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, 

 larvce of various descriptions forming their staple diet. The 

 young are principally fed on green larva, chironomidce, and 

 even chrysalides. 



The resemblance in the external characters of the Marsh 

 and Eeed 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 occur- 

 rence ? 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 con- 

 sideration 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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