272 MIMICRY AND NATURAL SELFXTION 



stront^ly developed as conii)ared with those in which they 

 were less strongly developed, and thus in [jrocess of time 

 a very hi<^h degree of resemblance has been attained. 



2. The Tlu'ory of ExtcDial Causes assumes that 

 IMimicr\' has been produced by the direct action upon the 

 organism of some one or more of the various inlUiences 

 which exist in the localit}', such as food, moisture, dryness, 

 heat, cold, ^c. 



3. 7 /ic Theory of InL }')ial Causes assumes that Mimetic 

 Resemblances are due to the independent arri\al of 

 different species at the same evolutionary stage, as regards 

 the characters in which such resemblances are manifest. 



4. TJie Theory of Sexual Selection has occasionall)' 

 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 l)y comparativel)' 

 few naturalists, although it was deemed to be worthy of 

 consideration by Charles Darwin and Fritz Miiller.^ The 

 theory of External Causes is probabl) more commonly 

 received as an explanation than an\' of the others except 

 Natural Selection itself. 



I now propose to bring before \ou several examples,^ 

 setting forth the main aspects of Mimetic Resemblance, 

 and to in(|uire how far each of them is consistent with 

 these four theories. 



The too-exclusiv^e stud)- of Mimicry in Lepidoptera 

 (butterflies and moths) alone is probably responsible for 

 a common belief in the theory of External Causes. Thus 

 wlu^n we contem[)late a group of man\- species of 

 Ithomiine, Danaine, Nymphaline, Heliconine, Erycinid, 

 and Pierine butterflies from British (niiana and Surinam, 

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

 plausil)le to suggest as an interpretation that we are 



' See a letlcr from Charles Darwin describing Fritz Miiller's thoughts 

 on this subject. Charles Darwin and the Theory of i\atural Selection. 

 Poulton, London, 1896, p. 202. 



* 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. 



