YELLOW-BREASTED MARSH-BIRD 135 



closely resembling the eggs of the Yellow-breast* I 

 was surprised to find five more eggs of the Cow-bird 

 on the ground, close together, and about three feet 

 from the bush ; and these five eggs were all pure 

 white and unspotted. Naturally I asked : How came 

 these eggs in such a position i They had not fallen 

 from the nest, which was very deep, contained few 

 eggs, and was scarcely thirty inches above the ground. 

 Then they were all white, while those in the nest 

 were mottled. That the eggs had been laid in the 

 nest I felt certain ; and the only way I can account 

 for their being in the place where I found them is 

 that the Yellow-breast itself removed them, taking 

 them up in its bill and flying with them to the ground* 

 If I am right, we must believe that this individual 

 Yellow-breast had developed an instinct unusual in 

 the species, which enabled it to distinguish, and cast 

 out of its nest, eggs very different from its own an 

 instinct, in fact, the object of which would be to 

 counteract the parasitical habit of Molothrus. What 

 would be the effect of such an instinct should the 

 species acquire its' Doubtless it would be highly 

 prejudicial to the parasitical birds laying white eggs, 

 but favourable to those laying mottled eggs. This 

 would be natural selection operating in a very unusual 

 manner ; for the Yellow-breast, or other species, 

 would improve another to its own detriment, since 

 the more the parasitical eggs assimilated to its own, 

 the greater would be the likelihood of their being 

 preserved. The perfect similarity of the eggs of 

 M. rufoaxillaris to those of M. badius was possibly 



