296 THE PEINCIPLES OF Part II. 



— we may conclude that one cause, though not the sole 

 cause, of characters being exclusively inherited by one 

 sex, is their development at a late age. And secondly, 

 that one, though apparently a less efficient, cause of 

 characters being inherited by both sexes is their deve- 

 lopment at an early age, whilst the sexes differ but 

 little in constitution. It appears, however, that some 

 difference must exist between the sexes even during an 

 early embryonic period, for characters developed at this 

 age not rarely become attached to one sex. 



Summary and concluding remarks. — From the fore- 

 going discussion on the various laws of inheritance, we 

 learn that characters often or even generally tend to 

 become developed in the same sex, at the same age, 

 and periodically at the same season of the year, in 

 which they first appeared in the parents. But these 

 laws, from unknown causes, are very liable to change. 

 Hence the successive steps in the modification of a 

 species might readily be transmitted in different ways ; 

 some of the steps being transmitted to one sex, 

 and some to both; some to the offspring at one age, 

 and some at all ages. Not only are the laws of inherit- 

 ance extremely complex, but so are the causes which 

 induce and govern variability. The variations thus 

 caused are preserved and accumulated by sexual selec- 

 tion, which is in itself an extremely complex affair, 

 depending, as it does, on ardour in love, courage, and \ 

 the rivalry of the males, and on the powers of percep- 

 tion, taste, and will of the female. Sexual selection will 

 also be dominated by natural selection for the general 

 welfare of the species. Hence the manner in which the 

 individuals of either sex or of both sexes are affected 

 through sexual selection cannot fail to be complex in 

 the highest degree. 



