xvrr 



by climatical influences. Even if such an evolution is brought about by external 

 influences or exciting causes — as in the case in question by the quantity or the 

 nutritiveness of the food — this can only happen when the susceptibiHty also 

 exists. Yet, this is, not always the case among the caterpillars that are 

 irritated. If not, such an individual insect retains notwithstanding that irritation, 

 the older form which has not proceeded further. Besides, some species, races, 

 sexes or even individual insects happen to be in the above-mentioned period of 

 cessation of development (epistasy) when no irritation makes them change. 



Already in 1895 ^'d Dr. L. Martin declare in his above-cited descrip- 

 tion of the Rhopalocera of Sumatra, in consequence of the thirteen years he 

 lived in that island, that at least in the north-eastern part which was known to 

 him, there did not exist a strong!}- separated dry and wet season, there being 

 no month in the year when it does not rain ; indeed it is rare for a week to 

 pass without a shower, consequently there are no dry-season forms of butterflies 

 to be found in Sumatra. Except — he remarkably adds — the dry season form 

 of Melanitis Ismene Cram, which, as also in Java, is found all the year 

 round equally common with the wet season ocellated form M. Determinata 

 Butler. The said butterfly happens to be the same as the form Cyllo Leda L. 

 just mentioned, whose pretended season forms do indeed not show that 

 character, as has been said already and therefore do not make an exception, 

 but on the contrary are quite compatible with Martin's observations. Dr. 

 Martin's just insight, founded on his 13 years' experience, could, however, 

 not prevent his collaborator De Niceville, though the latter had not that 

 experience about Sumatra, from expatiating in the same chapter in a very 

 superficial manner on his hobby of the drj' and the wet season forms. 



Observations which answer to the results of my researches in this respect 

 and thus also confirm Dr. Martin's views, have been made known more than 

 once. In the Proceedings of the Ent. Soc. of Londou, Febr. '^ 1879, Swinhoe 

 already informed us that both forms of the said butterfly occur all the year 

 round at Bombay, therefore in continental India. A good deal about it is still 

 to be found in the Transactions of the same Society of the years 1903, 1904 and 

 1905. At Aden, in a district where occasional showers are ver}^ rare and where, 

 therefore, a very drj' season is the rule, Yerbury found the so-called dry 

 and wet season forms as well as intermediate forms at the same time. 

 Marshall observed the same in Mashona land. Dr. Dixey mentions 

 butterflies collected in the dry season near the White Nile, several of which 

 had characteristics peculiar to those of the wet season. Longstaff had 

 the same experience in British India and Major Neville Manders mentions 

 of Ceylon that there the so-called dry and wet season forms of Callidryas 



III 



