, that there is some 

 evidence to render it probable, that at whatever age any 

 variation first appears in the parent, it tends to reappear 

 at a corresponding age in the offspring. Certain varia- 

 tions can only appear at corresponding ages, for in- 

 stance, peculiarities in the caterpillar, cocoon, or imago 

 states of the silk-moth ; or, again, in the horns of almost 

 full-grown cattle. But further than this, variations which, 

 for all that we can see, might have appeared earlier or 

 later in life, 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 a good 

 many cases of variations (taking the word in the largest 

 sense) winch have supervened at an earlier age in the 

 child than in the parent. 



These two principles, if their truth be admitted, will, 

 I believe, explain all the above specified leading facts 

 in embryology. But first let us look at a few analogous 

 cases in domestic varieties. Some authors who have 

 written on Dogs, maintain that the greyhound and bull- 

 dog, though appearing so different, are really varieties 

 most closely allied, and have probably descended from 



