314 Mr. Poulton's notes in 1886 



although, as a rule, the instinct is very accurate in this 

 respect. Thus it has occurred to me that the remarkable 

 association of sallow and apple, as the normal food-plants 

 of S. ocellatus, may have been due to the considerable 

 superficial resemblance between the wild crab (Pyrus 

 mains var. acerba) and some of the broad-leaved sallows 

 (such as Salix caprea or S. cinerea). There can be very 

 little doubt after the above-described observations that 

 holly is more recent as a food-plant of S. ligustri, and 

 laurel in the case of S. populi, and such cases help us to 

 understand how changes have occurred. The ready 

 growth of a complete specialisation between such larvae 

 and their more ancestral food-plants, and the less com- 

 plete specialisation to the more recent food-plants, is 

 probably the direct result of the far greater age and 

 frequency of the former relationship, so that in this case 

 heredity works with instead of against the specialisation 

 which grows up in the life of each individual. And 

 probably, for the same reason, the change from one 

 ancestral food-plant to another, or from a recent to an 

 ancestral food-plant, when larvae are half-grown, is 

 rendered possible in those cases in which it has been 

 proved to occur. But whether these suggestions be 

 well founded or not, the main facts of this section must 

 be held as established by observation, — that the newly- 

 hatched larva is free to form special relations with 

 occasional or rare food-plants which cannot be formed 

 by the more mature larva in which such relations have 

 already grown up towards a commoner food-plant. And 

 this observation obviously goes a long way towards the 

 explanation of those changes of food-plants which we 

 know must have often occurred. 



12. The origin of carnivorous habits in phyto- 

 phagous larv^. — Several observations make it probable 

 that cannibalism or carnivorous habits tend to arise in 

 larvae, and have probably arisen in the past, out of the 

 necessities which follow the scarcity of the normal food. 

 During the summer of 1886 I was keeping large numbers 

 of larvae under conditions which rendered it probable 

 that the food-supply would sometimes fall short. In 

 order to investigate the colour-relation between larvae 

 and their surroundings, coloured glass cages and bags 

 of coloured glazed lining were made use of ; within these 



