large domed nests, that whenever it can possess itself of one by dint of fighting, it will not build one for itself. Let us suppose that the Cow-bird also once acquired the habit of breeding in domed nests, and that through this habit its original nest-making instinct was completely eradicated, it is not difficult to imagine how in its turn this instinct was also lost. A diminution in the number of birds that built domed nests, or an increase in the number of species and individuals that breed in such nests, would involve _M. bonariensis_ in a struggle for nests, in which it would probably be defeated. In Buenos Ayres the White-rumped Swallow, the Wren, and the Yellow Seed-finch prefer the ovens of the _Furnarius_ to any other breeding-place, but to obtain them are obliged to struggle with _Progne tapera_; for this species has acquired the habit of breeding exclusively in the ovens. They cannot, however, compete with the _Progne_; and thus the increase of one species has, to a great extent, deprived three other species of their favourite building-place. Again, _Machetornis rixosa_ prefers the great nest of the _Anumbius_; and when other species compete with it for the nest they are invariably defeated. I have seen a pair of _Machetornis_ after they had seized a nest attacked in their turn by a flock of six or eight Bay-wings; but, in spite of the superior numbers, the fury of the _Machetornis_ compelled them to raise the siege. Thus some events in the history of our common _Molothrus_ have perhaps been accounted for, if not the most essential one--the loss of the nest-making instinct from the acquisition of the habit of breeding in the covered nests of other birds, a habit that has left a strong trace in the manners of the species, and perhaps in the pure white unmarked eggs of so many individuals; finally, we have seen how this habit may also have been lost. But the parasitical 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, and inherited with it a facility for acquiring their present one. _M. pecoris_ and _M. bonariensis_, though their instincts differ, are both parasitic on a great number of species; _M. rufoaxillaris_ on _M. badius_; and in this last species two or more females frequently 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