184 MIMICRY IN N. AMERICAN BUTTERFLIES 



It is highly probable that the earliest steps 

 in the direction of Mimicry in asterius and glaucus 

 were favoured by the appearance of partially 

 melanic varieties of the female, thus effecting 

 suddenly that essential change which enables a 

 butterfly with a yellow ground-colour to become 

 the mimic of one in which it is black. But this 

 transformation, immensely important as it is, 

 supplies nothing more than a tinted paper for the 

 new picture. That the melanic varieties were 

 partial is clearly shown by the persistence (in 

 glaucus) in a subdued and inconspicuous form of 

 certain ancestral features that do not contribute 

 to the Mimicry, but above all by the retention 

 of every element in the original pattern that can 

 be worked up into the new. By the modification 

 of these elements in form or colour, often in both 

 form and colour, the detailed mimetic pattern 

 has been wrought upon the darkened surface. 



Valuable confirmation of the history suggested 

 in the last paragraph is to be found in the dark 

 fonn melasina (Rothsch. and Jord.) found in both 

 sexes of P. polyxenes americus (Kollar), extending 

 from North Peru to Colombia and Venezuela. This 

 melanic variety probably represents the darkened 

 form of asterius before the initiation of the detailed 

 mimicry ofphilenor. The sub-species americus does 

 not enter the range of pliilenor, and those ancestral 

 elements which have been retained by its melanic 

 form have not developed into the mimetic likeness 

 seen in the more northern sub-species asterius. 



