BRITISH WARBLERS
correspond. Each year we are learning more about geographical races; we find slight differences in structure and colour where previously none were supposed to exist, and these peculiarities seem to be common to all the individuals of a species inhabiting a certain area. We also find peculiarities in the song of the same species in different districts. And if there are differences in structure, colour, and song, why not in the nervous system? It is generally, and probably rightly, supposed that the instincts of a species are more or less constant throughout the whole of its range. A motor reaction, however, is part of an emotion, and an emotion, though it accompanies an instinct, is a separate manifestation. An instinct is uniform just because it is of vital importance to the species as a whole and therefore subject to a rigid selection. But it is possible that these reactions may not be of selection value, and consequently subject to a greater variation than we imagine.
Before finally leaving this interesting phase of the bird's behaviour, we will analyse a little more closely the relation of attitude and vocal extravagance to emotion. This is a somewhat difficult undertaking, involving as it does the dangerous proceeding of attempting to put oneself in the place of the bird. Nevertheless it is important that we should attempt, as far as possible, to separate these activities, to trace out the emotion to which they correspond, and instead of taking a broad view of the behaviour as a whole, make some effort to reduce it to order. We are accustomed to use the term "display" to embrace the whole of the visible signs of the complex emotional system of a bird, and if there are facts which seem to be outside its province we find for them a ready explanation under the heading of "play"—practice, that is to say, for the more serious side of life. But we can scarcely make use of so loose a definition as "display" if we desire to resolve this emotional behaviour into its constituent parts. Let us take the song first, and for this purpose it is only necessary to consider it in its more emotional aspect. Violent
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