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 t 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 Dendrocolaptidae. We 



