296 
THE PRINCIPLES OF 
Part II. 
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. 
When variations occur late in life in one sex, and are 
