BRITISH WARBLERS
selection, and adjusted to this particular end. That the so-called delusion of the intruder may be complete, the response must be sufficiently intense, yet it must not exceed the point beyond which it might easily become prejudicial; the same measure of latitude which we gave to the interpretation of the sexual behaviour cannot well therefore be permitted. Now one might conceivably suppose that the response of the Blackcap would be of sufficient intensity to attract attention to itself and preserve the young from molestation, but that of the Willow Warbler is so weak that it is difficult to believe that it can answer this purpose. We might extend the number of such cases indefinitely; we can find examples wherein the emotional manifestation, when the parental instinct is dominant, ranges from almost zero up to remarkable extravagance, and yet trace a similar relation to the sexual response as that in the illustration which we have taken.
In the history of the Marsh Warbler some comparisons were made between the specific types of reaction in closely related forms which enabled us to form some idea of the complexity of the subject. Some species display their emotions far more intensely than others; some respond vigorously on the slightest provocation; others require a stimulus of a more prolonged kind to produce the customary reaction, whilst in a third class the point of motor release seems never to be reached. Why should the expressional movement visible to an external observer be marked in A and not in B? We have no answer to this question; we know nothing beyond the mere fact that it is so. But such differences must appeal to anyone who compares the behaviour of one family with that of another, or, better still, of one closely related species with that of another. Now instinct and emotion are believed to be two manifestations of one and the same process. Must we then say, just because the observed response varies in different species, that the emotion varies too, and can we link up the strength of the instinct with the intensity of the emotion with which it corresponds?
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