186 THE PAST AND FUTURE OF GEOLOGY. 



may assume that about 30 per ceut. of the orgauic remaius pass from 

 one stage to auother. Dividiug the 13,270 fossil species among the 

 thirty-eight stages, or omitting the lower stages and some others and 

 taking only thirty, we get a rough average of 442 species for each ; and, 

 allowing in addition for the number common to every two periods, we 

 obtain a mean of 030 species as the population of each of the thirty 

 periods against the 3,989 species of the present period. On this view, 

 the relative numbers are therefore reversed, as shown in the annexed 

 diagram, (Fig. 3,) where the number of living British species is compared 

 with the mean of the extinct species assumed for any one past period 

 of the same area. 



This gives a ratio for the fauna or flora of a past to that of the 

 present period of only as 1 : 0^. But it must be remembered that prob- 

 ably the actual as well as the relative numbers of the several classes 

 inter se in each and all of these several formations varied greatly at 

 the different geological periods. Still, we have no reason to sui)pose 

 but that, during the greater part of them, life of one form or another was 

 as prolific, or nearly so, in the British area then as at the present day, 

 and we may thus form some conception of how little relatively, though 

 so much really, we have yet discovered, and of how much yet remains 

 to be done, before we can re-establish the old lands and seas of each 

 successive period, with their full and significant populations. This we 

 cannot hope ever to succeed in accomplishing fully, for decay has been 

 too quick and the rock entombment too much out of our reach ever to 

 yield up all the varieties of past life. But although the limits of the 

 horizon may never be reached, the field may be vastly extended ; each 

 segment of that semicircle may yet be prolonged we know not how far; 

 and it is in this extension — in the filling-up of the blanks existing in 

 the life of each particular period — that lies one great work of the future. 

 The field which thus embraces the study of all the varied forms of life 

 in all past time has now, as we have just shown, attained such vast 

 dimensions as will long require for its due and continued cultivation 

 the active and unceasing co-operation of geologists and palaeontologists. 



We now come to the more especial ground of the geologist. Starting 

 with investigations connected with the origin of the globe, he has to 

 trace the changes it has undergone through the various phases of its 

 history, to determine the cause of those changes, and the manner in 

 which they were eflected. Besides investigating the character and dis- 

 tribution of all organized things inhabiting the earth in all former 

 periods — their order of succession, and the relation of the several and 

 successive groups one to auother — he has also to study various chemical 

 and physical questions connected with inorganic matter. 



In the infancy of the science, geologists generally sought to exi)laiu 

 the great mechanical phenomena exhibited on the surface of the globe 

 by energy rather than by length of action. The philosophy of llutton, 

 Playfair, and their successors checked this disposition, and has led to 



