224 THEORIES OF MIMICRY 



of Fritz Miiller's paper a great effect had been produced. 

 The immediate stimulus to the investigation of new 

 examples and fresh aspects of Mimicry which followed 

 Bates's memoir, must be ascribed to the fact that then for 

 the first time was offered a good working h)'pothesis — 

 a hypothesis which seemed to afford an adc([uate ex- 

 planation of one class of known facts, and challenged its 

 critics to find insuperable difficulties among facts as yet 

 unknown. In the forty-five years which have elapsed 

 since the appearance of this great memoir an immense 

 number of facts bearing upon the subject have been dis- 

 covered, and many naturalists consider that the Theories 

 o( Mimicr\-. associated with the names of H. \\\ Hates 

 and Fritz Mliller, have stood the test with complete 

 success, and that an interpretation based on the theory 

 o( Natural Selection is in a far stronger position than in 

 1 86 1. It is, I believe, true that this opinion is more 

 generall)- held among the students of other groups of the 

 animal kingdom than among those who are specially 

 devoted to entomology. 



2. 77/6' various I/ypol/icscs which have been proposed as 

 Substitutes for Natural Selection as the Explanation of 

 Mimicry and Coninio)i JFarnino- Colours. 



All the various suggestions which have been proposed 

 as substitutes for Natural Selection, may be included 

 imder three heads : — 



i. The direct effect of some physical or chemical cause 

 or causes connected with locality, such as climate, food, 

 &c. This may be called the Theory of External Causes. 



ii. The independent evolution of a similar appearance 

 in distinct species. Mimetic Resemblance, according to 

 this hypothesis, is due to internal developmental causes, 

 comijclling different sj;ecies to pass through similar phases. 

 My friend Professor Patrick Geddes has told me that he 

 is in favour of this view, and it is the central idea of 

 Professor G. H. Eimer's work.' The hypothesis that 

 * Laws of Growth ' may cause these resemblances also 

 falls into this category. The suggestion that such laws 

 ' Or//io^€fiesis der Sc/imeUcr/in^'e, Leipzig, 1898. 



