OVERLAPPING FORMS 163 



observing the consequences of the overlap of two closely related 

 types, but there are advantages in considering them separately. 

 In the examples hitherto given, with the possible exception of 

 the Papilios, 15 the two fixed types severally range over so ex- 

 tensive a region that it may fairly be supposed that in the dif- 

 ferent parts they are subject to considerable diversities of climate. 

 There is no outstanding difference that we know distinguishing 

 the habitats of the two forms; but in comparing Alpine with 

 Lowland forms, or essentially northern with essentially southern 

 forms we do know an external circumstance, temperature, that 

 may reasonably be supposed to have an influence, direct or indi- 

 rect, on the population. 



15 I think this case is fairly included because the machaon type is so widespread 

 that it cannot be regarded as a product of a Northern climate, nor can asterias be 

 claimed as especially a warm country form, seeing that brevicauda, which is scarcely 

 distinguishable from asterias, inhabits Newfoundland (having a curious phase 

 there in which the yellow is largely replaced by red) . 



