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uch interpretations is that instinctive behaviour and experience； its emotional accompaniments； and its expression； are but different aspects of the outcome of the same organic occurrences。 Such emotions are； therefore； only a distinguishable aspect of the primary tissue of experience and exhibit a like differentiation。 Here again a biological foundation is laid for a psychological doctrine of the mental development of the individual。
The intimate relation between emotion as a psychological mode of experience and expression as a group of organic conditions has an important bearing on biological interpretation。 The emotion； as the psychological accompaniment of orderly disturbances in the central nervous system profoundly influences behaviour and often renders it more vigorous and more effective。 The utility of the emotions in the struggle for existence can； therefore； scarcely be over…estimated。 Just as keenness of perception has survival… value； just as it is obviously subject to variation； just as it must be enhanced under natural selection； whether individually acquired increments are inherited or not； and just as its value lies not only in this or that special perceptive act but in its importance for life as a whole； so the vigorous effectiveness of activity has survival…value； it is subject to variation； it must be enhanced under natural selection； and its importance lies not only in particular modes of behaviour but in its value for life as a whole。 If emotion and its expression as a congenital endowment are but different aspects of the same biological occurrence； and if this is a powerful supplement to vigour effectiveness and persistency of behaviour； it must on Darwin's principles be subject to natural selection。
If we include under the expression of the emotions not only the premonitory symptoms of the initial phases of the organic and mental state； not only the signs or conditions of half…tide emotion； but the full…tide manifestation of an emotion which dominates the situation； we are naturally led on to the consideration of many of the phenomena which are discussed under the head of sexual selection。 The subject is difficult and complex； and it was treated by Darwin with all the strength he could summon to the task。 It can only be dealt with here from a special point of viewthat which may serve to illustrate the influence of certain mental factors on the course of evolution。 From this point of view too much stress can scarcely be laid on the dominance of emotion during the period of courtship and pairing in the more highly organised animals。 It is a period of maximum vigour； maximum activity； and； correlated with special modes of behaviour and special organic and visceral accompaniments； a period also of maximum emotional excitement。 The combats of males； their dances and aerial evolutions； their elaborate behaviour and display； or the flood of song in birds； are emotional expressions which are at any rate coincident in time with sexual periodicity。 From the combat of the males there follows on Darwin's principles the elimination of those which are deficient in bodily vigour； deficient in special structures； offensive or protective； which contribute to success； deficient in the emotional supplement of which persistent and whole…hearted fighting is the expression； and deficient in alertness and skill which are the outcome of the psychological development of the powers of perception。 Few biologists question that we have here a mode of selection of much importance； though its influence on psychological evolution often fails to receive its due emphasis。 Mr Wallace （〃Darwinism〃； pages 282； 283； London； 1889。） regards it as 〃a form of natural selection〃； 〃to it；〃 he says； 〃we must impute the development of the exceptional strength； size； and activity of the male； together with the possession of special offensive and defensive weapons； and of all other characters which arise from the development of these or are correlated with them。〃 So far there is little disagreement among the followers of Darwin for Mr Wallace； with fine magnanimity； has always preferred to be ranked as such； notwithstanding his right； on which a smaller man would have constantly insisted； to the claim of independent originator of the doctrine of natural selection。 So far with regard to sexual selection Darwin and Mr Wallace are agreed； so far and no farther。 For Darwin； says Mr Wallace （Ibid。 page 283。）； 〃has extended the principle into a totally different field of action； which has none of that character of constancy and of inevitable result that attaches to natural selection； including male rivalry； for by far the larger portion of the phenomena； which he endeavours to explain by the direct action of sexual selection； can only be so explained on the hypothesis that the immediate agency is female choice or preference。 It is to this that he imputes the origin of all secondary sexual characters other than weapons of offence and defence。。。In this extension of sexual selection to include the action of female choice or preference； and in the attempt to give to that choice such wide…reaching effects； I am unable to follow him more than a very little way。〃
Into the details of Mr Wallace's criticisms it is impossible to enter here。 We cannot discuss either the mode of origin of the variations in structure which have rendered secondary sexual characters possible or the modes of selection other than sexual which have rendered them； within narrow limits； specifically constant。 Mendelism and mutation theories may have something to say on the subject when these theories have been more fully correlated with the basal principles of selection。 It is noteworthy that Mr Wallace says （〃Darwinism〃； pages 283； 284。）： 〃Besides the acquisition of weapons by the male for the purpose of fighting with other males； there are some other sexual characters which may have been produced by natural selection。 Such are the various sounds and odours which are peculiar to the male； and which serve as a call to the female or as an indication of his presence。 These are evidently a valuable addition to the means of recognition of the two sexes； and are a further indication that the pairing season has arrived； and the production； intensification； and differentiation of these sounds and odours are clearly within the power of natural selection。 The same remark will apply to the peculiar calls of birds； and even to the singing of the males。〃 Why the same remark should not apply to their colours and adornments is not obvious。 What is obvious is that 〃means of recognition〃 and 〃indication that the pairing season has arrived〃 are dependent on the perceptive powers of the female who recognises and for whom the indication has meaning。 The hypothesis of female preference； stripped of the aesthetic surplusage which is psychologically both unnecessary and unproven； is really only different in degree from that which Mr Wallace admits in principle when he says that it is probable that the female is pleased or excited by the display。
Let us for our present purpose leave on one side and regard as sub judice the question whether the specific details of secondary sexual ch