Chap. XI V. 



AT CORRESPONDING PERIODS. 



to appear in the males of our domesticated animals than in 

 the females, and afterwards to be either exclusively or more 

 strongly inherited by the males. Finally, in accordance with 

 the principle of inheritance as limited by sex, the appearance 

 of secondary sexual characters in natural species offers no especial 

 difficulty, and their subsequent increase and modification, if of 

 any service to the species, would follow through that form of 

 selection which in my ' Origin of Species ' 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 perhaps the most remark- 

 able of all the 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 inhe- 

 rited at a corresponding age. As a consequence of this the 

 embryo, even when the parent-form undergoes a great amount 

 of modification, is left only slightly modified ; and the embryos 

 of widely-different animals which are descended from a common 

 progenitor remain in many important respects like each other 

 and their common progenitor. We can thus understand why 

 embryology should throw a flood of light on the natural system of 

 classification, for this ought to be as far as possible genealogical. 

 When the embryo leads an independent 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 each other and with 

 their parent-forms, present conspicuous differences, both in the 

 immature and mature states. Look at the seeds of the several 

 kinds of peas, beans, maize, which can be propagated truly, and 

 see how they differ in size, colour, and shape, whilst the full- 



