272 MIMICRY AND NATURAL SELECTION 



strongly developed as compared with those in which they 

 were less strongly developed, and thus in process of time 

 a very high degree of resemblance has been attained. 



2. The Theory of External Causes assumes that 

 Mimicry has been produced by the direct action upon the 

 organism of some one or more of the various influences 

 which exist in the locality, such as food, moisture, dryness, 

 heat, cold, &c. 



3. The Theory of Internal Causes assumes that Mimetic 

 Resemblances are due to the independent arrival of 

 different species at the same evolutionary stage, as regards 

 the characters in which such resemblances are manifest. 



4. The Theory of Sexual Selection has occasionally 

 been invoked to account for Mimicry, the assumption 

 being that the selection of mates has been influenced by 

 the colours and patterns of other species living in the 

 same country. 



The last-mentioned theory is held by comparatively 

 few naturalists, although it was deemed to be worthy of 

 consideration by Charles Darwin and Fritz Miiller. 1 The 

 theory of External Causes is probably more commonly 

 received as an explanation than any of the others except 

 Natural Selection itself. 



I now propose to bring before you several examples, 2 

 setting forth the main aspects of Mimetic Resemblance, 

 and to inquire how far each of them is consistent with 

 these four theories. 



The too-exclusive study of Mimicry in Lepidoptera 

 (butterflies and moths) alone is probably responsible for 

 a common belief in the theory of External Causes. Thus 

 when we contemplate a group of many species of 

 Ithomiine, Danaine, Nymphaline, Heliconine, Erycinid, 

 and Pierine butterflies from British Guiana and Surinam, 

 and find that all tend to develop dark hind-wings, it is 

 plausible to suggest as an interpretation that we are 



1 See a letter from Charles Darwin describing Fritz Miiller's thoughts 

 on this subject. Charles Darwin and the Theory of Natural Selection. 

 Poulton, London, 1S96, p. 202. 



2 These examples were illustrated by projections of three-colour illus- 

 trations prepared by Mr. Sanger-Shepherd from the actual specimens in 

 the Hope Department of the Oxford University Museum. 



