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ch goes on until it ultimately disappears altogether。 The process of dwindling cannot therefore be explained as due to panmixia alone； we can only find a sufficient explanation in germinal selection。
IV。 DERIVATIVES OF THE THEORY OF SELECTION。
The impetus in all directions given by Darwin through his theory of selection has been an immeasurable one； and its influence is still felt。 It falls within the province of the historian of science to enumerate all the ideas which； in the last quarter of the nineteenth century； grew out of Darwin's theories； in the endeavour to penetrate more deeply into the problem of the evolution of the organic world。 Within the narrow limits to which this paper is restricted； I cannot attempt to discuss any of these。
V。 ARGUMENTS FOR THE REALITY OF THE PROCESSES OF SELECTION。
（a） SEXUAL SELECTION。
Sexual selection goes hand in hand with natural selection。 From the very first I have regarded sexual selection as affording an extremely important and interesting corroboration of natural selection； but； singularly enough； it is precisely against this theory that an adverse judgment has been pronounced in so many quarters； and it is only quite recently； and probably in proportion as the wealth of facts in proof of it penetrates into a wider circle； that we seem to be approaching a more general recognition of this side of the problem of adaptation。 Thus Darwin's words in his preface to the second edition （1874） of his book； 〃The Descent of Man and Sexual Selection〃； are being justified： 〃My conviction as to the operation of natural selection remains unshaken；〃 and further； 〃If naturalists were to become more familiar with the idea of sexual selection； it would； I think； be accepted to a much greater extent； and already it is fully and favourably accepted by many competent judges。〃 Darwin was able to speak thus because he was already acquainted with an immense mass of facts； which； taken together； yield overwhelming evidence of the validity of the principle of sexual selection。
NATURAL SELECTION chooses out for reproduction the individuals that are best equipped for the struggle for existence； and it does so at every stage of development； it thus improves the species in all its stages and forms。 SEXUAL SELECTION operates only on individuals that are already capable of reproduction； and does so only in relation to the attainment of reproduction。 It arises from the rivalry of one sex； usually the male； for the possession of the other； usually the female。 Its influence can therefore only DIRECTLY affect one sex； in that it equips it better for attaining possession of the other。 But the effect may extend indirectly to the female sex； and thus the whole species may be modified； without； however； becoming any more capable of resistance in the struggle for existence； for sexual selection only gives rise to adaptations which are likely to give their possessor the victory over rivals in the struggle for possession of the female； and which are therefore peculiar to the wooing sex： the manifold 〃secondary sexual characters。〃 The diversity of these characters is so great that I cannot here attempt to give anything approaching a complete treatment of them； but I should like to give a sufficient number of examples to make the principle itself； in its various modes of expression； quite clear。
One of the chief preliminary postulates of sexual selection is the unequal number of individuals in the two sexes； for if every male immediately finds his mate there can be no competition for the possession of the female。 Darwin has shown that； for the most part； the inequality between the sexes is due simply to the fact that there are more males than females； and therefore the males must take some pains to secure a mate。 But the inequality does not always depend on the numerical preponderance of the males； it is often due to polygamy； for； if one male claims several females； the number of females in proportion to the rest of the males will be reduced。 Since it is almost always the males that are the wooers； we must expect to find the occurrence of secondary sexual characters chiefly among them； and to find it especially frequent in polygamous species。 And this is actually the case。
If we were to try to guesswithout knowing the factswhat means the male animals make use of to overcome their rivals in the struggle for the possession of the female； we might name many kinds of means； but it would be difficult to suggest any which is not actually employed in some animal group or other。 I begin with the mere difference in strength； through which the male of many animals is so sharply distinguished from the female； as； for instance； the lion； walrus； 〃sea…elephant；〃 and others。 Among these the males fight violently for the possession of the female； who falls to the victor in the combat。 In this simple case no one can doubt the operation of selection； and there is just as little room for doubt as to the selection…value of the initial stages of the variation。 Differences in bodily strength are apparent even among human beings； although in their case the struggle for the possession of the female is no longer decided by bodily strength alone。
Combats between male animals are often violent and obstinate； and the employment of the natural weapons of the species in this way has led to perfecting of these； e。g。 the tusks of the boar； the antlers of the stag； and the enormous； antler…like jaws of the stag…beetle。 Here again it is impossible to doubt that variations in these organs presented themselves； and that these were considerable enough to be decisive in combat； and so to lead to the improvement of the weapon。
Among many animals； however； the females at first withdraw from the males； they are coy； and have to be sought out； and sometimes held by force。 This tracking and grasping of the females by the males has given rise to many different characters in the latter； as； for instance； the larger eyes of the male bee； and especially of the males of the Ephemerids （May…flies）； some species of which show； in addition to the usual compound eyes； large； so…called turban…eyes； so that the whole head is covered with seeing surfaces。 In these species the females are very greatly in the minority （1… 100）； and it is easy to understand that a keen competition for them must take place； and that； when the insects of both sexes are floating freely in the air； an unusually wide range of vision will carry with it a decided advantage。 Here again the actual adaptations are in accordance with the preliminary postulates of the theory。 We do not know the stages through which the eye has passed to its present perfected state； but； since the number of simple eyes （facets） has become very much greater in the male than in the female； we may assume that their increase is due to a gradual duplication of the determinants of the ommatidium in the germ…plasm； as I have already indicated in regard to sense…organs in general。 In this case； again； the selection…value of the initial stages hardly admits of doubt； better vision DIRECTLY secures reproduction。
In many cases THE ORGAN OF SMELL shows a similar improvement。 Many l