COLOURS AND PATTERNS OF BIRDS 113 



that they appear as brilliant patches of distinct colours. And thus the 

 hues that depend on structure, the metallic sheens and the irides- 

 cences and blooms point to a greater delicacy of texture, a refine- 

 ment of minute structure, a replacement of the casual effects of 

 growth by more finely tempered products. In pattern, the primitive 

 spots and mottlings may be and probably are no more than an 

 accidental result of the composite nature of the skin, and of the 

 changes in the rate of growth, as the physiological activities of 

 the body rise and fall. These are replaced by patterns of a less 

 accidental character. Spots grow into stripes, or spread into patches ; 

 some areas expand, others are suppressed. Flaps and outgrowths 

 of skin, ridges and patches of hair, tufts and mantles of feathers 

 expand and lie over their neighbouring areas, producing arrangements 

 that do not conform with the primitive contours and uniform 

 characters of the body. 



The set of changes has been attended and made possible by an 

 increase in the vigour of the body and a heightening of the vital 

 activities, so that respiration,, excretion and all the chemical changes 

 in the living laboratory have become more exuberant. The changes 

 are an expression of surplus vital activity, for if animals are to succeed 

 they must on the average be a little more vigorous than is absolutely 

 necessary to attain their purposes. Now and again a successful 

 runner may faint at the goal, but in most cases he can run a little 

 beyond it. In the affairs of animals, as of men, some reserve is 

 requisite. And so it is natural to find the beginnings of more 

 brilliant colour and more vivid pattern associated with the breeding 

 season, for in the brfeeding season the strength and vigour of animal 

 life are most acute. It might be argued, as not a few naturalists 

 have urged, that the cumulative beauty of animals is in itself acci- 

 dental and inevitable, the mere result of their increasing strength and 

 vitality, and that there is no need to try to account for it by theories 

 of natural selection. 



Darwin was always careful to insist that natural and sexual 

 selection were not the actual causes of the wonderful patterns and 

 colourg that are displayed in the animal kingdom. They were out- 

 crops of the constitution of the body, by-products of its activities, 

 and what happened was that when a colour or pattern that was useful 

 appeared, it was favoured in the struggle for existence, or in the eyes 

 of choosing mates, whilst colours and patterns that were harmful or 

 that were displeasing were slowly eliminated. I do not think it can 

 be doubted that in many cases the spottings and mottlings and duU 



C.A. H 



