DEVELOPMENT AND EMBRYOLOGY 483 



ology, — namely, the very general, though not universal, dif- 

 ference in structure between the embryo and the adult; — ■ 

 the various parts in the same individual embryo, which ulti- 

 mately become very unlike and serve for diverse purposes, 

 being at an early period of growth alike ; — the common, but 

 not invariable, resemblance between the embryos or larvae 

 of the most distinct species in the same class; — the em- 

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

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

 later period of life; on the other hand larvae, which have to 

 provide for their own wants, being perfectly adapted to the 

 surrounding conditions; — and lastly the fact of certain larvae 

 standing higher in the scale of organisation than the mature 

 animal into which they are developed? I believe that all 

 these facts can be explained, as follows. 



It is commonly assumed, perhaps from monstrosities affect- 

 ing 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 varia- 

 tion 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 genera- 

 tion. 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 reappear at 

 a corresponding age in the offspring. Certain variations can 



