1909] Mimicry in the Butterflies of North America 205 



In accordance with the facts briefly summarized in the above 

 statements, we find that better and more numerous examples of 

 rapid recent change are to be found in mimetic patterns than in 

 those which promote concealment. Not only is this evident 

 when we trace the geographical changes of model and mimic over 

 a wide continuous area, but in many cases the same genus 

 includes both mimetic and non-mimetic species, the latter enabling 

 us to infer with more or less certainty the ancestral appearance of 

 the former. The history thus unravelled may often be further 

 confirmed by a study of the non-mimetic males of mimetic 

 females. 



Many naturalists at the present day incline to return to the 

 old belief that the history of evolution has been 'discontinuous,' 

 proceeding by "mutations" or large and definite steps of change. 

 The comprehensive and detailed study of Mimicry as a piece of 

 biological history certainly provides one of the best and safest 

 means — perhaps the very best — of forming a judgment 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. Nev- 

 ertheless the evidence at present available yields much support 

 to the theory of Natural Selection as the motive cause of evolu- 

 tion. The facts certainly do not point to any other interpreta- 

 tion. They negative the conclusion that mimetic resemblances 

 have been produced by the direct action of external forces 

 (Hypothesis of External Causes) or by variation uhguided by 

 selection (Hypothesis of Internal Causes) . Nor do they support 

 Fritz Mliller's earlier and daring speculation that female prefer- 

 ences were influenced by the sight of the patterns displayed by 

 the models (Hypothesis of Sexual Selection). The only hypoth- 

 eses which are in any way consistent with the body of facts, con- 

 sidered 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 total organism 

 as well as in the rapidity of the results achieved, the operation of 

 Natural Selection in the production of Mimicry is more than 



