74 BIRDS OF LA PLATA 
mind they appear like theéfherest 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 over- 
sight 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 un- 
expected 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 
