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. rufoaxillaria on M. badius ; 
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 
