BEDROCK 



hypotheses, the first and the third, but Fritz Miiller never could 

 accept any explanation of Mimicry based on External Forces, and he 

 proved* in answer to a paper by Wallacef that physico-chemical 

 influences did not adequately explain the detail of mimetic 

 resemblance. 



(4) The last suggested explanation of Mimicry with which I pro- 

 pose to deal may be called the hypothesis of Internal Forces, formerly 

 " Internal Causes." This interpretation is based on the supposition 

 of an " internal creative force," upon the idea that progressive 

 change is produced by forces resident within the organism itself — 

 in fact, upon that conception of evolution of which Bergson's works 

 supply the latest as well as the most distinguished exposition. 

 According to this hypothesis, " Creative Evolution " has made both 

 Model and Mimic, and has made them so that they are alike in the 

 characters that can be seen, but in no other respect. 



It now remains to test these hypotheses by the detailed considera- 

 tion of a single example in which it is possible to make out the origin 

 and history of the resemblance with a high degree of probability. 

 In the selected example, which has only recently been fully worked 

 out and understood, the ancestral pattern is still borne by the non- 

 mimetic male, and we can judge of the changes that have been 

 necessary in order to produce the mimetic female pattern. The 

 example will, I venture to hope, furnish not only solid evidence of 

 the fact of evolution, but also a clear indication of its causes. These 

 results we owe to the combined study of Mimicry and geographical 

 distribution by keen and enthusiastic naturalists in the tropics — 

 men who are observing, collecting, breeding, and all the time keeping 

 in close touch with a museum at home, where the material can be 

 worked out and co-ordinated, and whence suggestions for fresh 

 enquiries can be continually sent. It will be seen that I have been 

 extraordinarily fortunate in the colleagues with whom the study has 

 been pursued ; and this is not only true of the localities under con- 

 sideration, but of many other localities also, some African, some in 

 other parts of the world. 



The left-hand column of butterflies (Figs. 1 — 4) represents 



* Kosmos, x., Oct., 1SS1— Mar., 1882, pp. 157—167. 



t Presidential Address to Sect. I)., Brit. Assoc, Glasgow, 1876 : reprinted in 

 Tropical Nature, 1878, p. 256. 



57 



