318 THE PKINCIPLES OF Part II. 



would be the most likely to survive, and would inherit 

 the same tendency not to produce superfluous males, 

 whilst retaining their full fertility in the production of 

 females. So it would be with the converse case of the 

 female sex. Any slight excess, however, of either sex 

 could hardly be checked in so indirect a manner. Nor 

 indeed has a considerable inequality between the sexes 

 been always prevented, as we have seen in some of the 

 cases given in the previous discussion. In these cases 

 the unknown causes which determine the sex of the 

 embryo, and which under certain conditions lead to 

 the production of one sex in excess over the other, have 

 not been mastered by the survival of those varieties 

 which were subjected to the least waste of organised 

 matter and force by the production of superfluous indi- 

 viduals of either sex. Nevertheless we may conclude 

 that natural selection will always tend, though some- 

 times inefficiently, to equalise the relative numbers of 

 the two sexes. 



Having said this much on the equalisation of the 

 sexes, it may be well to add a few remarks on the regula- 

 tion through natural selection of the ordinary fertility 

 of species. Mr. Herbert Spencer has shewn in an able 

 discussion 74 that with all organisms a ratio exists be- 

 tween what he calls individuation and genesis; whence 

 it follows that beings which consume much matter or 

 force in their growth, complicated structure or activity, 

 or which produce ova and embryos of large size, or 

 which expend much energy in nurturing their young, 

 cannot be so productive as beings of an opposite nature. 

 Mr. Spencer further shews that minor differences in fer- 

 tility will be regulated through natural selection. Thus 

 the fertility of each species will tend to increase, from 



r-i < 



Principles of Biology,' vol. ii. 18G7, chaps, ii.-xi. 



