ix] PAST AND PRESENT 115 



of detail, the general explanation of insect meta- 

 morphosis as the result of divergent evolution in 

 the two active stages of the life-story must assuredly 

 be accepted. No other explanation accords with the 

 increasing degree of divergence to be observed as 

 we pass from the lower to the higher insect orders. 



The successive incidents of the life-story of most 

 insects are largely connected with the acquisition of 

 wings. Wings, and the power of flight wherewith 

 they endow their possessors, are evidently beneficial 

 to the race in giving power of extending the range 

 during the breeding period and thus ensuring a wide 

 distribution of the eggs. In no case are wings fully 

 developed until the closing stage of the insect's life, 

 they are always acquired after hatching or birth. 

 We have already noticed (p. 40) how Sharp (1899) 

 has laid stress on the essential difference between the 

 exopterygote and endopterygote insects, the wing- 

 rudiments of the former growing outwards through- 

 out life while those of the latter remain hidden until 

 the pupal instar. Sharp considers that there is some 

 difficulty in bridging, in thought, the gap between 

 these two methods of wing-growth, and has put 

 forward an ingenious suggestion to meet it (1902). 

 Reference has already been made to insects of various 

 orders in which one sex is wingless, the Vapourer 

 Moth (p. 96) for example, or all the individuals of 

 both sexes are wingless, as the aberrant cockroaches 



82 



