168 DISTRIBUTION OF EXTINCT ANIMALS. [PART II. 
and recognisable. It is only when we go further back still, into 
the Paleozoic 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 Zphemera,” “near Blatta,” “near 
Phasmide ;” and in deposits of the same age at Saarbriick near 
Tréves, a well-preserved wing of a grasshopper or locust has been 
found, as well as a beetle referred to the Scarabeide. 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 Saturniide ; 
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 Myriapoda, 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 Paleozoic 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 
