246 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 



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

 provide for their own wants, being perfectly adapted 

 to the surrounding conditions ; — and lastly the fact of 

 certain larvse standing higlier 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 eifects 

 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 import- 

 ance 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 

 whetlier 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-appea,r at a corresponding age in the offspring. 



