Facts like this bring before us with startling vividness the struggle for existence, showing what great issues in the life of a species may depend on matters so trivial, seemingly, that to the uninformed mind they appear like the merest dust in the balance, which is not regarded. And how tremendous and pitiless is that searching law of the survival of the fittest in its operations when we see a species like this _Synallaxis_, in the fashioning and perfecting of which nature seems to have exhausted all her art, so exquisitely is it adapted in its structure, coloration, and habits to the one great object of concealment, yet apparently doomed to destruction through this one petty oversight--the irrepressible garrulity of the fledglings in their nest! It is, however, no oversight at all; since the law of natural selection is not prophetic in its action, and only preserves such variations as are beneficial in existing circumstances, 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+, _Burm. La-Plata Reise_, ii. p. 434 (La Plata); _Durnford, Ibis_, 1877, p. 40 (Chupat). +Polyborus tharus+, _Scl. et Salv. Nomencl._ p. 123; _Durnford, Ibis_, 1877, p. 188 (Buenos Ayres), et 1878, p. 398 (Centr. Patagonia); _Gibson, Ibis_, 1879, p. 415 (Buenos Ayres); _White, P. Z. S._ 1883, p. 41 (Cordova); _Barrows, Auk_, 1884, p. 111 (Entrerios); _Sharpe, Cat. B._ i. p. 31. +Polyborus brasiliensis+, _Darwin, Zool. Beagle_, iii. p. 9.