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Show 1874.] MOLOTHRI O F B U E N O S A Y R E S . 169 when they are abundant. It hovers over the grass, and pounces hawklike on its prey ; but this does not suffice, the mouse being too large to be swallowed entire and the bird's bill too straight and weak to tear it in pieces. To remedy this defect or want of structural adaptation to its requirements, it has acquired a supplementary habit, and, carrying its prey to a tree and dexterously swinging it by its hind feet or tail, beats it with violence against a branch until it is bruised into a soft pulp. But however much the instincts of a species may have become altered-the habits of a species being widely different from those of its congeners, also a want of correspondence between structure and habits (the last being always more suited to conditions than the first) being taken as evidence of such alteration-traces of ancient and disused habits frequently reappear. Seemingly capricious actions too numerous, too vague, or too insignificant to be recorded, improvised definite actions that are not habitual, apparent imitations of the actions of other species, a perpetual inclination to attempt something that is never attempted, and attempts to do that which is never done-these and other like motions are, I believe, in many cases to be attributed to the faint promptings of obsolete instincts. To the same cause many of the occasional aberrant habits of individuals m a y possibly be due-such as of a bird that builds in trees occasionally laying on the ground. If recurrence to an ancestral type be traceable in structure, coloration, language, it is reasonable to expect something analogous in instincts. But even if such casual and often harmless motions as I have mentioned should guide us unerringly to the knowledge of the old and 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 m y meaning with a supposititious case. Should all or many species of Columba 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 birds 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, C. livia, does lay in holes in rocks, it would be easy to believe that the habit of this species was once common to the genus. W e should conclude that an insufficiency of proper breeding- places, i. e. new external conditions, first induced Doves to build in trees. The C. livia also builds in trees where there are no rocks; but when able, returns to the 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. Still it will be asked, what, in faint and uncertain habits of species or in the occasional actions of individuals, is the criterion to distinguish those due to the laws of variation from those due merely to recurrence 1 I presume that PROC. ZOOL. Soc-1874, NO. XII. 12 |