INTRODUCTION 21 



I am not bold enough to say that the case is theoreti- 

 cally impossible, but if it is ever realised by the dis- 

 covery of actual examples, we may expect to find that 

 the whole advantage of the transformation has been 

 sacrificed. For transformation is merely a step to 

 the acquisition of wings, and wings would be useless 

 except in air. The power of flight implies great 

 muscular activity, keen senses, food which can be 

 quickly absorbed and highly nutritious in proportion 

 to its bulk, mouth-parts adapted to such food. Hence 

 the striking difference which so often arises between 

 the slow, heavy and voracious larva and the nimble, 

 honey-sucking imago. There is a division of labour 

 between the two stages, and each becomes specialised 

 for its own purposes until the extremes can only be 

 reconciled by a prolonged resting-stage. The pupa 

 and all that relates to it are subordinate to the 

 acquisition of wings. 



The acquisition of wings is not however an ultimate 

 purpose, but only a means to an end. That end is, 

 primarily, the dispersal of the eggs in fresh sites. 

 Where the food consists of the foliage of a particular 

 thinly distributed plant, all the eggs must not be laid 

 together, lest the larvae, being over-crowded, should 

 consume their whole supply of food and perish of 

 starvation, leaving other plants of the same species 

 untouched. If the female were endowed with the 

 power of flight, able to absorb some highly nutritious 

 food which would neither require much time to col- 

 lect nor over-load her body, and gifted with adequate 

 powers of perception, she might find out a number of 

 scattered plants, and lay her eggs upon them a few at 



