The geographical distribution of animals, with a study of the relations ...

168 DISTRIBUTION OF EXTINCT ANIMALS. [part II.and recognisable.It is only when we go further back still, intothe Palseozoic formations, that the insect forms begin to show thatgeneralization of type which renders it impossible to classifythem in any existing groups. Yet even in the coal formation ofNova Scotia and Durham, the fossil insects are said by competententomologists to be " allied to Ephemera" " near Blatta,"Phasmidce ; "" nearand in deposits of the same age at Saarbruck nearTreves, a well-preserved wing of a grasshopper or locust has beenfound, as well as a beetle referred to the Scarabeidse. Moreremarkable, however, is the recent discovery in the carboniferousshales 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—includingthose supposed to be the most highly developed and the mostrecent—well represented at this very remote epoch. Even theoldest insects, from the Devonian rocks of North America, canmostly be classed as Neuroptera or Myriapoda, but appear toform new families.We may consider it, therefore, as proved, that many of thelarger and more important genera of insects date back to thebeginning of the Tertiary period, or perhaps beyond it ;but thefamily types are far older, and must have been differentiated veryearly in the Secondary period, while some of them perhaps goback to Palseozoic times. The great comparative antiquity ofthe genera is however the important fact for us, and we shallhave occasion often to refer to it, in endeavouring to ascertainthe true bearing of the facts of insect distribution, as elucidatingor invalidating the conclusions arrived at from a study of thedistribution of the higher animals.Antiquity of the Genera of Land and Fresh-WaterShells.The remains of land and fresh-water shells are not much morefrequent than those of insects. Like them, too, their forms arevery stable, continuing unchanged through several geological