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could fill many pages with instances of this habit of M. bonariensis, 

 which, useless though it be, is as strong an affection as the bird 

 possesses. That it is a recurrence to a long disused habit, I can 

 scarcely doubt ; at least, to no other cause that I can imagine can it 

 be attributed; and, besides, it seems to me that if M. bonariensis, 

 when once a nest-builder, had acquired the semiparasitical habit of 

 breeding in domed nests of other birds, such a habit might conduce to 

 the formation of the instinct which it now possesses. I may mention 

 that twice I have seen birds of this species attempting to build nests, 

 and that on both occasions they failed to complete the work. So 

 universal is the nest-making instinct, that one might safely say the M. 

 bonariensis had once possessed it, and that in the cases I have mentioned 

 it was a recurrence, too weak to be efficient, to the ancestral habit. 

 Another interesting circumstance may be adduced as strong presump- 

 tive evidence that M. bonariensis once made itself an open exposed 

 nest as M. badius occasionally does viz., the difference in colour of 

 the male and female ; for whilst the former is rich purple, the latter 

 possesses an adaptive resemblance in colour to nests and to the shaded 

 interior twigs and branches on which nests are usually built. How 

 could such an instinct have been lost ? To say that the Cow-bird 

 occasionally dropped an egg in another bird's nest, and that the young 

 hatched from these accidental eggs possessed some (hypothetical) 

 advantage over those hatched in the usual way, and that the para- 

 sitical habit so became hereditary, supplanting the original one, is an 

 assertion without any thing to support it, and seems to exclude the 

 agency of external conditions. Again the want of correspondence in 

 the habits of the young parasite and its foster-parents would in reality 

 be a disadvantage to the former ; the unfitness would be as great in 

 the eggs and other circumstances ; for all the advantages the parasite 

 actually possesses in the comparative hardness of the egg-shell, rapid 

 evolution of the young, &c., already mentioned, must have been acquired 

 little by little through the slowly accumulating process of natural selec- 

 tion, but subsequently to the formation of the original parasitical 

 inclination and habit. I am inclined to believe that M. bonariensis 

 lost the nest-making instinct by acquiring that semiparasitical habit, 

 common to so many South- American birds, of breeding in the large 

 covered nests of the Dendrocolaptidse. We have evidence that this 

 semiparasitical habit does tend to eradicate the nest-making one. The 

 Synallaxes build great elaborate domed nests, yet we have one species 

 (S. (Bgithaloides] that never builds for itself, but breeds in the nests of 

 other birds of the same genus. In some species the nesting-habit is in 

 a transitional state. Machetornis rixosa sometimes makes an elaborate 



