OF THE MALAYAN REGION. 9 



the intermediate steps in that process which has been accidentally preserved in company 

 with its more favoured rivals, though its extreme rarity (only one specimen having been 

 seen to many hundreds of the other form) would indicate that it may soon become extinct. 



The only other case of polymorphism in the genus Fapilio, at all equal in interest to 

 those I have now brought forward, occurs in America ; and we have, fortunately, accu- 

 rate information about it. PctpiUo Turmts, L., is common over almost the Avhole of 

 temperate Xorth America ; and the female resembles the male very closely. A totally 

 different-looking insect both in form and colour, Papilio Glaucus, L., inhabits the same 

 region ; and though, down to the time when Boisduval published his ' Species General,' 

 no connexion was supposed to exist between the two species, it is now well ascertained that 

 P. Glaucus is a second female form of P. Turmis. In the ' Proceedings of the Entomological 

 Society of Philadelphia,' Jan. 1863, Mr. "Walsh gives a very interesting account of the 

 distribution of this species. He teUs us that in the New England States and in New York 

 all the females are yellow, while in Illinois and further south all are black ; in the inter- 

 mediate region both black and yellow females occur in varying proportions. Lat. 37° is 

 approximately the southern limit of the yellow form, and 42° the northern limit of the black 

 form ; and, to render the proof complete, both black and yellow insects have been bred 

 from a single batch of eggs. He fiu'ther states that, out of thousands of specimens, he 

 has never seen or heard of intermediate varieties between these forms. In this interesting 

 example we see the effects of latitude in determining the proportions in which the indi- 

 \dduals of each form should exist. The conditions are here favourable to the one form, there 

 to the other ; but we are by no means to suppose that these conditions consist in climate 

 alone. It is highly probable that the existence of enemies, and of competing forms of life, 

 may be the main determining influences ; and it is much to be wished that such a com- 

 petent observer as Mr. Walsh would endeavour to ascertain what are the adverse causes 

 which are most eflB.cient in keeping do^vn the numbers of each of these contrasted forms. 



Dimorphism of this kind in the animal kingdom does not seem to have any direct 

 relations to the reproductive powers, as Mr. Darwin has shown to be the case in plants, nor 

 does it appear to be very general. One other case only is known to me in another family 

 of my eastern Lepidoptera, the Pieridce ; and but few occiu* in the Lepidoptera of other 

 countries. The spring and autumn broods of some European species differ very remarkably; 

 and this must be considered as a phenomenon of an analogous though not of an identical 

 nature*. Araschnia jyrorsa, of Central Europe, is a striking example of this alternate or 

 seasonal dimorphism. Mr. Pascoe has pointed out two forms of the male sex in some 

 species of Coleoptera belonging to the family Anthribidse, in seven species of the two 

 genera Xenocerus and Mecocerus (Proc. Ent. Soc. Lond., 1862, p. 71) ; and no less than 

 six European Water-beetles, of the genus Dytiscus, have females of two forms, the most 

 common having the elytra deeply sulcate, the rarer smooth as in the males. The three, 

 and sometimes four or more, forms under which many Hymenopterous insects (especially 

 Ants) occur must be considered as a related phenomenon, though here each form is spe- 

 cialized to a distinct function in the economy of the species. Among the higher animals, 



* Among our nocturnal Lepidoptera, I am informed, many analogous cases occur ; and as the whole history of 

 many of these has been uivestigated by breeding successive generations from the egg, it is to be hoped that some of 

 our British Lepidopterists will give us a connected account of all the abnormal phenomena which they present. 



VOL. XXV. C 



