X] MIMICRY AND VARIATION 127 



interesting points are brought out by a comparison of 

 the occurrence and variation of the species on the 

 mainland with what is found on Bugalla Island in the 

 Sesse Archipelago. On the mainland the Pseud- 

 acraeas are abundant but the Planemas even more so, 

 outnumbering the former by about 5 : 2\ Moreover, 

 it is rare to find individuals more or less intermediate 

 between the three forms, though they are known to 

 occur. On Bugalla Island, however, a different state 

 of things is found. The Pseudacraeas are very abund- 

 ant, whereas the Planemas, owing doubtless to the 

 scarcity of their food plant, are relatively rare, and are 

 very greatly outnumbered by the Pseudacraeas. At 

 the same time the proportion of transitional forms 

 among the Pseudacraeas is definitely higher than on 

 the mainland. These facts are interpreted by Car- 

 penter as follows: — 



On the mainland where the models are abundant 

 there is a vigorous action on the part of natural selection. 

 The mimetic forms have a strong advantage and the 

 non-mimetics have been gradually weeded out. But 

 on the island, where the Pseudacraeas outnumber the 

 models, the advantage obtained through mimicry is not 

 so great. The so-called transitional forms are little, 

 if at aU, worse off than those closely resembling the 

 scarce models, and consequently have as good a chance 

 of surviving as any of the typical mimetic forms. On 



1 Cf. Poulton, E. B., /"■ Congr. Internat. d'Entomol., Bruxelles 1911. 

 This proportion is founded on several hundreds caught at random. 

 Observers are agreed that Paevdacraea is both a warier insect and a 

 stronger flyer than the various Planemas which it resembles. 



