224 SEXUAL SELECTIOX : BIRDS. Part II. 



nearly tlie same clangers. It is therefore probable 

 that strongly-pronounced colours have been acquired 

 by tree-hauntinof birds throuo'h sexual selection, but 

 that screen tints have had an advantaoe throu^'h 

 natural selection over other colours for the sake of 

 protection. 



In regard to birds which live on the ground, every- 

 one admits that they are coloured so as to imitate the 

 surrounding surface. How difficult it is to see a par- 

 tridge, snipe, woodcock, certain plovers, larks, and 

 night-jars wlien crouched on the ground. Animals in- 

 habiting: deserts offer the most strikino^ instances, for the 

 bare surface affords no concealment, and all the smaller 

 quadrupeds, reptiles, and birds depend for safety on 

 their colours. As ]\Ir. Tristram has remarked,^^ in 

 regard to the inhabitants of the Sahara, all are pro- 

 tected bv their " isabelline or sand-colour." Callino- to 

 my recollection the desert-birds which I liad seen in 

 South America, as well as most of the ground-birds 

 in Great Britain, it appeared to me that both sexes 

 in such cases are generally coloured nearly alike. Ar- 

 corlingly I applied to Mr. Tristram, with respect to the 

 birds of the Sahara, and he has kindly given me the 

 following information. There are twenty-six species, 

 belonging to fifteen genera, which manifestly have had 

 their plumage coloured in a protective manner ; and 

 this colouring is all the more striking, as with most 

 of these birds it is different from that of their con- 

 geners. Both sexes of thirteen out of the twenty-six 

 species are coloured in the same manner ; but these 

 belong to genera in which this rule commonly })re- 

 vails, so that they tell us nothing about the protective 

 colours being the same in both sexes of desert-birds. Of 



■^1 ' Ibis,' 1859, vol. i. p. 429, et seq. 



