Now colour, Mr. Wallace continues, is proportionate to integumentary development, and is most conspicuous in the wings of butterflies and the feathers of birds, for the reason that, just as “the spots and rings on a soap-bubble increase with increasing tenuity,” similarly the delicately-organised surface of feathers and scales is highly favourable to the production of varied colour-effects. Colour being thus proportionate to integumentary development, we find next that integumentary development is, in turn, proportionate to vigour and vitality; the strongest animals having the largest feathers, scales, horns, etc. Hence the most vigorous and healthy animals are also the most beautiful, the most brilliantly coloured. And this correlation between healthful vigour and beauty is still more strikingly shown in this, that “The colours of an animal usually fade during disease or weakness, while robust health and vigour adds to their intensity.... In all quadrupeds a ‘dull coat’ is indicative of ill-health or low condition; while a glossy coat and sparkling eye are the invariable accompaniments of health and energy. The same rule applies to the feathers of birds, whose colours are only seen in their purity during perfect health; and a similar phenomenon occurs even among insects, for the bright hues of caterpillars begin to fade as soon as they become inactive preparatory to their undergoing transformation. Even in the vegetable kingdom we see the same thing: for the tints of foliage are deepest, and the colours of flowers and fruits richest, on those plants which are in the most healthy and vigorous condition.” Add to all these considerations that “this intensity of coloration becomes most developed during the breeding season, when the vitality is at a maximum,” and we shall be prepared for Mr. Wallace’s summing up of his case:— “If now we accept the evidence of Mr. Darwin’s most trustworthy correspondents, that the choice of the female, so far as she exerts any, falls upon ‘the most vigorous, defiant, and mettlesome male’; and if we further believe, what is certainly the case, that these are as a rule the most highly-coloured and adorned with the finest developments of plumage, we have a real and not a hypothetical cause at work. For these most healthy, vigorous, and beautiful males will have the choice of the finest and most healthy females; and will be able best to protect and rear those families. Natural Selection, and what may be termed Male Selection, will tend to give them the advantage in the struggle for existence; and thus the fullest and the finest colours will be transmitted, and tend to advance in each succeeding generation.”