216 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



few and probably rare. Under these circumstances any given insect 

 would gain l)ut little advantage by resembling merely the general 

 type of the coloration of its fellows. For the relative advantage 

 gained by such imitation, according to Fritz Miiller's law, increases 

 inversely as the square of the fraction whose numerator is the actual 

 numberof the imitating form and whose denominator is the actual 

 number of the imitated. Therefore when the insects were still rare 

 there would be few to imitate and consequently but little advantage 

 would be gained by the imitation. Imagine, for example, that a 

 single insect happens to imitate the color-pattern of a group of 100, 

 and that the advantage gained thereby is represented by the number 

 1 ; it is evident from Fritz Miiller's law that, if it happened to 

 imitate the coloration of a group of 1,000, its relative advantage 

 would be 100 instead of 1. We see, then, that mimicry within the 

 group of the Danaoid Heliconidae became an important factor only 

 after the group was well established and the insects became common. 

 During the early history of the race, then, there would be but little 

 tendency towards conservatism of color-patterns, and when the 

 "Ithomia" and "Melinaea" types of coloration made their appear- 

 ance, they both survived and now serve as the patterns for mimicry ; 

 and this accounts very well for the remarkable fact, that there 

 are no other types of coloration than these two to be found within 

 the whole group wnth its 450 species ! 



(2) 3Iimicry among the Danaoid Heliconidae. The genus 

 Ithomia with its 230 species is the dominant genus of the Danaoid 

 group, and in nearly all of the other genera individual species are 

 found which have departed widely from their generic type of 

 coloration and have assumed the clear wings of the Ithomias. A 

 good idea of how far these interesting individuals may depart from 

 the coloration of their type may be gained by comparing Fig. 53, 

 Plate 4, which represents Melinaea gazoria, Avith Fig. 48, which 

 represents a typical Melinaea (M. paraiya). It is evident that 

 Melinaea gazoria is startlingly like an Ithomia both in size apd 

 coloration, although it retains the venation and generic charac- 

 teristics of a Melinaea, 



In Mechanitis, which is the most independent genus of the 

 .Melinaea type of coloration, all of the species are fair examples of 

 the Melinaea type, except JMechanitis ortygia Druce, from Peru. 

 Druce (76) in his description of this curious Uttle species states in 

 astonishment that it possesses the venation of a Mechanitis, but the 

 size and coloration of an Ithomia ! 



