134 DARWINISM STATED BY DARWIN HIMSELF. 

 TWO PRINCIPLES THAT EXPLAIN THE PACTS. 



™ „,>~ How, then, can we explain these several 



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facts in embryology — namely, the very general, 

 though not universal, difference in structure between the 

 embryo and the adult ; the various parts in the same in- 

 dividual embryo, which ultimate become very unlike and 

 serve for diverse purposes, being at an early period of 

 growth alike ; the common, but not invariable, resem- 

 blance between the embryos or larva? of the most distinct 

 species in the same class ; the embryo often retaining, 

 while 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, larvae, which have to provide for their 

 own wants, being perfectly adapted to the surrounding 

 conditions ; and, lastly, the fact of certain larvae stand- 

 ing higher in the scale of organization 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, can not 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 chil- 

 dren ; we can not 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 de- 



