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Show 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 development 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 foregoing 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 the3e laws, from unknown causes, are Yery 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 offsprii:tg at one age, and some at all ages. Not only are the laws of inheritance extremely complex, but so are the cmises which induce and govern variability. The variations thus caused are preserved and accumulated by sexual selection, 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 perception, taste, and will of the female. Sexual selection will also be dominated by natural selection f~r the general welfare of the species. Hence the manner in which the individuals of either sex or of both sexes are affected through sexnul selection cannot fail to be complex in the highest degree. 'Vhen variations occur late in life in one sex, and are CHAP. VIII. SEXUAL SELECTIO:Y. 297 transmitted to the same sex at the same age, the other sex and the young are necessarily left unmodified. When they occur late in life, but are transmitted to both sexes at the same age, the young alone are left unmodified. Variations, however, may occur at any period <>f life in op.e sex or in both, and be transmitted to both sexes at all ages, and then all the individuals of the .species will be similarly modified. In the following chapters it will be seen that all these cases frequently <.>ccur under nature. Sexual selection can never act on any animal whilst young, before the age for 1·eproduction has arrived. From the great eagerness of the male it has generally acted on this sex and not ·on the females. The males have thus become provided with weapons for fighting with their rivals, or with organs for discovering and secui·ely holding the female, or for exciting and charming her." vVhen the sexes differ in these respects, it is also, as we have seen, an extremely general law that the adult male differs more or less from the young male ; and we. may conclude from this fact that the successive variations, by which the adult male became modified, cannot have occurred much before the age for reproduction. How then are we to .account for this general and remarkable coincidence b~tween the period of variability and that of sexual selection,-principles which are quite independent of each other? I think we can see the cause: it is not that. the males have never varied at an early age, but that such variations have commonly been lost, whilst those occurring at a later age have been preserved. All animals produce more offspring than can survi,,e to maturity; and we have every reason to believe that death falls heavily on the weak and inexperienced young. If then a certain proportion of the offspring |
