256 DARWINISM chap. 



closely allied forms — those that require minute examination 

 to discriminate them as distinct species — are most generally 

 found in separate areas, and are what are termed representative 

 forms. 



The extension Ave have now given to the theory of mimicry 

 is important, since it enables us to explain a much "sWder 

 range of colour phenomena than those which were first im- 

 puted to mimicry. It is in the richest butterfly region in the 

 world — the Amazon valley — that we find the most abundant 

 evidence of the three distinct sets of facts, all depending on 

 the same general principle. The form of mimicry first 

 elucidated by Mr. Bates is characterised by the presence in 

 each locality of certain butterflies, or other insects, themselves 

 edible and belonging to edible groups, Avhich derived protec- 

 tion from having acquired a deceptive resemblance to some 

 of the inedible butterflies in the same localities, which latter 

 were believed to be wholly free from the attacks of in- 

 sectivorous birds. Then came the extension of the principle, 

 by Dr. F. JMiiller, to the case of species of distinct genera 

 of the inedible liutterflies resembling each other quite as 

 closely as in the former cases, and like them always found 

 in the same localities. They derive mutual benefit from 

 becoming, in appearance, one species, from which a certain 

 toll is taken annually to teach the young insectivorous birds 

 that they are uneatable. Even when the two or more species 

 are approximately equal in numbers, they each derive a 

 considerable benefit from thus combining their forces ; but 

 Avhen one of the species is scarce or verging on extinction, the 

 benefit becomes exceedingly great, being, in fact, exactly appor- 

 tioned to the need of the species. 



The third extension of the same principle explains the 

 grouping of allied species of the same genera of inedible 

 butterflies into sets, each having a distinct type of coloration, 

 and each consisting of a number of species which can hardly 

 be distinguished on the "\nng. This must be useful exactly 

 in the same way as in the last case, since it di\ades the 

 inevitable toll to insectivorous birds and other animals 

 among a number of species. It also explains the fact of the 

 great similarity of many species of inedible insects in the 

 same locality — a similarity Avhich does not obtain to anything 



