354 



SEXTD8 EMPIRICUS 



SEXUAL SELECTION 



new parUln". under Church-building Act - ; the 

 In ty is to keep the church clean, swept, and 

 adorned, to open the pews, to make ami (ill up 

 the graven, and to prevent any disturbance in 

 church. The salary u usually paid by the church- 

 wardens, and as to amount depend* on custom. 

 In Scotland he may also ! beadle or ' church- 

 officer,' although the tatter's duties are usually 

 connected with attendance on the minister. 



s<- Mil-; Knipirirus. physician and philo- 

 sopher, lived at Alexandria and Athens about 

 200-250 A.D. As physician he was * representa- 

 tive of the Empirics ( hence his second name; see 

 MEDICINE); as philosopher he was the chief 

 exponent of the later Scepticism (q. v.) of the Old 

 World, which was professedly a continuation of 

 Pyrrhonism. In his two works still extant the 

 Hypotyposes and Adversus Mathematicos he has 

 left a prodigious battery of arguments and excep- 

 tions against dogmatism in grammar, rhetoric, geo- 

 metry, arithmetic, music, astrology, logic, physics, 

 ethics. There are monographs by Jourdain ( Paris, 

 1858) and Pappenheim (Berlin, 1875). 



Sexual Selection is a term applied by Darwin 

 to the process of favouring and eliminating which 

 to some extent occurs in the mating of many 

 animals. It is a special case of natural selection, 

 depending upon a competition between rival males, 

 in which a premium is set upon those qualities which 

 favour their possessors in securing mates. This 

 competition takes two forms : on the one hand, 

 rival males, for instance stags and gamecocks, fight 

 with one another, and the conquerors have natur- 

 ally the preference in mating ; on the other hand, 

 rival males sometimes seem to vie with one another 

 in displaying their attractive qualities liefore their 

 desired mates, who, according to Darwin, choose 

 those that please them best. 



Darwin gives the following summary of his 

 theory : ' It has been shown that the largest 

 numl>er of vigorous offspring will be reared from 

 the pairing of the strongest and best-formed males, 

 victorious in contests over other males, with the 

 most vigorous and best-nourished females, which 

 are the Hirst to breed in the spring. If such females 

 select the more attractive and, at the same time, 

 vigorous males, they will rear a larger number of 

 offspring than the retarded females, which must 

 pair with the less vigorous and less attractive 

 males. So it will > if the more vigorous males 

 select the more attractive and, at the same time, 

 healthy and vigorous females ; and this will especi- 

 ally hold good if the male defends the female, and 

 aids in providing fcxxl for the young. The advan- 

 tage thus gained by the more vigorous pairs in 

 rearing a larger number of offspring has apparently 

 sufficed * render sexual selection efficient. 



Where there is direct competition between males, 

 the weakest will tend to be eliminated, either 

 directly by death or injury in the struggle, or 

 indirectly by diminished success in reproduction. 

 In the same way, if a male be lacking in the 

 qualities necessary to find a mate e.g. in senses 

 acute enough to find out her wherealmuts that 

 male may remain unreproductive. But there is 

 not enough of evidence to enable us to compute 

 how many males do remain unmated in consequence 

 of non-success in competition. 



In regard to the second aspect of sexual selection, 

 in which the females are lielieved to exercise some 

 choice, giving the preference to those suitors which 

 have brighter colours, more graceful forms, sweeter 

 voices, or greater charms of some kind, there is no 

 little difference of opinion. Darwin- indeed belie\ <-<\ 

 Htrongly in the female's choice, and referred to this 

 process of selection many of the qualities which 

 distinguish male animals. The females 'have by 



a long selection of the more attractive males added 

 to their beauty or other attractive qualities.' ' If 

 any man can in a short time give elegant carriage 

 and beauty to his bantams, according to his 

 standard of l>eautv, I can see no reason to doubt 

 that female birds, by selecting during thousands of 

 rations the most melodious or )>eautiful males, 

 according to their standard of Ixsauty, might pro- 

 duce a marked effect.' On the other hand, Alfred 

 Kussel Wallace maintains a very different ]xi-ition. 

 ' There is,' he says, ' a total absence of any evidence 

 that the females admire or even notice the display 

 of the males. Among butterflies there is literally 

 not one particle of evidence that the female is 

 influenced by colour or even that she has any 

 power of choice, while there is much direct evi- 

 dence to the contrary.' Against this, G. W. and 

 E. G. Peckham, in their careful essay on sexual 

 selection in spiders, state that they have in the 

 Attidic 'conclusive evidence that tin- females pay 

 close attention to the love dances of the males, and 

 also that they have not only the jiower, but the 

 will, to exercise a choice among the suitors for their 

 favour.' Some observers of birds are also confident 

 that the females choose the more musical or other- 

 wise attractive males. But again Wallace main 

 tains that the fact that every male bird finds a 

 mate ' would almost or quite neutralise any effect 

 of sexual selection of colour or ornament ; since 

 the less highly coloured birds would be at no dis- 

 advantage as regards leaving healthy offspring.' 

 In spiders, however, it seems that the more bril- 

 liant males may be selected again and again while 

 the mating season lasts. 



The theory of sexual selection is of considerable 

 importance in a general theory of evolution. This 

 may l>e illustrated in reference to the bright plum- 

 age of many birds. If we ]x>stulate MICCI^MM- 

 crops of variations (which cannot at present be 

 completely rationalised), if we acknowledge that 

 there is really ' preferential mating ' among birds 

 (which is not readily proved or disproved), if we 

 believe that the females are sensitive to the slight 

 excellences which distinguish one suitor from 

 another and that their choice of mates is deter- 

 mined by these excellences (which Wallace em- 

 phatically denies), then we may say that the 

 greater brightness of male birds may have been 

 evolved by sexual selection. This was Darwin's 

 opinion. The brighter males succeeded better than 

 their rivals in the art of courtship; the variations 

 which gave them success were transmitted to the 

 offspring ; gradually the qualities were established 

 and enhanced as secondary sexual characters of the 

 species. But Wallace interpreted the facts quite 

 otherwise. The relatively plain plumage of the 

 female birds was due to natural selection, elimin- 

 ating those whose conspicuousness during incuba- 

 tion was fatal, fostering those whose colouring was 

 protective. Just as Dailies Barrington, a natur- 

 alist still remembered us tin- correspondent of 

 Gilbert White, suggested ( 1773) that singing -birds 

 were small and hen-birds mute for safety's sake. v,> 

 Wallace maintained that female birds had forfeited 

 brightness as a ransom for life. 



But, leaving the birds, let us take a cose which 

 seems to afford better illustration of Darwin's 

 theory of sexual selection that of spiders. The 

 courtship of these animals has l>een observed 

 and described by G. W. and E. G. Peckham in a 

 manner so careful that their paper ranks as one of 

 the most important contributions yet made to the 

 theory of sexual selection. ' The fact that in the 

 Attidie the males vie with each other in making an 

 elaliorate display, not only of their grace and agility 

 but also of their beauty, before the females, and 

 that the females, after attentively watching the 

 dances and tournaments which have been executed 



