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Show 318 THE PRINCIPLES OF PART II. had wasted least force in producing superfluous males 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 organise<.l matter and force by the production of superfluous individuals of either sex. Nevertheless we may conclude that natural selection will. always tend, though sometimes 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 regulation 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 between what he calls individuation and genesis; whence it follows that beings which consume much matter or force i~ their growth, complicated structure or activity, or .which produce ova and embryos of large size, or whwh expend much energy in nurturing their young, cannot be so productive as beings of an opposite nature. Mr. Spencer furthet shews that minor differences in fertility will be regulated through natural selection. Thus ; 4 'Principles of Biology,' vol. ii. 1867, chaps. ii.-xi. CuAP. VIII. SEXUAL SETJECTION. 319> the fertility of each species will tend to increase, from the more fertile pairs producing a larger number of offspring, and these from their mere number will have the best chance of surviving, and will transmit their tendency to greater fertility. The only check to a continued augmentation of fertility in each organism seems to be either the expenditure of more power and the greater risks run by the parents that produce a more· numerous progeny, or the contingency of very numerous eggs and young being produced of smaller size, or less vigorous, or subsequently not so well nurtured. To strike a balance in any case between the disadvantages which follow from the production of a numerous progeny, and the advantages (such as the escape of at least. some individuals from various dangers) is quite beyonc1 our power of judgment. When an organism has once been rendered extremely fertile, how its fertility can be reduced through natura1 selection is not so clear as how this capacity was first acquired. Yet it is obvious that if individuals of a. species, from a decrease of their natural enemies, were habitually reared in larger numbers than could be supported, all the members would suffer. Nevertheless the offspring from the less fertile parents would have no ' direct advantage over the offspring from the more fertile parents, when all were mingled together in the same district. All the individuals would mutually tend to starve each other. The offspring indeed of the less fertile parents would lie under one great disadvantage, for from the simple fact of being produced in smaller numbers, they would be the most bable to extermination. Indirectly, however, they would partake of one great advantage ; for under the supposed conditio~ ~f severe competition, when all were pressed for foocl, It IS extremely probable that those individuals which from |
