viii] THE CASE OF PAPILIO POLYTES 97 



exhibiting ordinary Mendelian inheritance. For the 

 present we may consider one of these factors, X, which 

 involves the proportion of mimetic to non-mimetic 

 forms. It is generally agreed among observers who 

 have studied this species that of the three forms of 

 female the M form is distinctly the most common, 

 while of the other two the H form is rather more 

 numerous than the A form. The two dominant 

 mimetic forms taken together, however, are rather more 

 numerous than the recessive M form. The most 

 recent observer who studied this question, Mr Fryer, 

 captured 155 specimens in the wild state as larvae. 

 When reared 66 turned out to be males, while of the 

 females there were 49 of the two mimetic forms and 40 

 of the M form, the ratio of dominants to recessives 

 being closely 5 : 4 1 . Now as has already been pointed 

 out the ratio 5:4 of dominants and recessives is 

 characteristic of a population exhibiting simple Men- 

 delian inheritance when in a state of stable equilibrium. 

 The natural deduction from Mr Fryer's figures is that 

 with regard to the factor that differentiates the mimetic 

 forms from the non-mimetic, the polytes population is, 

 for the moment at any rate, in a position of stable 

 equilibrium. This may mean one of two things. 

 Either the population is definitely in a state of equi- 

 librium which has lasted for a period of time in the past 



1 As these larvae were for the most part found simply over a consider- 

 able time it follows that they are the offspring of different females 

 and represent the relative proportions of the three forms in the general 

 population. 



P. M. 7 



