706 Some breeding experiments on Catopsilia pyranthe. 
instinct might have arisen from the necessity for constantly 
seeking new feeding-grounds for the larve. As the 
species increased this tendency to expand would not only 
preserve the species, but would cause in time its very 
material increase; the necessity for constantly enlarging 
the feeding-grounds would in time produce an inherited 
tendency to migrate. But in due course, when all available 
feeding-grounds were occupied, as they soon would be 
in a small island like Ceylon, some check would be 
required to keep the enormous number of resulting 
butterflies within due bounds, otherwise the species 
would be in danger of annihilation from their very numbers. 
This appears to me to be effected in the following 
manner: the insects of the wet-season migration are 
mostly composed of females, and provided that the males 
can successfully impregnate more than one female, the 
result would be an enormous number of eggs laid, and 
this 1 have shown to be the case. The migratory in- 
stinct 1s so strong that the females are precluded from 
taking any precautions for their future offspring, as the 
females of most butterflies do; and the result is that the 
struggle for existence among the multitude of larve sub- 
sisting on the food-plant, which is quickly diminishing by 
their voracity, and also slowly by the heat and dry weather, 
is so great that the larve which would produce female 
butterflies succumb, and a great majority of males are pro- 
duced which form the dry-weather flights. This majority 
of males would also be another factor in checking the 
increase of the species. During the intervening portion 
of the year the species would gradually increase, until 
the wet months at the fall of the year favour a luxuriant 
vegetation, and all the female larve then survive, and 
possibly being stronger, crowd out the male larve. These 
larve produce the overwhelming proportion of females 
in the next wet-season flight, with the result shown 
above. This migratory instinct, originally due to a 
necessity for the increase of the species, is now become 
a means of preventing its undue propagation. 
