MARSH WARBLER 



utmost of its power, and at the same time rapidly raising, 

 and as rapidly contracting, its fully expanded wings. By the 

 fact of the fuller expansion of the wings we know that the bird 

 is a Marsh Warbler, though it is only with difficulty that we 

 can detect the subtle difference in the colour of the plumage. 

 The behaviour of the two birds may seem to be very dissimilar, 

 but a close examination of their actions reveals the fact that 

 the difference is only one of degree. So that the motor 

 reactions of the most closely related forms within this one 

 family may be alike, or unlike, or differ only in degree. 

 Shall we then say that each reaction has some special 

 part to fulfil, and that that part is dependent upon just the 

 particular way in which the reaction is to be found setting ? 

 Or must we take a broader view and say that the reactions as 

 a whole have a part to fulfil, but that their utility is not 

 necessarily consequent upon their being cast in any particular 

 mould? Or yet a third view which w T ould regard them as 

 mechanical results of the way in which this or that nervous 

 system has been framed? There are some who uphold the 

 first of these propositions ; who say that there is some relation 

 between the plumage, i.e. the secondary sexual characters, and 

 the attitudes assumed; they regard the antics of the male 

 not as a physiological result of excitation but as the outcome 

 of intelligence, and assert consequently that they are performed 

 as a means to an end and that end is the fascination of the 

 female. But if it be true that there is some relation between 

 the plumage and the attitudes assumed, it must surely be 

 the case that identity of structure and colour ought to corre- 

 spond with identity of movement. Yet of none of the 

 examples into which we have been inquiring can this be said 

 to be true, for in the one that most nearly approaches it — the 

 Grasshopper and Savi's Warbler — there is identity of structure 

 and expression but not of colour. When, therefore, we con- 

 sider that the attitudes may be alike when the plumage is 

 different or wholly unlike when the plumage is similar, and 

 at the same time bear in mind the manifold and diverse forms 



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