The Darwinian theory threw new light on organic colour--The problem to be solved--The constancy of animal colour indicates utility--Colour and environment--Arctic animals white--Exceptions prove the rule--Desert, forest, nocturnal, and oceanic animals--General theories of animal colour--Variable protective colouring--Mr. Poulton's experiments--Special or local colour adaptations--Imitation of particular objects--How they have been produced--Special protective colouring of butterflies--Protective resemblance among marine animals--Protection by terrifying enemies--Alluring coloration--The coloration of birds' eggs--Colour as a means of recognition--Summary of the preceding exposition--Influence of locality or of climate on colour--Concluding remarks. Among the numerous applications of the Darwinian theory in the interpretation of the complex phenomena presented by the organic world, none have been more successful, or are more interesting, than those which deal with the colours of animals and plants. To the older school of naturalists colour was a trivial character, eminently unstable and untrustworthy in the determination of species; and it appeared to have, in most cases, no use or meaning to the objects which displayed it. The bright and often gorgeous coloration of insect, bird, or flower, was either looked upon as having been created for the enjoyment of mankind, or as due to unknown and perhaps undiscoverable laws of nature.