290 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
few and probably rare. Under these circumstances any given insect 
would gain but 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 Muller’s law, increases 
inversely as the square of the fraction whose numerator is the actual 
number of 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 Muller’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 with its 450 species! 
(2) Mimicry 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, with Fig. 48, which 
represents a typical Melinaea (M. paraiya). It is evident that 
Melinaea gazoria is startlingly like an Ithomia both in size and 
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 Mechanitis ortygia Druce, from Peru. 
Druce (’76) in his description of this curious little species states in 
astonishment that it possesses the venation of a Mechanitis, but the 
size and coloration of an Ithomia ! 
