Chap. XIV.] DEVELOPMENT AITD EMBRYOLOGY. o91 



same individual embryo, which, ultimately become very unlike and 

 serve for diverse purposes, being at an early period of growth alike ; 

 — the common, but not invariable, resemblance between the em- 

 bryos or larva? of the most distinct species in the same class ; — 

 the embryo often retaining whilst within the egg or womb, struc- 

 tures which are of no service to it, either at that or at a latei 

 period of life ; on the ether hand larva?, which have to provide for 

 their own wants, being perfectly adapted to the surrounding condi- 

 tions ; — and lastly the fact of certain larva? standing higher in the 

 scale of organisation than the mature animal into which they are de- 

 veloped ? I believe that all these facts can be explained, as follows. 



It is commonly assumed, perhaps from monstrosities affecting the 

 embryo at a very early period, that slight variations or individual 

 differences necessarily appear at an equally early period. We have 

 little evidence on this head, but what we have certainly points the 

 other way ; for it is notorious that breeders of cattle, horses, and 

 various fancy animals, cannot positively tell, until some time after 

 birth, what will be the merits or demerits of their young animals. 

 We see this plainly in our own children; we cannot tell whether a 

 child will be tall or short, or what its precise features will be. The 

 question is not, at what period of life each variation may have been 

 caused, but at what period the effects are displayed. The cause 

 may have acted, and I 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 whilst 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 re-appear at a corresponding 

 age in the offspring. Certain variations can only appear at corres- 

 ponding 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 re- 

 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 exceptional cases of variations (taking the word in the 

 largest sense) which have supervened at an earlier age in the chilb 

 than in the parent. 



