1874.] MOLOTHRI OF BUENOS AYRES. 10'J 



when they are abundant. It hovers over the grass, and pounces hawk- 

 like 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 adap- 

 tation 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 corre- 

 spondence 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 insigni- 

 ficant to be recorded, improvised definite actions that are not habi- 

 tual, 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, 1 

 believe, in many cases to be attributed to the faint promptings of 

 obsolete instincts. To the same cause many of the occasional aber- 

 rant habits of individuals may 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 illus- 

 trate my 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. We should conclude that an insufficiency of proper breed- 

 ing-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 aucestral 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 



