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By E.B. POULTON.
Hope Professor of Zoology in the University of Oxford.
INTRODUCTION.
The following pages have been written almost entirely from the historical stand-point. Their principal object has been to give some account of the impressions produced on the mind of Darwin and his great compeer Wallace by various difficult problems suggested by the colours of living nature. In order to render the brief summary of Darwin's thoughts and opinions on the subject in any way complete, it was found necessary to say again much that has often been said before. No attempt has been made to display as a whole the vast contribution of Wallace; but certain of its features are incidentally revealed in passages quoted from Darwin's letters. It is assumed that the reader is familiar with the well-known theories of Protective Resemblance, Warning Colours, and Mimicry both Batesian and Mullerian. It would have been superfluous to explain these on the present occasion; for a far more detailed account than could have been attempted in these pages has recently appeared. (Poulton, "Essays on Evolution" Oxford, 1908, pages 293-382.) Among the older records I have made a point of bringing together the principal observations scattered through the note-books and collections of W.J. Burchell. These have never hitherto found a place in any memoir dealing with the significance of the colours of animals.
INCIDENTAL COLOURS.
Darwin fully recognised that the colours of living beings are not necessarily of value as colours, but that they may be an incidental result of chemical or physical structure. Thus he wrote to T. Meehan, Oct. 9, 1874: "I am glad that you are attending to the colours of dioecious flowers; but it is well to remember that their colours may be as unimportant to them as those of a gall, or, indeed, as the colour of an amethyst or ruby is to these gems." ("More Letters of Charles Darwin", Vol. I. pages 354, 355. See also the admirable account of incidental colours in "Descent of Man" (2nd edition), 1874, pages 261, 262.)Incidental colours remain as available assets of the organism ready to be turned to account by natural selection. It is a probable speculation that all pigmentary colours were originally incidental; but now and for immense periods of time the visible tints of animals have been modified and arranged so as to assist in the struggle with other organisms or in courtship. The dominant colouring of plants, on the other hand, is an essential element in the paramount physiological activity of chlorophyll.
In exceptional instances, however, the shapes and visible colours of plants may be modified in order to promote concealment.
TELEOLOGY AND ADAPTATION.
In the department of Biology which forms the subject of this essay, the adaptation of means to an end is probably more evident than in any other;and it is therefore of interest to compare, in a brief introductory section, the older with the newer teleological views.
The distinctive feature of Natural Selection as contrasted with other attempts to explain the process of Evolution is the part played by the struggle for existence. All naturalists in all ages must have known something of the operations of "Nature red in tooth and claw"; but it was left for this great theory to suggest that vast extermination is a necessary condition of progress, and even of maintaining the ground already gained.
Realising that fitness is the outcome of this fierce struggle, thus turned to account for the first time, we are sometimes led to associate the recognition of adaptation itself too exclusively with Natural Selection.
Adaptation had been studied with the warmest enthusiasm nearly forty years before this great theory was given to the scientific world, and it is difficult now to realise the impetus which the works of Paley gave to the study of Natural History. That they did inspire the naturalists of the early part of the last century is clearly shown in the following passages.
In the year 1824 the Ashmolean Museum at Oxford was intrusted to the care of J.S. Duncan of New College. He was succeeded in this office by his brother, P.B. Duncan, of the same College, author of a History of the Museum, which shows very clearly the influence of Paley upon the study of nature, and the dominant position given to his teachings: "Happily at this time (1824) a taste for the study of natural history had been excited in the University by Dr Paley's very interesting work on Natural Theology, and the very popular lectures of Dr Kidd on Comparative Anatomy, and Dr Buckland on Geology." In the arrangement of the contents of the Museum the illustration of Paley's work was given the foremost place by J.S. Duncan: