x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 297 
to the most familiar facts of nature. Much yet remains to he 
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 lamellae or in fine 
surface striae which, by the laws of interference, produce the 
wonderful metallic hues of so many birds and insects. 
progeny. And liere would come in, as it appears to ine, tlie proper 
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 he checked where it became really detrimental in some 
respect or other to the individual.” 
This passage, giving the independent view's 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. 
