viii] THE CASE OF PAPILIO POLYTES 101 



century to produce an appreciable diminution in the 

 proportion of non-mimetic females. For even if the 

 mimetic resemblance brings about but the survival of 

 one additional protected form in 100 as compared with 

 the unprotected, this means a marked diminution in 

 the proportion of M females in 50 years — a diminution 

 such as there are no grounds for supposing to have 

 taken place. 



It has been argued that in populations exhibiting 

 Mendelian heredity even a relatively low selection rate 

 must bring about a rapid change in the constitution of 

 a mixed population. Have we any grounds for sup- 

 posing that populations of this sort can undergo such 

 rapid changes? In cases where mimetic resemblances 

 are involved we have no examples of the sort. But 

 some interesting evidence as to the rate at which a 

 population may change is to be gathered from the 

 study of melanism in certain moths. It is well 

 known that in some parts of England the common 

 peppered moth, Amphidasys betularia has been almost 

 entirely supplanted by the darker melanic form double- 

 dayaria. It first made its appearance near Manchester 

 in 1850, and from that centre has been gradually 

 spreading over northern England, the Midlands, and 

 the south-eastern counties. At Huddersfield, for 

 instance, fifty years ago only the type form betularia 

 existed; to-day there is nothing but doubledayaria. 

 In Lancashire and Cheshire the type is now rare. 

 On the continent, too, there is the same story to 

 be told. The melanic form first appeared in Rhenish 



