220 The Descent of Man. Part II. 



exception, and this species is believed by Montagu to be a 

 polygamist. Hence it appears that amongst birds there often 

 exists a close relation between polygamy and the development of 

 strongly-marked sexual differences. I asked Mr. Bartlett, of the 

 Zoological Gardens, who has had very large experience with 

 birds, whether the male tragopan (one of the GallinaceaB) was 

 polygamous, and I was struck by his answering, " I do not 

 " know, but should think so from his splendid colours." 



It deserves notice that the instinct of pairing with a single 

 female is easily lost under domestication. The wild-duck is 

 strictly monogamous, the domestic-duck highly polygamous. 

 The Eev. W. I). Fox informs me that out of some half- tamed 

 wild-ducks, on a large pond in his neighbourhood, so many 

 mallards were shot by the gamekeeper that only one was left for 

 every seven or eight females ; yet unusually large broods were 

 reared. The guinea-fowl is strictly monogamous ; but Mr. Fox 

 finds that his birds succeed best when he keeps one cock to two 

 or three hens. Canary-birds pair in a state of nature, but the 

 breeders in England successfully put one male to four or five 

 females. I have noticed these cases, as rendering it probable 

 that wild monogamous species might readily become either 

 temporarily or permanently polygamous. 



Too little is known of the habits of reptiles and fishes to enable 

 us to speak of their marriage arrangements. The stickle -back 

 (Gasterosteus), however, is said to be a polygamist ; 17 and the 

 male during the breeding season differs conspicuously from the 

 female. 



To sum up on the means through which, as far as we can 

 judge, sexual selection has led to the development of secondary 

 sexual characters. It has been shewn that the largest number 

 of vigorous offspring will be reared from the pairing of the 

 strongest and best-armed males, victorious in contests over 

 other males, with the most vigorous and best-nourished females, 

 which are the first 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 be if the more vigorous males select the more 

 attractive and at the same time healthy and vigorous females ; 

 and this will especially hold good if the male defends the 

 female, and aids in providing food for the young. The ad- 

 vantage thus gained by the more vigorous pairs in rearing a 

 larger number of offspring has apparently sufficed to render 

 sexual selection efficient. But a large numerical preponderance 

 iT Kop.1 Humphreys, ' River Gardens,' 1857. 



