C32 THE DESCENT OF MAN 



by every one who has ever tried, how difficult it is to dis- 

 tinguish parrots in a leaf-covered tree. Nevertheless, we 

 must remember that many parrots are ornamented with 

 crimson, blue, and orange tints, which can hardly be pro- 

 tective. Woodpeckers are eminently arboreal, but besides 

 green species there are many black, and black-and-white 

 kinds — all the species being apparently exposed to nearly 

 the same dangers. It is therefore probable that with tree- 

 haunting birds strongly pronounced colors have been ac- 

 quired through sexual selection, but that a green tint has 

 been acquired oftener than any other, from the additional 

 advantage of protection. 



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

 admits that they are colored so as to imitate the surrounding 

 surface. How difficult it is to see a partridge, snipe, wood- 

 cock, certain plovers, larks, and night-jars when crouched 

 on ground. Animals inhabiting deserts offer the most 

 striking cases, for the bare surface affords no concealment, 

 and nearly all the smaller quadrupeds, reptiles and birds 

 depend for safety on their colors. Mr. Tristram has re- 

 marked, in regard to the inhabitants of the Sahara, that all 

 are protected by their "isabelline or sand-color."" Call- 

 ing to my recollection the desert-birds of South America, 

 as well as most of the ground-birds of Great Britain, it 

 appeared to me that both sexes in such cases are generally 

 colored nearly alike. Accordingly, I applied to Mr. Tris- 

 tram 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 mani- 

 festly have their plumage colored in a protective manner; 

 and this coloring is all the more striking, as with most of 

 these birds it differs from that of their congeners. Both 

 sexes of thirteen out of the twenty-six species are colored 

 in the same manner; but these belong to genera in which 



''" "Ibis," 1859, vol. i. p. 429 et seq. Dr. Eohlfs, however, remarks to 

 me in a letter that, according to Ms experience of the Sahara, this statement 

 is too strong. 



