142 CONCLUSION [ch. 



the relative scarcity of mimicry in the male sex and the 

 existence of cases of polymorphism among females of 

 a species which cannot possibly be explained on mimetic 

 lines have already been mentioned. But while the 

 main objections remain it is hardly necessary to insist 

 upon these others. Looked at critically in the light 

 of what we now know about heredity and variation the 

 mimicry hypothesis is an unsatisfactory explanation of 

 the way in which these remarkable resemblances be- 

 tween different species of butterflies have been brought 

 about. Sometimes this is admitted by those who never- 

 theless embrace the theory with a mild aloofness. 

 For they argue that even though it does not explain 

 all the facts no other theory explains so many. Others 

 have sought an explanation in what has sometimes 

 been termed the hypothesis of external causes, regarding 

 these resemblances as brought about by similar con- 

 ditions of soil and climate, and so forth. It is not 

 inconceivable that certain types of colour and pattern 

 may be the expression of deep-seated physiological 

 differences, which place their possessors at an advan- 

 tage as compared with the rest of the species. Were 

 this so it is but reasonable to suppose that they would 

 become established through the agency of natural 

 selection. But it is difficult, if not impossible, to regard 

 this as a satisfactory solution, if for no other reason than 

 that it offers no explanation of polymorphism. For 

 example, each of the three forms of polytes female 

 holds its own and all must, therefore, be regarded as 

 equally well adapted to the circumstances under which 



