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The Descent of Man

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sed to nearly the same dangers. It is therefore<br />

probable that with tree-haunting birds, strongly-pronounced<br />

colours have been acquired<br />

through sexual selection, but that a green tint<br />

has been acquired <strong>of</strong>tener than any other, from<br />

the additional advantage <strong>of</strong> protection.<br />

In regard to birds which live on the ground,<br />

every one admits that they are coloured so as to<br />

imitate the surrounding surface. How difficult<br />

it is to see a partridge, snipe, woodcock, certain<br />

plovers, larks, and night-jars when crouched on<br />

ground. Animals inhabiting deserts <strong>of</strong>fer the<br />

most striking cases, for the bare surface affords<br />

no concealment, and nearly all the smaller quadrupeds,<br />

reptiles, and birds depend for safety<br />

on their colours. Mr. Tristram has remarked in<br />

regard to the inhabitants <strong>of</strong> the Sahara, that all<br />

are protected by their "isabelline or sandcolour."<br />

(50. 'Ibis,' 1859, vol. i. p. 429, et seq. Dr.<br />

Rohlfs, however, remarks to me in a letter that<br />

according to his experience <strong>of</strong> the Sahara, this

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