EVOLUTION 345 



Coleoptera originated, and are called Protocoleoptera." 

 From this highly interesting discovery it is clear that the 

 specialised sheathing forewings (elytra) of beetles were 

 modified from somewhat firm forewings (tegmina) re- 

 sembling those of the Orthoptera and many Hemiptera, and 

 this modification must have been at least begun in Car- 

 boniferous times. Possibly other early fossil forms yet to 

 be unearthed, will make it clear that the specialised neuration 

 of the Protocoleoptera can be derived from that of the 

 most primitive Mecopteroid insects. If so we shall be 

 convinced that all the Endopterygota are related among 

 themselves and form a truly " natural " group, a conclusion 

 that seems justified in view of their distinctive common 

 type of life-history. Tillyard (1926) inclines, however, to 

 the view that the beetles originated from some exopterygote 

 group independently of the other orders which undergo 

 complete transformation. 



The facts summarised in the preceding pages are enough 

 to convince us that notwithstanding many considerable 

 gaps in the geological record of the insect life of the past, 

 there has been from the Carboniferous to the Recent Period 

 general progress from the simpler to the more specialised 

 types of structure and life-history within the class. We 

 realise that the various forms of larvae adapted to particular 

 modes of life have arisen by divergence, often clearly de- 

 generative, from the adult forms as these became more 

 highly elaborated. Nor must it be forgotten that the 

 existence of groups of wingless insects related to winged 

 orders, tells of degeneration as well as progress in the 

 evolutionary history of the race. The greatest gap in our 

 knowledge of this subject is evident when we think about 

 the origin of insects' wings. As the oldest fossil insects 

 known all possessed the power of flight, there is no definite 

 evidence how the primitive Exopterygota acquired their 

 wings. In the stages of development in the individual 

 insect the wings grow out from the tergal region of the 

 thoracic segments as hollow, flattened pouches ; this fact 

 indicates the morphological nature of the wing, but it 



