Phenomena of Variation and Geographical Distribution. 169 



has no male specimen of P. Poli/'es yet been found, but the female 

 {Polytes) has never yet been found in localities to which the male 

 (Pammon) does not extend. In this case, as in the last, distinct species, 

 local forms, and dimorphic specimens have been confounded under the 

 common appelation of varieties. 



But, beside the true B. Polytes, there are several allied forms of 

 females to be considered, namely, P. Theseus, Cr., P. Melanides, De 

 Haiin, P. Elyros, G. R. G., and P. Romulus, L. The dark female 

 jfigured by Cramer as P. Theseus seems to be the common and perhaps 

 the only form in Sumatra, whereas in Java, Borneo and Timor, along 

 with males quite identical with those of Sumatra, occur females of the 

 Polytes form, although a single specimen of the true P. Theseus, C. R., 

 taken at Lombock, would seem to show that the two forms do occur 

 together. In the allied species found in the Philippine Islands (P. 

 Alphenor, Cr., P. Ledebouria, Eschsch., P. Elyros, G. R. G.), forms 

 corresponding to these extremes occur along with a number of inter- 

 mediate varieties, as shown by a fine series in the British Museum. 

 We have here an indication of how dimorphism may be produced ; for 

 let the extreme Philippine forms be better suited to their conditions 

 of existence than the intermediate connecting links, and the latter will 

 gradually die out, leaving two distinct forms of the same insect, each 

 adapted to some special conditions. As these conditions are sure to 

 vary in different districts, it will often happen, as in Sumatra and Java, 

 that the one form will predominate in the one island, the other in the 

 adjacent one. In the island of Borneo there seems to be a third form ; 

 for P. Melanides, De Haan, evidently belongs to this group, and has 

 all the chief characteristics of T. Theseus, with a modified coloration 

 of the hind wings. I now come to an insect which, if I am correct, 

 ofiers one of the most interesting cases of variation yet adduced. 

 Papilio Romulus, L., a butterfly found over a large part of India and 

 Ceylon, and not uncommon in collections, has always been considered 

 a true and independent species, and no suspicions have been expressed 

 regarding it. But a male of this foi'm does not, I believe, exist. I 

 have examined the fine series in the British Museum, in the East India 

 Company's Museum, in the Hope Museum at Oxford, in Mr. Hewit- 

 son's and several other private collections, and can find nothing but 

 females; and for this common butterfly no male partner can be found 

 except the equally common P. Pammon, a species already provided 

 with two wives, and yet to whom we shall be forced, I believe, to as- 

 sign a third. On carefully examining P. Romulus, I find that in all 

 essential characters — the form and texture of the wings, the length of 

 the antenna}, the spotting of the head and thorax, and even the 



