40 THE THEORY OF EVOLUTION 



The numerical relations differ also from those of 

 to-day. 



That which is lacking in the Jura ' we find already 

 richly represented in the Tertiary deposits. All fami- 

 lies now agree, as do very many genera and even several 

 species, with those now existent, so that the difference 

 consists now almost entirely in the numerical relations 

 and in the geographical distribution/ l 



According to the above the whole of the present 



FIG. 7. BEETLE FIG. 8. PRIMARY WOOD FIG. 9. DRAGON-FLY 



(Jura). WASP (Jura Hymenoptera). (Jura). 



FIGS. 7-9. (After Handlirsch.} 



insect orders with their subdivisions have arisen by 

 differentiation of one original type. This occurred 

 through changes in the originally similarly segmented 

 body for instance, in the Beetles and Hymenoptera, 

 into three clearly divided main sections : head, breast, 

 and rear segments. Other recent offshoots, like the 

 Termites, show a still more uniform segmentation of 

 the whole body. In the Thysanura this is complete. 

 The formation of the antennae, also the number of 

 tarsi and the venation of the wings afford opportunities 



1 Die, Umschau, p. 590. 



