DEVELOPMENT AND EMBRYOLOGY. 461 



It is commonly assumed, perhaps from Dionstrosities 

 affecting the embr}^© 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 and demerits of their young 

 animals. We see this plainly in our own children; 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 par- 

 ents 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 

 while young it possessed a beak of this shape, as long as it 

 was fed by its parents. 



1 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 vari- 

 ations 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 varia- 

 tions (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 gen- 

 erally appear at a not very early period of life, and are in- 

 herited at a corresponding not early period, explain, as I 

 believe, all the above specified leading facts in embryology. 

 But first let us look to a few analogous cases in our domes- 



