Chap. VIII. Sexual Selection. 219 



greater number of females. It is an interesting fact, as Dr. 

 Gill remarks, 15 that in the monogamons species, " or those 

 " living in small communities, there is little difference in size 

 " between the males and females ; in the social species, or rather 

 " those of which the males have harems, the males are vastly 

 " larger than the females." 



Amongst birds, many species, the sexes of which differ greatly 

 from each other, are certainly monogamous. In Great Britain 

 we see well-marked sexual differences, for instance, in the wild- 

 duck which pairs with a single female, the common blackbird, 

 and the bullfinch which is said to pair for life. I am informed 

 by Mr. Wallace that the like is true of the Chatterers or 

 Cotingidae of South America, and of many other birds. In several 

 groups I have not been able to discover whether the species are 

 polygamous or monogamous. Lesson says that birds of paradise, 

 so remarkable for their sexual differences, are polygamous, but Mr. 

 Wallace doubts whether he had sufficient evidence. Mr. Salvin 

 tells me he has been led to believe that humming-birds are 

 polygamous. The male widow-bird, remarkable for his caudal 

 plumes, certainly seems to be a polygamist. 1G I have been 

 assured by Mr. Jenner Weir and by others, that it is somewhat 

 common for three starlings to frequent the same nest ; but 

 whether this is a case of polygamy or polyandry has not been 

 ascertained. 



The Gailinacese exhibit almost as strongly marked sexual 

 differences as birds of paradise or humming-birds, and many of 

 the species are, as is well known, polygamous ; others being 

 strictly monogamous. What a contrast is presented between the 

 sexes of the polygamous peacock or pheasant, and the mono- 

 gamous guinea-fowl or partridge ! Many similar cases could be 

 given, as in the grouse tribe, in which the males ot the poly- 

 gamous capercailzie and black-cock differ greatly from the 

 females; whilst the sexes of the monogamous red grouse and 

 ptarmigan differ very little. In the Cursores, except amongst 

 the bustards, few species offer strongly-marked sexual dif- 

 ferences, and the great bustard (Otis tarda) is said to be poly- 

 gamous. With the Grallatores, extremely few species differ 

 sexually, but the ruff (Machetes jwjnax) affords a marked 



15 The Eared Seals, 'American Great Bustard, see L. Lloyd, ' Game 



Naturalist,' vol. iv., Jan. 1871. Birds of Sweden,' 1867, p. 19, and 



1C 'The Ibis,' vol. iii. 1861, p. 182. Montagu and Selby speak of 



133, on the Progne Widow-lard. the Black Grouse as polygamous 



See also on the Vidua axill.iris, and of the Red Grouse as mono* 



.bid. vol. ii. 1860, p. 211. On the gamjus. 

 polygamy of the Capercailzie and 



