224 
SEXUAL SELECTION: BIRDS. 
Part II. 
nearly the same dangers. It is therefore probable 
that strongly-pronounced colours have been acquired 
by tree-haunting birds through sexual selection, but 
that green tints have had an advantage through 
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 when crouched on the ground. Animals in- 
habiting deserts offer the most striking instances, for the 
bare surface affords no concealment, and all the smaller 
quadrupeds, reptiles, and birds depend for safety on 
their colours. As Mr. Tristram has remarked,^^ in 
regard to the inhabitants of the Sahara, all are pro- 
tected by their isabelline or sand-colour.” Calling to 
my recollection the desert-birds which I had seen in 
South America, as well as most of the ground-birds 
in Grreat Britain, it appeared to me that both sexes 
in such cases are generally coloured nearly alike. Ac- 
cordingly 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 pre- 
vails, so that they tell us nothing about the protective 
colours being the same in both sexes of desert-birds. Of 
‘ Ibis/ 1859, vol. i. p. 429, et seq. 
