706 Some breeding experiments on Catopsilia pyranthe. 



instinct might Lave arisen from the necessity for constantly 

 seeking new feeding-grounds for the larvae. As the 

 species increased this tendency to expand would not only 

 preserve the spocies, 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 T have shown to be the case. The migratory in- 

 stinct is 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 larvae 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 larvae 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 larvae then survive, and 

 possibly being stronger, crowd out the male larvae. These 

 larvae 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. 



