YELLOW-BREASTED MARSH-BIRD 125 
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’ 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 it? 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 
