There is one other fact to which attention must be drawn--the variation in the song of the same species in different districts. As an illustration let us take the case of the Chaffinch. In Worcestershire the bird sings what I imagine to be a normal song--the notes are clear and the phrases are distinct and combined in numerous ways. With the notes fresh in mind I leave them and go to the west of Donegal, where I am at once conscious of a difference; not a subtle difference that perplexes the mind and is difficult to trace, but a change so remarkable that one is conscious of a passing doubt as to whether after all the voice is the voice of the Chaffinch; the song is pitched in a lower key, certain phrases are absent, the notes lack tone and are sometimes even harsh, and the bird seems wholly incapable of reaching the higher notes to which I am accustomed.
Now the immature Reed-Bunting, though to our ears its song is but a poor representation of that of the adult, gains a mate; the Yellow Bunting pairs, and the discharge of the sexual function may even have taken place before its voice attains what we judge to be its full development; and there are no grounds for supposing that the Donegal Chaffinch, with its less musical notes, has on that account any the less chance of procreating its kind--facts which demonstrate that the biological value of song is neither to be sought in the purity of tone, nor in the variety and combination of phrases, nor, indeed, in any of those qualities by which the human voice gains or loses merit, and which leave us with no alternative but to dismiss from our minds all æsthetic considerations in the attempt to estimate its true significance.
What, then, determines its value? Are there any qualities which, whether the bird is mature or immature, whether it is untrained or has acquired fuller expression by practice, whether it inhabits this district or that, are alike constant? Well, no matter how great the variation, no matter how much this voice falls below or exceeds the standard, judged from the human standpoint, attained by that, even we, with our duller perception, have no difficulty in recognising the species to which the owner of the voice belongs; in other words, the song is always specific, and this is the most noticeable, as it is the most remarkable, characteristic.