100 THE CASE OF PAPILIO POLYTES [ch. 



It may, however, be argued that even an exceedingly 

 low selection rate is able to bring about the elimination of 

 one or other type provided that it acts for a sufficiently 

 long time. This is perfectly true. A selective rate 

 of '001% would reduce the proportion of recessives 

 to dominants from 4 : 5 down to 1 : 40 in the course 

 of about 1,400,000 generations where the mimetic 

 resemblance is already established. Such a form of 

 selection entails the death of but one additional non- 

 mimetic in 100,000 in each generation. If, however, 

 the mimetic resemblance is not fully established and 

 the mimic bears only what supporters of the mimicry 

 theory term a "rough" resemblance to the model, it is 

 clear that it will have far less chance of being mistaken 

 for the model. Its advantage as compared with the 

 non-mimetic form will be very much less. Even 

 supposing that the slight variations concerned are in- 

 herited, an intensity of selection which would produce 

 a certain change in 1,400,000 generations where a 

 mimetic resemblance is already established must be 

 supposed to take an enormously greater time where 

 an approach to a model has to take place from a 

 "rough" resemblance. 



From the data as to the relative proportions of the 

 polymorphic females of P. polytes during the past and 

 at present, and from the behaviour of their different 

 forms in breeding, the following conclusions only can 

 be drawn. Either natural selection, from the point of 

 view of mimicry, is non-existent for this species in 

 Ceylon, or else it is so slight as to be unable in half a 



