98 THE CASE OF PAPILIO POLYTES [ch. 



and may be expected to endure for a further period in 

 the future, or else the population is in a condition of 

 gradual change as regards the numerical proportion of 

 mimetics and non-mimetics, progressing towards the 

 elimination of the one or the other, the present state 

 of equilibrium being merely transitory and accidental. 

 In this connection a few scraps of historical evidence 

 are of interest. Of the various forms of P. polytes the 

 A form of female was the first to be described in 1758, 

 and not long after (1776) the H form was registered as 

 a species under the name of Papilio Eques Trojanus 

 romulus. Later on the female resembling the male 

 found its way into the literature as Papilio pammon. 

 From the fact that the mimetic forms were known before 

 the non-mimetic, it is unlikely that they can have been 

 scarce a century and a half ago. As P. polytes cer- 

 tainly produces at least four broods a year in Ceylon 

 this period of time represents something like 600 

 generations in the life of the species, and we have 

 already seen that even if the mimetic forms have but 

 a 1 % advantage over the non-mimetic the proportion 

 of the latter would decrease from nearly equality 

 down to but 1 in 40 in about 700 generations. 

 Actually for P. polytes the decrease would not be so 

 marked because the male is non-mimetic. Owing to 

 this peculiar feature the rapidity of change in the 

 proportion of the different forms is reduced to about 

 one-half of what it would be if the males were also 

 mimetic. Nevertheless the change from nearly equality 

 to about one non-mimetic in 40 would have taken place 



