Chap. XX.] GENEEAL SUCCESSION OF FOEMER LIFE. 
481 
Edward Forbes considered that the Silurian rocks constitute the Lower 
Palaeozoic, — the Devonian, Carboniferous, and Permian rocks the Upper 
Palaeozoic ; but whether this ancient series be divided into two or into 
three classes (some palaeontologists preferring to hold the Devonian as a 
separate and intermediate type, — the Lower and Upper Silurian rocks 
constituting truly an older natural system, just as the Carboniferous and 
Permian deposits form a younger one), the result of the researches of the 
numerous authors appealed to in this volume has unquestionably justified 
the application of the more comprehensive term ' system ' to the Silurian 
rocks. 
At the close of the Permian or Supra-Carboniferous era, an infinitely 
greater change took: place in organic life than that which marked the 
ascent from the Silurian system to the overlying Palaeozoic groups. Nearly, 
if not quite, all the species of the earlier races then disappeared, and were 
replaced in the Trias by a new series, the types of which were continued 
through those long epochs which geologists term Secondary or Mesozoic 
(the ' mediaeval age' of extinct beings). In these, again, the reader will 
learn, by consulting the works of numerous writers, how one formation fol- 
lowed another, each characterized by different creatures,— many of them, 
however, exhibiting near their downward and upward limits certain fos- 
sils which link one reign of life on to another. 
Whilst it is beside my present aim to enter upon descriptions of such 
Secondary deposits (still less of those called Tertiary, which intervened be- 
tween the Secondary rocks and the sediments of the present day), we may 
still cast a glance over the general order admitted by all geologists, to see 
how it harmonizes with what has been related of the succession of animals 
belonging to the older rocks, and as also indicating a progression from 
lower to higher grades of life. 
Let me, then, again remind the reader that he has been presented in this 
volume with clear proofs of an immensely long period having elapsed 
during which, with countless remains of nearly all other inhabitants of the 
sea, no trace of a Fish has been discovered, to represent the prototype of 
that vertebrate succession which terminated in Man. Next he will have 
noted that, after Fishes first appeared, long ages elapsed before a Beptile 
was added to the creatures of the epoch ; and then we have seen that, 
notwithstanding the demonstration of the frequent submergences of enor- 
mous breadths of land, clothed with a rich and copious vegetation on which 
Insects fed, no sign whatever has been met with, in the spoils of these well- 
explored Carboniferous and Permian domains, that they were inhabited by 
a single Mammal ! 
Proceeding upwards into the Secondary strata, the earliest traces of 
Mammalia are first detected in the uppermost member of the Trias (the 
Rhoetic beds or upper part of the Keuper Sandstone of Wurtemberg, which 
2 i 
