viii] THE CASE OF PAPILIO POLYTES 99 



during the time P. polytes has been known if a 2% 

 selection advantage had operated during that period 

 in favour of the mimetic. If there has been any 

 appreciable selection going on during that time mi- 

 metics must have been far rarer when the species was 

 first discovered, but the fact that both the mimetic 

 forms made their way into collections before the 

 non-mimetic tells distinctly against this supposition. 

 Nor is there any reason to suppose that the non-mimetic 

 form has been dwindling in numbers relatively to the 

 mimetics during the last half century. Moore 1 in 1880 

 records an earlier observation of Wade's that " These 

 three butterflies are very common, especially those of 

 the first form; the second being perhaps least so." 

 The first form alluded to is the M form, and the 

 second is the A form, so that at the time Wade wrote 

 the relative proportions of these three forms must 

 have been very much what they are to-day. Even 

 during half a century and with such a relatively weak 

 selection rate as 2% in favour of the mimetics, the 

 proportion of non-mimetics should drop from about 

 4 : 5 down to about 1 : 5. Therefore we must either 

 infer that in respect of mimetic resemblances natural 

 selection does not exist for P. polytes in Ceylon, or else 

 we must suppose its force to be so slight that in half a 

 century certainly, and perhaps in a century and a 

 half, it can produce no effect appreciable to the neces- 

 sarily rough method of estimation employed. 



1 The Lepidoptera of Ceylon, 1880. 



7—2 



y 



