398 ON THE ORIGIN OF SPECIES 



which ultimately become very unlike and serve for 

 diverse purposes, being at this early period of growth 

 alike ; — of embryos of different species within the same 

 class, generally, but not universally, resembling each 

 other ; — of the structure of the embryo not being closely 

 related to its conditions of existence, except when the 

 embryo becomes at any period of life active and has to 

 provide for itself; — of the 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 embryos 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 variation 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. Nevertheless 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 communi- 

 cated 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 



