56 MIMICRY AND HOMOPLASY 



a special environment produces likenesses among 

 different animals which are incidental and ana- 

 logous to those convergent resemblances "caused 

 by functional adaptation, such as the mole-like 

 forms produced in the burrowing Insectivora, 

 Rodentia, and Marsupialia " [Poulton]. 



We may safely claim that the possession by 

 noxious animals of common warning coloration is 

 as much due to convergence as is the possession 

 by harmless animals of a common protective 

 coloration ; and both of these colour - schemes 

 are referable to conceivable though indefinite 

 reactions. On the other hand, the resemblances 

 and associations between palatable and unpalat- 

 able insects are hard to explain on the tropism 

 theory, unless we suppose that they arose by 

 ordinary convergence before advantage was taken 

 of them by natural selection. We may there- 

 fore differentiate usefully between passive or con- 

 vergent mimicry (including Poulton's syncryptic 

 and synaposematic categories) and active or 

 selective mimicry (including true mimetic re- 

 semblances) ; and we may add that while in 

 typical cases the categories are sharply divided, 

 in less obvious cases it is often as difficult to 

 separate pure convergence from either form of 

 mimicry as it is to distinguish it from homology ; 

 the temptation being to describe the more or 

 less sensational instances and to ignore those 

 which are less convincing, thereby imparting a 



