Page:The British Warblers A History with Problems of Their Lives - 7 of 9.djvu/91

 in certain districts at the commencement of May. Thus it seems as if the period of arrival were not yet adjusted to the environment in which the bird now finds itself, but that a gradual adjustment is taking place.

Impressed with the possibility of some change having recently been effected, we turn our attention to the points of similarity or difference in the characters of the two species, hoping thereby to find some indication of the nature of the change. Structure and colour tell us but little beyond the intimacy of the bond; they point to the probability of the two forms having originally been the same, but they afford no clue as to which is the more recent development. The emotional behaviour gives greater hope of success, for although the song and motor reactions of the Marsh Warbler are widely differentiated from those of the Reed Warbler, we nevertheless find in the former the rudiments of the special features characteristic of the latter. We may say of the Marsh Warbler that there is a general heightening of the manifestation at all emotional periods. The song is more vigorous, possesses a wider range of tone, includes a greater number of imitations and is produced with but little hesitation. The visible organic symptoms of the emotions which accompany the instinct of pugnacity and the reproductive and parental instincts are more suffused with feeling tone, more frequent in occurrence, and appear to be more readily provoked. May we not therefore speak of its nervous system as a higher and probably more recent development? Now we concluded from our examination of the dates of its arrival in different countries, the variation in its nest-building instinct, and the small dimensions of its breeding territory, that its environment may have been at one time similar to that now inhabited by the Reed Warbler. Assuming then that we are thus far correct in our speculations we shall find it difficult to conceive of such a development of the nervous system and such a change of environment as a coincidence and nothing more. We are justified in believing that there must be some relation between