THE THEORY OF MIMICRY 157 



tion of further adornment or display, which may be due 

 to sexual selection or to overwhelming hereditary Im- 

 pulse to change. Whichever may be the impelling force, 

 there is no doubt that Ihe males have generally taken the 

 lead in these developments. 



The constant migration of butterflies to and from a 

 given region must cause great changes in the directive 

 tendencies of variation, and this might account for the 

 numerous examples of species of the same genera which 

 exhibit developments in opposite directions such as occur 

 for instance in Pseudacraea trimenii, Butler and P. imi- 

 tator, Tr. Polymorphism, such as is met with among 

 the females of Papillo clardaniis, may be reversions due 

 to instability brought about by the same causes (migra- 

 tion) acting on constitutions exceedingly susceptible to 

 such influences. 



The uniformity of coloration of desert animals I attri- 

 bute to direct physical causes which react upon the con- 

 stitutions. Incidentally, cases occur among birds and 

 small mammals in which the assimilative coloring is of 

 value to them as disguises against the attacks of pre- 

 dacious birds that hunt by sight. On the other hand, it 

 seems likely that it is of no use to these and others 

 against the attacks of carnivora that seek their prey by 

 scent. 



Much capital in favour of mimicry has been made out 

 of the fact that the Planemas and Pseudacrseas vary 

 analogously according to the countries or districts they 

 frequent. The same ])henomenon occurs even more 

 markedly in South America, where Bates has stated that 

 '' numerous series of gaily coloured butterflies and moths 

 of very difi'erent families all change their hues and mark- 

 ings together, as if by the touch of an enchanter's wand 

 at every hundred miles." Pro. Ent. Soc. Lond., 1879 

 page XXIX. 



> Is not this just as good evidence of the direct effects 

 of similar climatic conditions, acting on various organ 

 isms susceptible to the same chemi co-physical stimuli, 



