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 w T e 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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