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382 TROPICAL NATUREhere, not only is sexual selection out of the question, butthe need for recognition and identification by others of thesame species seems equally unnecessary. We can then onlyimpute this variety to the normal production of colour inorganic forms when fully exposed to light and air and undergoinggreat and rapid developmental modification. Amongmore perfect animals, where the need for recognition has beenadded, we find intensity and variety of colour at its highestpitch among the South American butterflies of the familiesHeliconidse and Danaidse, as well as among the Nymphalidseand Erycinidse, many of which obtain the necessary protectionin other ways. Among birds also, wherever the habits aresuch that no special protection is needed for the females, andwhere the species frequent the depths of tropical forests, andare thus naturally protected from the swoop of birds of prey,we find almost equally intense coloration, as in the trogons,barbets, and gapers.Local Causes of Colour-developmentAnother real, though as yet inexplicable cause of diversityof colour is to be found in the influence of locality. It isobserved that species of totally distinct groups are colouredalike in one district, while in another district the alliedspecies all undergo the same change of colour. Cases of thiskind have been adduced by Mr. Bates, by Mr. Darwin, andby myself, and I will here give the more curious and importantexamples which I 1have been able to collect.The Influence of Locality on Colour in Butterflies and BirdsOur first example is from tropical Africa, where we findtwo unrelated groups of butterflies belonging to two verydistinct families (Nymphalidse and Papilionidse) characterised bya prevailing blue-green colour not found in 2any other continent.Again, we have a group of African Pieridse which are whiteor pale yellow with a marginal row of bead-like black spots ;and in the same country one of the Lycsenidae (Leptena1 These were first given in my Address to the Biological Section of theBritish Association at Glasgow in 1876.2Romaleosoma and Euryphene (Nymphalidae), Papilio zalmoxis andseveral species of the Nireus-group (Papilionidae).