MARSH WARBLER
heard during courtship, when an intruder enters the territory, or when the parents are anxious about their young. A similar sound therefore seems to be indicative of different emotional states, but although similar to our ears it may in reality be specifically distinct to the keener perceptual powers of the bird.
In the summer their food consists entirely of insects, larvæ of various descriptions forming their staple diet. The young are principally fed on green larvæ, chironomidæ, and even chrysalides.
The resemblance in the external characters of the Marsh and Reed Warblers and the contrast which is presented to us in much of their behaviour only serves to stimulate our interest in the two birds. We wish to know more of their relationship, why they resemble one another in this particular feature or differ in that. We wish, in fact, to be able to interpret more of the mystery of their development. Have they diverged from some early ancestor along separate paths? Or has the separation been of more recent occurrence? In the pursuit of evidence bearing upon this question of development we must not lose sight of specific behaviour. The behaviour is the outcome of an inherited nervous system; it is the output, so to speak, of a certain type of machine. And surely if we desire to trace the course which the evolution of any particular machine has taken, we do not fix our attention solely upon the material of which that machine is composed and its outward appearance, but take into our consideration also the quality of the work turned out, the details of its mechanism, the productive capacity, and the changes in the relations between the machine and the surrounding conditions which have made development imperative. So ought it to be with regard to the development of species. Not only must we take into account structure and colour but also analyse and carefully compare the output of each inherited nervous system as expressed in song, in the reproductive parental and migratory instincts and the emotional behaviour
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