ILLUSTRATIVE OF NATURAL SELECTION. 149 
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 P. Theseus, with a 
modified colouration of the hind wings. I now come 
to an insect which, if I am correct, offers one of the 
most interesting cases of variation yet adduced. Pa- 
pilio Romulus, 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 regard- 
ing it. But a male of this form 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. Hewitson’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 
assign a third. On carefully examining P. Romulus, 
f 
