168 DISTRIBUTION OF EXTINCT ANIMALS. [part ii. 



and recognisable. It is only when we go further back still, into 

 the Palaeozoic formations, that the insect forms begin to show that 

 generalization of type which renders it impossible to classify 

 them in any existing groups. Yet even in the coal formation of 

 Nova Scotia and Durham, the fossil insects are said by competent 

 entomologists to be " allied to Ephemera" " near Blatta" " near 

 Pliasmidce ; " and in deposits of the same age at Saarbruck near 

 Treves, a well-preserved wing of a grasshopper or locust has been 

 found, as well as a beetle referred to the Scarabeidae. More 

 remarkable, however, is the recent discovery in the carboniferous 

 shales of Belgium, of the clearly-defined wing of a large moth 

 {Breyeria borinensis), closely resembling some of the Saturniidae ; 

 so that we have now all the chief orders of Insects — including 

 those supposed to be the most highly developed and the most 

 recent — well represented at this very remote epoch. Even the 

 oldest insects, from the Devonian rocks of North America, can 

 mostly be classed as Neuroptera or Myriapocla, but appear to 

 form new families. 



We may consider it, therefore, as proved, that many of the 

 larger and more important genera of insects date back to the 

 beginning of the Tertiary period, or perhaps beyond it ; but the 

 family types are far older, and must have been differentiated very 

 early in the Secondary period, while some of them perhaps go 

 back to Palaeozoic times. The great comparative antiquity of 

 the genera is however the important fact for us, and we shall 

 have occasion often to refer to it, in endeavouring to ascertain 

 the true bearing of the facts of insect distribution, as elucidating 

 or invalidating the conclusions arrived at from a study of the 

 distribution of the higher animals. 



Antiquity of the Genera of Land and Fresh-Water 



Shells. 



The remains of land and fresh-water shells are not much more 

 frequent than those of insects. Like them, too, their forms are 

 very stable, continuing unchanged through several geological 



