,24 THEORIES OF MIMICRY 



of Fritz Muller'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 hypothesis — 

 a hypothesis which seemed to afford an adequate 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 

 of Mimicry, associated with the names of H. W. Bates 

 and Fritz Miiller, have stood the test with complete 

 success, and that an interpretation based on the theory 

 of Natural Selection is in a far stronger position than in 

 1861. It is, I believe, true that this opinion is more 

 generally held among the students of other groups of the 

 animal kingdom than among those who are specially 

 devoted to entomology. 



2. The various Hypotheses which have been proposed as 

 Substitutes for Natural Selection as the Explanation of 

 Mimicry and Common Warning Colours. 



All the various suggestions which have been proposed 

 as substitutes for Natural Selection, may be included 

 under 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, 

 compelling different species 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. 1 The hypothesis that 

 ' Laws of Growth ' may cause these resemblances also 

 falls into this category. The suggestion that such laws 

 1 Orthogenesis der Schmcttcrlingc, Leipzig, 1898. 



