With the results just recorded I felt more than satisfied, though so much still remained to be known; and I looked forward to the next summer to work out the rich mine on which I had stumbled by chance. Unhappily, when spring came round again ill-health kept me a prisoner in the city, and finding no improvement in my condition, I eventually left Buenos Ayres at the close of the warm season to try whether change of climate would benefit me. Before leaving, however, I spent a few days at home, and saw enough then to satisfy me that my conclusions were correct. Most of the birds had finished breeding, but while examining some nests of _Anumbius_ I found one which Bay-wings had tenanted, and which for some reason they had forsaken leaving _ten_ unincubated eggs. They were all like Bay-wings' eggs, but I have no doubt that five of them were eggs of _M. rufoaxillaris_. During my rides in the neighbourhood I also found two flocks of Bay-wings, each composed of several families, and amongst the young birds I noticed several individuals beginning to assume the purple plumage, like those of the previous autumn. I did not think it necessary to shoot more specimens. The question, why _M. badius_ permits _M. rufoaxillaris_ to use its nest, while excluding the allied parasite, _M. bonariensis_, must be answered by future observers; but before passing from this very interesting group (_Molothrus_) I wish to make some general remarks on their habits and their anomalous relations to other species. It is with a considerable degree of repugnance that we regard the parasitical instincts in birds; the reason it excites such a feeling is manifestly because it presents itself to the mind as--to use the words of a naturalist of the last century, who was also a theologian, and believed the Cuckoo had been created with such a habit--"a monstrous outrage on the maternal affection, one of the first great dictates of nature." An _outrage_, since each creature has been endowed with this all-powerful affection for the preservation of its own, and not another, species; and here we see it, by a subtle process, an unconscious iniquity, turned from its purpose, perverted and made subservient to the very opposing agency against which it was intended as a safeguard! The formation of such an instinct seems indeed like an unforeseen contingency in the system of nature, a malady strengthened, if not induced, by the very laws established for the preservation of health, and which the _vis medicatrix_ of nature is incapable of eliminating. Again, the egg of a parasitical species is generally so much larger, differing also in coloration from the eggs it is placed with, whilst there is such an unvarying dissimilarity between the young bird and its living or murdered foster-brothers, that, unreasoning as we know instinct, and especially the maternal instinct, is, we are shocked at so glaring and flagrant an instance of its blind stupidity.