154 THE MALAYAN PAPILIONIDH AS 
of eggs. He further states that, out of thousands 
of specimens, he has never seen or heard of inter- 
mediate varieties between these forms. In this in- 
teresting example we see the effects of latitude in 
determining the proportions in which the individuals 
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 competent ob- 
server 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 con- 
trasted 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 Pier- 
ide; 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 analo- 
gous though not of an identical nature, while the 
Araschnia prorsa, of Central Europe, is a striking 
example of this alternate or seasonal dimorphism. 
Among our nocturnal Lepidoptera, I am informed, 
