ix] PAST AND PRESENT 117 



insects which grew their wings from visible external 

 rudiments, and that in later times re-acquiring 

 wings, they developed these organs in a new way, 

 from inwardly directed rudiments or imaginal buds. 



This theory of Sharp's is original, daring, and 

 ingenious, but the loss and re-acquisition of wings 

 which it presupposes is difficult to imagine in large 

 groups during a prolonged evolutionary history, 

 while the sudden appearance of a totally new mode 

 of wing-growth in the offspring of wingless insects 

 would be an extreme example of discontinuity in 

 development. 



On the whole the most probable suggestion which 

 can be made as to the origin of ' complete' trans- 

 formation in insects is that the instar in which wings 

 were first visible externally became later and later 

 in the course of the evolution of the more highly 

 organised groups. In this way a gradual transition 

 from the exopterygote to the endopterygote type 

 of life-story is at least conceivable. It will be 

 remembered that a may -fly (p. 33) undergoes a moult 

 after acquiring functional wings, emerging into the 

 air as a 'sub-imago.' In not a few endopterygote 

 insects, the pupa shows more or less activity, swim- 

 ming through water intermittently (gnats) or just 

 before the imago has to emerge (caddis-flies); work- 

 ing its way out of the ground (crane-flies) or coming 

 half-way out of its cocoon (many moths). The pupa 



