( 27 ) 



furthermore we have no cause to assign for the change except 

 the vague and unsatisfactory one of isolation. The relation- 

 ship of the two north-western species is also opposed to this 

 interpretation. We may infer fi'om the differences between 

 them that comorarum has been isolated for a much longer period 

 than mayoUensis, and yet the tint of their yellowish markings 

 is almost precisely the same. It therefore appears to me more 

 likely that saclava formerly possessed in Madagascar, and 

 probably also in Africa, markings of a tawny colour, and that 

 these have been gradually changed to white on both areas 

 as the result of a synaposematic approach to other black 

 .and white species of Neptis. The four outlying species did 

 snot encounter any such causes of change. According to this 

 hypothesis the eastern species represent an early emigration 

 -when the markings of saclava were of a deep tawny tint, 

 while the north-western species repr-esent a much later phase 

 when they were far advanced towards white. Mimetic 

 approach commonly advances more rapidly in the female sex ; 

 and the faint cream tint of the male saclava may be a lingering 

 trace of an earlier tawny shade. The difference between the 

 pale markings of male and female is, however, so minute that 

 a long series of fresh specimens should be compared before it 

 can be accepted as firmly established. If the suggestion made 

 xli] 



on p. vi that Lihythea cinyras, Trim., is a mimic of Xeptis 

 frobenia be sound, it would support the conclusion that the 

 tawny markings ai^e of great age. 



b. Chaxges Subsequent to Isolation. — Although it is 

 evident that the two north-western species are specially 

 related together, and the two eastern species also specially 

 related, so that they represent two distinct emigrations from 

 Madagascar, it is nevertheless true that more complete and 

 ■ probably far longer isolation in the ovitermost island has 

 been followed by certain changes which have much in common 

 on the two sides. In .some characters the north-westernmost 

 iV. comorarum differs from N>. mayoUensis in much the same 

 way that the easternmost frobPMxa differs from dumetorum. 

 Both east and west of Madaga.scar the more remote and 

 isolated species has undergone simplification in the details of 



