Page:The British Warblers A History with Problems of Their Lives - 7 of 9.djvu/29

 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 would 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 correspond 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 consider 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