Chap. XIII. EMBRYOLOGY. 443 



embryo becomes at any period of life active and has to 

 provide for itself; — of tlie embryo apparently having 

 sometimes a higher organisation than the mature 

 animal, into which it is developed. I believe that all 

 these facts can be explained, as follows, on the view of 

 descent with modification. 



It is commonly assumed, perhaps from monstrosities 

 often affecting the embyro at a very early period, 

 that slight variations necessarily appear at an equally 

 early period. But we have little evidence on this head — 

 indeed the evidence rather points the other way ; for it 

 is notorious that breeders of cattle, horses, and various 

 fancy animals, cannot positively tell, until some time 

 after the animal has been born, what its merits or form 

 will ultimately turn out. "We see this plainly in our own 

 children ; we cannot always tell whether the child will 

 be tall or short, or what its precise features will be. The 

 question is not, at what period of life any variation has 

 been caused, but at what period it is fully displayed. 

 The cause may have acted, and I believe generally has 

 acted, even before the embryo is formed ; and the varia- 

 tion may be due to the male and female sexual elements 

 having been affected by the conditions to which either 

 parent, or their ancestors, have been exposed. Never- 

 theless an effect thus caused at a very early period, even 

 before the formation of the embryo, may appear late in 

 life ; as when an hereditary disease, which appears in 

 old age alone, has been communicated to the offspring 

 from the reproductive element of one parent. Or 

 again, as when the horns of cross-bred cattle have been 

 affected by the shape of the horns of either parent. For 

 the welfare of 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, it must be quite 

 unimportant whether most of its characters are fully 



