ARGENTINE COW-BIRD 95 



this habit may also have been lost. But the parasi- 

 tical habit of the M. bonariensis may have originated 

 when the bird was still a nest-builder. The origin 

 of the instinct may have been in the occasional habit, 

 common to so many species, of two or more females 

 laying together ; the progenitors of all the species of 

 Molothrus may have been early infected with this 

 habit, which eventually led to the acquisition of the 

 present one, M, pecoris and M, bonariensis, though 

 their instincts differ, are both parasitic on a great 

 number of species ; M, mfoaxillaria on M. badins ; 

 and in this last species two or more females fre- 

 quently lay together. If we suppose that the M, 

 bonariensis, when it was a nest-builder or reared its 

 own young in the nests it seized, possessed this 

 habit of two or more females frequently laying 

 together, the young of those birds that oftenest 

 abandoned their eggs to the care of another would 

 probably inherit a weakened maternal instinct. The 

 continual intercrossing of individuals with weaker 

 and stronger instincts would prevent the formation 

 of two races differing in habit ; but the whole race 

 would degenerate, and would only be saved from 

 final extinction by some individuals occasionally 

 dropping their eggs in the nests of other species, 

 perhaps of a Molothrus, as M, rufoaxillaris still does, 

 rather than of birds of other genera. Certainly in 

 this way the parasitic instinct may have originated 

 in M, bonariensis without that species ever having 

 acquired the habit of breeding in the covered dark 

 nests of other birds, I have supposed that they once 



