DISTRIBUTION , 347 



General Conclusions. Unfortunately, the earliest fossils with 

 rhich we are acquainted shed much less light upon the subject of insect 

 >hylogeny than one might expect. The few Devonian forms, though 

 rnthetic indeed as compared with their modern allies, are at the same 

 Lime highly organized, or far from primitive, and their ancestors have 

 >een obliterated. 



The general plan of wing structure, as Scudder finds, has remained 

 unaltered from the earliest times, though the Devonian specimens 

 exhibit many peculiarities of venation, in which respect some of them are 

 more specialized than their nearest living allies, while none of them 

 have much special relation to Carboniferous forms. 



Carboniferous insects are more nearly related to recent forms than are 

 le Devonian species, but present a number of significant generalized 

 Features. Generally speaking, the thoracic segments were similar and 

 iconsolidated, and the two pairs of diaphanous wings were alike in 

 ivery respect in groups that have since developed tegmina and dis- 

 similar thoracic segments. The Carboniferous precursors of our cock- 

 >aches, phasmids and May flies have been mentioned. Palaeozoic 

 isects were grouped by Scudder into a single order, Palaeodictyoptera, on 

 tccount of their synthetic organization, though other authors have tried 

 to distribute them among the modern orders. This disagreement will 

 mtinue until, with increasing knowledge, our classification becomes 

 less arbitrary and more natural. 



Mesozoic insects are interesting chiefly as evolutionary links, 

 notably so in the case of cockroaches the only insects whose ancestry 

 is continuously traceable. In this era the large families became differen- 

 tiated out. 



Most of the Tertiary species are referable to recent genera, peculiar 

 families being highly exceptional, while all the Quaternary species 

 belong to recent genera. 



Hemiptera appear in the Silurian; Neuroptera (in the old sense) in 

 the Devonian; Thysanura and Orthoptera, Carboniferous; Coleoptera 

 and Hymenoptera, Triassic; Diptera, Jurassic; and Lepidoptera not 

 until the Tertiary. 



Since Scudder's day, considerable additions to the knowledge of our 

 fossil insects have been made by Professor T. D. A. Cockerell and 

 by Professor H. F. Wickham. 



A comprehensive and richly illustrated account of fossil insects is 

 given by Handlirsch in his great work, Die fossilen Insekten. 



