SIS 
THE PKINCIPLES OF 
Part U- 
would be the most likely to survive, and would inherit 
the same tendency not to produce superfluous males? 
whilst retaining their lull fertility in the production of 
females. So it woidd be with the converse case of the 
female sex. Any slight excess, however, of either seS 
could hardly be checked in so indirect a manner. Nor 
indeed has a considerable inequality between the sexes 
been always prevented, as wo 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 regular 
lion through natural selection of the ordinary fertility 
of species. Mr. Herbert Spencer has shewn in an abl® 
discussion ‘ 4 that with all organisms a ratio exists be- 
tween what he calls individuation and genesis; whence 
it follows that beings which consume much matter of 
force in their growth, complicated structure or activity? 
or which produce ova and embryos of large size, ° r 
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 let' 
tility will be regulated through natural selection. Th« s 
the fertility of each species will tend to increase, from 
‘ Principles of Biology,’ vol. ii. 1867, chaps, ii.-xi. 
