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Show 256 THE PRINCIPLES OF PART II. or walking, if she gradually acquired habits which rendered such powers useless. V.l e are, however, here concerned only with. that kin~ of selec6on which I have called sexual selectwn. Th1s depends on ~he advantage which certain individual~ ha~e over other individuals of the same sex and spec1es, m exclusive relation to reproduction. When the two sexes differ in structure in relation to different habits of life, as in the cases above mentioned, they have no doubt been modified through natural selection, aecompanied by inheritance limited to one and the same sex. . So again the primary sexual organs, and thos~ for nounshing or protecting the young, come under th1s s~me hea~; for those individuals which generated or nounshed thmr offspring best, would leave, ea;_ter_is pan'b.us, the great.est number to inherit their supenonty; whilst those whwh generated or nourished their offspring badly, would leave but few to inherit their weaker powers. As the male has to search for the female, he requires for this purpose organs of sense and locomotion, but if these organs are necessary for the other purposes of life, as is generally the case, they will have been developed through natural selection. When the male has found the female he sometimes absolutely requires prehensile organs to hold her; thus Dr. vVallace informs me that the males of certain moths cannot unite with the females if their tarsi or feet are broken. The males of many oceanic crustaceans have their legs and antennre modified in an extraordinary manner for the prehension of the female ; hence we may suspect that owing to these animals being washed about by the waves of the open sea, they absolutely require these organs in order to propagate their kind, and if so their develo1)ment will have been the result of ordinary or natural selection. 'Vhen the two sexes follow exactly the same habits CHAP. VIII. SEXUAL SELECTION. 237 of life, and the male has more highly developed sense or locomotive organs than the female, it may be that these in their perfected state are indispensable to the male for finding the female ; but in the vast majority of cases, they serve only to give one male an advantage over another, for the less well-endowed males, if time were allowed them, would succeed in pairing with the females ; and they would in all other respects, judging from the structure of the female, be equally well adapted for their ordinary habits of life. In such cases sexual selection must have come into action, for the males have acquired their present structure, not from being better fitted to survive in the struggle for existence, but from having gained an advantage over other males, and from having transmitted this advantage to their male offspring alone. It was the importance of this distinction which led me to designate this form of selection as sexual selection. So again, if the chief service rendered to the male by his prehensile organs is to prevent the escape of the female before the arrival of other males, or when assaulted by them, these organs will have been perfected through sexual selection, that is by the advantage acquired by certain males over their rivals. But in most cases it is scarcely possible to distinguish between the effects of natural and sexual selection. vVhole chapters could easily be filled with details on the differences between the sexes in their sensory, locomotive, and prehensile organs. As, however, these structures are not more interesting than others adapted for the ordinary purposes of life, I shall almost pass them over, giving only a few instances under each class. There are many other structures and instincts which must have been developed through sexual selectionsuch as the weapons of offence and the means of defence VOL. I. S |