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 Papilio, at all equal in interest to 
those I have now brought forward, occurs in America; and we have, fortunately, accu- 
rate information about it. Papilio Turnus, L., is common over almost the whole of 
temperate North America; and the female resembles the male very closely. A totally 
different-looking insect both in form and colour, Papilio Glaueus, L., inhabits the same 
region; and though, down to the time when Boisduval published his * Species Général,’ 
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. Turnus. 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 tells 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 further 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- 
viduals 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 efficient in keeping down 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 Pieride ; and but few occur 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 prorsa, 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 Anthribidæ, 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 investigated by breeding suecessive 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. 
TIL. XXY. C ; 
