EMBRYOLOGY 399 



parent, it must be quite unimportant whether most of 

 its characters are fully 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 modifications, by which each species 

 has acquired its present structure, may have super- 

 vened 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 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 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 varia- 

 tions (taking the word in the largest sense) which 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 1 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 de- 

 scended 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 



