CHAP. XIV.] DEVELOPMENT AND EMBRYOLOGY. 367 



in the 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 embryos or larvae of the most distinct species in the 

 same class ; the embryo often retaining whilst within the egg 

 or womb, structures which are of no service to it, either at that 

 or at a later period of life ; on the other hand larva?, 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 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 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 

 only appear at corresponding 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 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 generally 



