MARSH WARBLER
next we must. I think, assume that this is the case, and that each individual acquires the imitations during its lifetime. An interesting experiment might be made with young Marsh Warblers if they could be successfully reared in confinement.
It is difficult to understand what part, if any, imitation in the limited sense in which we are here discussing it can play in furthering the life of the individual. The only explanation we have is that these modifications of the song evoke in different degrees the pairing impulse of the female, but even when we interpret this in a very broad sense it is not altogether easy to reconcile it with the facts we have at our command. If these diverse modifications of song have differential effects, such effects must be due to the quality, quantity, or both, of the imitations. Those individuals, in brief, which start life with an instrument capable of more perfect reproductions will acquire a greater number of imitations and consequently be more likely to secure mates. But we can scarcely assume that imitations, however perfect they may be, will arouse the pairing instinct of the female in a greater degree than the true song; and so the development of the imitative faculty must run parallel with the development of the vocal powers in other directions, the one supplementing the other from the point of view of this theory. Now since the acquirement of imitations is a matter of experience, and since observation shows that some imitations are susceptible of revival, the older birds will have predominant opportunities of securing mates, the younger, no matter how well they may be endowed congenitally, being at a disadvantage in competition with their more experienced rivals. This factor of experience then will to some extent neutralise the beneficial effects arising from the possession of a slightly more perfect instrument, though perhaps not altogether so, for it may be argued that over a long period of time those individuals which were not so perfectly endowed would on the average be at a disadvantage; the point however is a debatable one. Another difficulty arises from the peculiar similarity in the strains
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