cliff we cannot distinguish between the voices of different individuals. And the more highly developed the song, the greater the range of variation appears to be; but notwithstanding this—notwithstanding the fact that the pitch may differ, the phrase combination may differ, and the timbre may differ—the song remains nevertheless specific. So that the two principal features of "song," broadly speaking, are "diversity" and "specific character."
In contrast with the call-notes, the majority of which can be heard at all times of the year, the song is restricted as a rule to one season, and that one the season of reproduction. It is true, of course, that some birds sing during the autumn, and, if the climatic conditions are favourable, in the winter also, just as others betray, in the autumn, symptoms of emotional manifestation peculiar to the spring; but just as the manifestation of the latter is feeble and vestigial, so, too, does the song of the former lack the vigour and persistency which is characteristic of the spring. Again, in contrast with the call-notes, which are common alike to both sexes, song is confined to one sex—a peculiar property of the males.
Now all. I think, will agree that it must serve some biological purpose—this at least seems to be the conclusion to be drawn from the two outstanding features of "diversity" and "specific character"; and since the voices of different individuals of the same species vary, it has been suggested that, by creating a more
