PCLYBORUS THARUS. 81 



all; since the law of natural selection is not prophetic in its action, and 

 only preserves such variations as are beneficial in existing circum- 

 stances, without anticipating changes in the conditions. The settlement 

 of the country has, no doubt, caused a great increase of Chimangos, and 

 in some indirect way probably has served to quicken their intelligence; 

 thus a change in the conditions which have moulded this Synallaxis 

 brings a danger to it from an unexpected quarter. The situation of the 

 nest exposes it, one would imagine, to attacks from snakes and small 

 mammals, from bird-killing spiders, beetles, and crickets, yet these 

 subtle ground foes have missed it, while the baby-laughter of the little 

 ones in their cradle has called down an unlooked-for destroyer from 

 above. It might be answered that this must be a very numerous 

 species, otherwise the Chimango could not have acquired the habit of 

 finding the nests ; that when they become rarer the pursuit will be 

 given over, after which the balance will readjust itself. But in numbers 

 there is safety, especially for a feeble hunted species, unable from its 

 peculiar structure to vary its manner of life. To such, the remark made 

 by Darwin, that " rarity is the precursor to extinction/' applies with 

 peculiar force. 



310. POLYBORUS THARUS (Mol.). 

 (CARANCHO CARRION-HAWK.) 



Polyborus vulgaris, Sunn. La-Plata Reise, ii. p. 434 (La Plata) ; Durnford, 



Ibis, 1877, p. 40 (Chupat). Polyborus tharus, Sol. et Salv. Nomenel. p. 123 ; 



Durnford, Ibis, 1877, p. 188 (Buenos Ayres), et 1878, p. 398 (Centr. Pata- 

 f gonia) ; Gibson, Ibis, 1879, p. 415 (Buenos Ayres) j White, P. Z. S. 1883, 



p. 41 (Cordova) ; Barrows, Auk, 1884, p. Ill (Entrerios) ; Sharpe, Cat. B. 



i. p. 31. Polyborus brasiliensis, Darwin, Zool. Beagle, iii. p. 9. 



Description. Above dark brown with whitish mottlings ; head blackish ; wings 

 and tail greyish white, with numerous greyish-brown cross bars and blackish 

 tips : beneath dark brown, varied with white ; throat and sides of head fulvous 

 white ; bill yellow, bluish at the base ; cere orange ; feet brown ; claws black : 

 whole length 20 inches, wing 15' 5, tail 9*0. Female similar. 



Hob. South America. 



This bird, which combines the raptorial instincts of the Eagle with 

 the base carrion-feeding propensities of the Vulture, has already had so 

 lany biographers that it might seem superfluous to speak again at any 

 reat length of it ; only it happens to be one of those very versatile 

 species about which there is always something fresh to be said ; and, 

 >esides, I do not altogether agree with the very ignoble character 

 mlly ascribed to it by travellers. It is, however, probable that it 



VOL. II. O 



