Chap. XIV. AT CORRESPONDING PERIODS. 51 



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

 to appear in the males of our domesticated animals than in 

 the females, 29 and afterwards to be inherited exclusively 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. 



Inheritance at corresponding 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 embryo 

 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 embryo, 

 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 many 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 sj^stem 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 ft> 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 



29 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 



E 2 



