x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 297 



to the most familiar facts of nature. Much yet remains to be 

 done, both in the observation of new facts as to the relations 

 between the colours of animals and their habits or economy, 

 and, more especially, in the elucidation of the laws of growth 

 which determine changes of colour in the various groups ; but 

 so much is already known that we are able, with some 

 confidence, to formulate the general principles which have 

 brought about all the beauty and variety of colour which 

 everywhere delight us in our contemplation of animated 

 nature. A brief statement of these principles will fitly con- 

 clude our exposition of the subject. 



1 . Colour may be looked upon as a necessary result of the 

 highly complex chemical constitution of animal tissues and 

 fluids. The blood, the bile, the bones, the fat, and other 

 tissues have characteristic, and often brilliant colours, which 

 we cannot suppose to have been determined for any special 

 purpose, as colours, since they are usually concealed. The 

 external organs, with their various appendages and integu- 

 ments, would, by the same general laws, naturally give rise to 

 a greater variety of colour. 



2. We find it to be the fact that colour increases in variety 

 and intensity as external structures and dermal appendages 

 become more differentiated and developed. It is on scales, 

 hair, and especially on the more highly specialised feathers, 

 that colour is most varied and beautiful ; while among insects 

 colour is most fully developed in those whose wing membranes 

 are most expanded, and, as in the lepidoptera, are clothed 

 with highly specialised scales. Here, too, we find an additional 

 mode of colour production in transparent lamella? or in fine 

 surface striae which, by the laws of interference, produce the 

 wonderful metallic hues of so many birds and insects. 



progeny. And here would come in, as it appears to me, the jnroper 

 application of Darwin's theory of Natural Selection ; for the possessors 

 of greatest vital power being those most frequently produced and repro- 

 duced, the external signs of it would go on developing in an ever-increasing 

 exaggeration, only to be checked where it became really detrimental in some 

 respect or other to the individual." 



This passage, giving the independent views of a close observer — one, 

 moreover, who has studied the species of an extensive group of animals 

 both in the field and in the laboratory— very nearly accords with my own 

 conclusions above given ; and, so far as the matured opinions of a competent 

 naturalist have any weight, afford them an important support. 



