570 Revieivs — Phillips's Manual of Geology — 



tion is no doubt tlie fact tbat life now existing is substantially the 

 same as life has bee^ in all the past ages of time. The combination 

 of the different grou ps of organisms is of like character, though the 

 genera and species have varied. There is no trace of a beginning. 

 There is evolution, but it is only the evolution of genera and of 

 ordinal groups, and not of classes. 



" It is chiefly by means of extinct families and orders that strata 

 are characterized, and the periods of past time separated from each 

 other ; but when we bear in mind what the circumstiinces are which 

 are causing extinction at the present day, we may doubt whether a 

 classification so made is the best possible. Its method is unphilo- 

 sophical. At least of equal importance with the occurrence of 

 extinct types is the first apjoearance as elements in a fauna of genera 

 and j orders which still survive; for both are connected with the 

 changed distribution of land and water which time has developed. 

 The first appearance of organisms as a characteristic featui'e in a 

 fauna would divide the strata differently from the extinct types, and 

 would show how local are all the phenomena of the succession of 

 life. Many groups of organisms which still survive appear plenti- 

 fully in the Ci'etaceous rocks, so that a palfBontological division 

 might be drawn on the evidence of plants and fishes and many 

 intermediate groups of organisms, which link the Lower Greensand 

 with strata below, and the Gault with strata above. The Trias is 

 sharply cut off from the Lias above and from the Permian rocks 

 below. The Primary period is certainly divided into two, by a gap 

 in succession of species between the uppermost beds of the Silurian 

 and the lower part of the Devonian, which is not less marked than 

 the other great changes in life, such as divide the Secondary and 

 Tertiary rocks, or the Trias and Lias. 



" The names Palaeozoic, Mesozoic, and Cainozoic, therefore, do not 

 represent completely palseontological facts, and the divisions which 

 they indicate are artificial when studied in the light of the groups 

 of animals composing the several faunas. The Sponges give no 

 indication of the larger divisions of time ; the Foraminifera intro- 

 duce their new types gradually, so that we look to the Carboniferous 

 rocks, the Trias, and the Chalk as furnishing the majority of existing 

 genei'a. Amongst the corals, the Alcyonarians are scantily developed, 

 yet date back to the older Primary rocks. The Rugosa are chiefly, 

 though not exclusively, of Primary age ; the Sclerobasic corals are 

 not known prior to the Tertiary period ; and the Perforata, which 

 are common corals of the present day, date from the Cambrian 

 rocks ; the Aporosa are more numerous in the newer rocks than in 

 the Primary period. The Sea-urchins would tend to unite Secondary 

 and Tertiary rocks together, while some urchin type shows a remark- 

 able connection between the Cretaceous and Tertiai'y periods. The 

 Crinoids are an asthenoid ^ group most numerous in the Primary 

 period ; but otherwise have little value in stratigraphical classifica- 

 tion. The living groups of Crustacea do not suggest any of the 



^ This term seems to have hitherto been applied in medicine only, to define diseases 

 marked by dvbility ; literally = weakly, feeble, infirm. 



