MIMETIC AND CRYPTIC COLOUES 147 



change, while the models of cryptic resemblance 

 appear by comparison to be stereotyped and 

 permanent. Furthermore the models as weU as 

 their mimics within the same area are liable to 

 changes of distribution, whereas the models of 

 cryptic resemblance are as a rule comparatively 

 fixed. A mimetic species may often be found 

 passing into an area where its model exists in 

 a different form or does not exist at aU, and 

 highly instructive conclusions may be drawn 

 from the study of the corresponding changes. 



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 inchne to 

 return to the old belief that the history of evolu- 

 tion 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 



l2 



