Chap. XIV. AT COREESPONDING PERIODS. 51 



wliole, as far as I can judge, new characters are more apt 

 to appear in the males of our domesticated animals than in 

 the females, ^^ and afterwards to be inherited exclusivelv or 

 more strongly by the males. Finally, in accordance with 

 the principle of inheritance as limited by sex, the preserva- 

 tion and augmentation of secondary sexual characters in 

 natural species offers no especial difficulty, as this would 

 follow through that form of selection which I have called 

 sexual selection. 



InJieritance at corresj>ondin(j periods of Life. 



This is an important subject. Since the publication of my 

 ' Origin of Species,' I have seen no reason to doubt the truth 

 of the explanation there given of one of the most remarkable 

 facts in biology, namely, the difference between the embr^^o 

 and the adult animal. The explanation is, that variations 

 do not necessarily or generally occur at a very early period 

 of embryonic growth, and that such variations are inherited 

 at a corresponding age. As a consequence of this the embrj^o, 

 even after the parent-form has undergone great modification, 

 is left only slightly modified ; and the embryos of widely- 

 different animals which are descended from a common pro- 

 genitor remain in manj^ important respects like one another 

 and probably like their common progenitor. We can thus 

 understand why embryology throws a flood of light on the 

 natural system of classification, as this ought to be as far as 

 possible genealogical. When the embryo leads an inde- 

 pendent life, that is, becomes a larva, it has to be adapted 

 to the surrounding conditions in its structure and instincts, 

 independently of those of its parents; and the principle 

 of inheritance at corresponding periods of life renders this 

 possible. 



This principle is, indeed, in one way so obvious that it 

 escapes attention. We possess a number of races of animals 

 and plants, which, when compared with one another and with 



^^ I have given in my ' Descent of usually more variable than the fe* 

 Man ' (2nd edit. p. 223) sufficient males, 

 evidence that male animals are 



