148 MIMICRY IN N. AMERICAN BUTTERFLIES 



one of the best and safest means — perhaps the 

 very best — of forming a judgement between this 

 revived opinion and Darwin's conclusion that, 

 although the rate of transformation varied greatly 

 and might slow down to nothing for long periods, 

 the steps of change were small, forming a gradual 

 and ' continuous ' transition between the successive 

 forms in the same evolutionary history.^ 



The study of the causes of Mimicry is more 

 difficult than that of the history of Mimicry, the 

 conclusions far less certain. Nevertheless the 

 evidence at present available yields much support 

 to the theory of Natural Selection as the motive 

 cause of evolution. The facts certainly do not 

 point to any other interpretation. They negative 

 the conclusion that mimetic resemblances have 

 been produced by the direct action of external 

 forces (Hypothesis of External Causes) or by 

 variation unguided by selection (Hypothesis of In- 

 ternal Causes). Nor do they support Fritz Mtiller's 

 earlier and daring speculation (see pp. 127-8) 

 that female preferences were influenced by the 

 sight of the patterns displayed by the models 

 (Hypothesis of Sexual Selection). The only 

 hypotheses which are in any way consistent with 

 the body of facts, considered as a whole, are those 

 which assume that the resemblances in question 

 have been built up by the selection of variations 

 beneficial in the struggle for life. 



In its concentration on a minute fraction of the 



' See pp. 42-51; also Appendix B, p. 254. 



