( XXV ) 



rate the latter are so extremely scarce that they can have 

 little protective value, and the Pseudacraea would gain little 

 by resembling models that are less common than itself. Con- 

 sequently any form of Pseudacraea that is produced will have 

 as much chance of surviving as the most perfect mimic, and 

 the transitional forms appear almost as abundantly as the 

 types. On the mainland, however, conditions are very 

 different. Owing to the abundance of Planemas, their presence 

 is of definite protective value to the Pseudacraeas, and varieties 

 that are produced which do not conform rigidly to the types 

 of the models are put at a disadvantage in the struggle for 

 existence, and are destroyed by enemies in preference to the 

 types. On the mainland the mimics are kept rigidly up to the 

 mark [italics mine], and the transitional varieties between 

 hohleiji, tirikensis and terra are by comparison rarely to be 

 found. It may perhaps be argued that there is some con- 

 dition productive of greater variability on the island, but not 

 on the mainland. But though intermediate varieties are 

 scarce on the mainland, yet they do occur, and it is difficult 

 not to believe that they are rarely caught by collectors, because 

 they are so much more destroyed by enemies than are those 

 which more closely resemble models. If, as I believe, this 

 explanation be the correct one, it supplies the strongest 

 possible proof of the reality of mimicry and the power of 

 natural selection to preserve it — indeed, it is a crucial test." 



Inasmuch as the localities where these insects occur are 

 unfamiliar to the majority of English entomologists, let me 

 transfer the case to the British Islands. 



Let the county of Sussex distant from the mainland twenty- 

 five miles represent Bugalla Island ; the Isle of Wight would be 

 connected with France by a chain of small islands stretching 

 across the Channel, but in place of a sea subject for half the 

 year to the storms of winter we have a warm equable climate 

 such as exists under the equator. 



In Sussex we have a butterfly, let us say Melitaea aurinia, 

 occurring in great abundance, and in its many forms familiar 

 to all of us, some of them more or less resembling another 

 butterfly Hamearis lucina, which for some cause not quite 

 clear is very scarce. On the other side of the Channel at Calais 



