154 THE MALAYAN PAPILIONIDJE 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, tJiere 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- 

 idas ; 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, 



