Page:The British Warblers A History with Problems of Their Lives - 7 of 9.djvu/33

 of the female. But it can scarcely be intended in this latter sense, as that would imply that the reactions came into being independently of the utility which the theory suggests. The central fact of the theory appears to be this, that the reactions are the agency by which the mating of the stronger individuals is secured. Let us then carry our minds back to some earlier period, and try to picture the appearance of a variation which possessing greater vitality reflected it in its emotions, stimulated the female, and thus gained an advantage in securing a mate. The weaker members insufficiently endowed with overt expressional movement would gradually disappear and the whole species become transformed. But since, on the average, it would only be those which possessed the power of reflecting their emotions in the highest degree which would attain to reproduction, competition would still continue, and the reactions would slowly increase in intensity pari passu with the selection of the stronger individuals until a level were reached when perhaps further increase might become harmful. This level would then be maintained by the elimination of those that failed to reach the necessary standard. The reactions have accordingly been developed just in so far as they represent increased emotion and thereby increased strength. Now with regard to some of the warblers the facts are as follow: There are ten species which for purposes of comparison can be divided into five pairs, the two Whitethroats, Blackcap and Garden Warbler, Grasshopper and Savi's Warbler, Willow Warbler and Chiff-Chaff, and the Reed and Marsh Warbler. All these pairs are severally very closely allied, but in the case of two of them the relationship is so close that one can only be distinguished from the other with some difficulty, and all may be said to be fairly equally distributed over Western Europe. Regarding these ten species as a whole we find it difficult to trace any uniformity in the intensity with which they manifest their emotions, and equally difficult to satisfy ourselves of any general resemblance in the particular form such manifestations take;