iv.] METAMORPHOSES OF INSECTS. 75 



breeding. If this were so, the development of wings 

 would gradually have been relegated to a late period 

 of life ; and by the tendency to the inheritance of 

 characters at corresponding ages, which Mr. Darwin 

 has pointed out, 1 the development of wings would 

 have thus become associated with the maturity of 

 the insect. Thus the late acquisition of wings in the 

 Insecta generally seems to be itself an indication of 

 their descent from a stock which was at one period, 

 if not originally, aquatic, and which probably re- 

 sembled the present larvae of Chloeon in form, but 

 had thoracic as well as abdominal branchiae. 



Finally, from the subject of metamorphosis we 

 pass naturally to that most remarkable phenomenon 

 which is known as the u Alternation of Generations : " 

 for the first systematic view of which we are indebted 

 to my eminent friend Prof. Steenstrup. 2 



I have always felt it very difficult to understand 

 why any species should have been created in this 

 double character ; nor, so far as I am aware, has any 

 explanation of the fact yet been attempted. Never- 

 theless insects offer, in their metamorphoses, a phe- 

 nomenon not altogether dissimilar, and give a clue to 

 the manner in which alternation of generations may 

 have originated. 



The caterpillar owes its difference from the butter- 

 fly to the undeveloped state in which it leaves the 

 egg; but its actual form is mainly due to the influence 

 of the conditions under which it lives. If the cater- 



1 Origin of Species, 4th ed., pp. 14 and 97. 



2 On the Alternation of Generations. By J. J. Steenstrup. Trans, 

 by G. Busk, Esq. Ray Society. 1842. 



