92 BIRDS OF LA PLATA 
Another interesting circumstance may be adduced 
as strong presumptive 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) advan- 
tage over those hatched in the usual way, and that 
the parasitical habit thus became hereditary, sup- 
planting the original one, is all conjecture, 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, etc., already mentioned, must have 
been acquired little by little through the slowly 
accumulating process of natural selection, subse- 
quently to the formation of the original parasitical 
inclination and habit. I am inclined to believe that 
M. bonariensis lost the nest-making instinct by ac- 
quiring that semi-parasitical habit, common to so 
many South American birds, of breeding in the 
large covered nests of the Dendrocolaptide. We 
