MOLOTHRUS BONARIENSIS. 83 



disused instincts of a species, this knowledge of itself would not enable 

 us to discover the origin of present ones. But assuming it as a fact that 

 the conditions of existence, and the changes going on in them, are in 

 every case the fundamental cause of alterations in habits, I believe that 

 in many cases a knowledge of the disused instincts will assist us very 

 materially in the inquiry. I will illustrate my meaning with a suppo- 

 sititious case. Should all or many species of Columbidce manifest an 

 inclination for haunting rocks and banks, and for entering or peering 

 into holes in them, such vague and purposeless actions, connected with 

 the facts that all Doves build simple platform-nests (like Columba livia 

 and others that build on a flat surface), also lay white eggs (the rule 

 being that eggs laid in dark holes are white, exposed eggs coloured), 

 also that one species, G. livia, does lay in holes in rocks, would lead 

 us to believe that the habit of this species was once common to the 

 genus. We should conclude that an insufficiency of proper breeding- 

 places, i. e. new external conditions, first induced Doves to build in 

 trees. Thus C. livia also builds in trees where there are no rocks ; but, 

 when able, returns to its ancestral habits. In the other species we 

 should believe the primitive habit to be totally lost from disuse, or 

 only to manifest itself in a faint uncertain manner. 



Now, in Molothrus bonariensis we see just such a vague, purposeless 

 habit as the imaginary one I have described. Before and during the 

 breeding-season the females, sometimes accompanied by the males, are 

 seen continually haunting and examining the domed nests of some of 

 the Dendrocolaptidse. This does not seem like a mere freak of curiosity, 

 but their persistence in their investigations is precisely like that of birds 

 that habitually make choice of such breeding-places. It is surprising 

 that they never do actually lay in such nests, except when the side or 

 dome has been accidentally broken enough to admit the light into the 

 interior. Whenever I set boxes up in my trees, the female Cow-birds 

 were the first to visit them. Sometimes one will spend half a day 

 loitering about and inspecting a box, repeatedly climbing round and 

 over it, and always ending at the entrance, into which she peers curi- 

 ously, and when about to enter starting back, as if scared at the 

 obscurity within. But after retiring a little space she will return 

 again and again, as if fascinated with the comfort and security of such 

 an abode. It is amusing to see how pertinaciously they hang about 

 the ovens of the Oven-birds, apparently determined to take possession 

 of them, flying back after a hundred repulses, and yet not entering 

 them even when they have the opportunity. Sometimes one is seen 

 following a Wren or a Swallow to its nest beneath the eaves, and then 

 clinging to the wall beneath the hole into which it, disappeared. I 



