27 



vironments, yet we must agree with Pratt that the principal sig- 

 nificance of the pupal period, and the attendant phenomenon of 

 metamorphosis is that " it is the time when the larval characters 

 which were adapted for use during a period of free life, and which 

 would be valueless to the imago, are corrected or abandoned." 



In conclusion, one may be asked what was the probable initial 

 cause of metamorphosis. We see that it does not exist in apterous 

 insects, and that it is confined to winged insects. There can be no 

 doubt that the winged condition has enabled such insects to become 

 the most successful type of life among the invertebrates, as up to a 

 point birds are the most successful type of terrestrial Vertebrata. 

 The existence of a period during which insects can retire from their 

 ordinary environment, already provided with an abundant supply of 

 stored food to last for a considerable period of time, and there undergo 

 the changes which will enable them to take at once to a new environ- 

 ment, has given them an enormous advantage over their competitors 

 in the struggle for existence. 



There can be little doubt that insects were first driven to the air to 

 enable them to compete successfully with other animals occupying 

 the same ground, and in order to enable them to avoid the attacks 

 of their numerous enemies. The mere development of wings, how- 

 ever, was only successful against certain classes of enemies ; but the 

 insects were competitors against each other, and hence those insects 

 which could hide themselves during the critical period of their 

 existence, i. e. whilst developing wings, had a very strong point in 

 their favour, a point so favourable that it has ended, as we have said, 

 in their becoming numerically the most successful type of life in 

 existence. 



