226 DARWINISM chap. 



on the head or neck, often not interfering with the generally 

 protective character of their plumage. Such are the bright 

 patches of blue, red, or yellow, by which the usually green 

 Eastern barbets are distinguished ; and similar bright patches 

 of colour characterise the separate species of small green 

 fruit-doves. To this necessity for specialisation in colour, by 

 which each bird may easily recognise its kind, is probably due 

 that marvellous variety in the peculiar beauties of some groups 

 of birds. The Duke of Argyll, speaking of the humming 

 birds, made the objection that "A crest of topaz is no 

 better in the struggle for existence than a crest of sapphire. 

 A frill ending in spangles of the emerald is no better in the 

 battle of life than a frill ending in spangles of the ruby. A 

 tail is not affected for the purposes of flight, whether its 

 marginal or its central feathers are decorated with white ;" 

 and he goes on to urge that mere beauty and variety for 

 their own sake are the only causes of these differences. But, 

 on the principles here suggested, the divergence itself is useful, 

 and must have been produced pari passu with the structural 

 differences on Avhicli the differentiation of species depends ; 

 and thus we have explained the curious fact that prominent 

 difterences of colour often distinguish species otherwise very 

 closely allied to each other. 



Among insects, the principle of distinctive coloration for 

 recognition has probably been at work in the production of 

 the wonderful diversity of colour and marking we find every- 

 where, more especially among the butterflies and moths ; and 

 here its chief function may have been to secure the pairing 

 together of individuals of the same species. In some of the 

 moths this has been secured by a jjeculiar odour, Avhich 

 attracts the males to the females from a distance : but there is 

 no evidence that this is universal or even general, and among 

 butterflies, especially, the characteristic colour and marking, 

 aided by size and form, aftbrd the most probable means of 

 recognition. That this is so is shown by the fact that "the 

 common white butterfly often flies down to a bit of paper on 

 the ground, no doubt mistaking it for one of its own species ;" 

 while, according to Mr. Collingwood, in the Malay Ai'chipelago, 

 " a dead butterfly ])iinicd upon a conspicuous twig Avill often 

 arrest an insect of the same species in its headlong flight, and 



