DISTRIBUTION 



389 



species, the continuity between Palaeozoic and recent cock- 

 roaches is clearly established which can be said of no other 

 insects; and in fact of no other animals, the only comparable 

 cases being those of the horse and the molluscan genus Planor- 

 bis. In the Triassic period occur the first fossils that can be 



FIG. 299. 



reon bockingi. Three quarters natural size. After DOHRN. 



referred indisputably to Coleoptera and Hymenoptera, the lat- 

 ter order being represented first, as it happens, by some of 

 its most specialized members, namely ants. 



Jurassic. At length, in the Jurassic, all the large orders 

 except Lepidoptera occur; Diptera appear for the first time, 

 and Odonata are represented by many well-preserved speci- 

 mens, while the Liassic Coleoptera studied by Heer number 

 over one hundred species. The Cretaceous has yielded but 

 few insects, as might be expected. 



Tertiary. In the rich Tertiary deposits all orders of insects 

 occur. Baltic amber has yielded Collembola, some remarkable 

 Psocidae, many Diptera, and ants in abundance. Of 844 spe- 



