1870.] 



The Bate of Geological Change. 



327 



superiority in early times of a slowly changing physical condition 

 of the whole globe over the partial and irregularly varying local 

 conditions, which were continually augmenting, and are still 

 augmenting in influence with the lapse of time." 



It may be objected to Professor Phillips's figures that he 

 includes only the species belonging to the eight classes of animals 

 which have been represented in each of the three great periods. I 

 therefore give in the following Table the approximate total number 

 of species (excluding plants only) that have been discovered in 

 British stratified rocks up to the present time, with the result per 

 1000 feet, and the average number of feet per species in each 

 great period : — 



Periods. 



Number of 

 Species. 



Maximum 

 Thickness. 



Cainozoic 

 Mesozoic . . 

 Palaeozoic 



1,500 

 4,000 

 3,500 



2,240 

 23,190 

 57,124 



Relative Number 



of Species per 



1000 feet. 



670 



173 



61 



Number of 

 feet per 

 Species. 



1-5 



5-8 



16-3 



The evidence yielded by the analysis of these figures is even more 

 in favour of the conclusion that the rate of geological change, 

 according to the evidence of animal life, has progressively increased 

 from the earliest to the latest times. 



With one other numerical illustration I shall close the argument 

 drawn from the organic phase of the question. The late Professor 

 Bronn, in his prize essay, and in the third edition of his ' Lethaea 

 Geognostica,' gives the following as the total numbers of known 

 species in 1850, just twenty years ago : — Palaeozoic 6681, Mesozoic 

 10,879, and Cainozoic 15,138. If we assume the maximum 

 thickness of the deposits of these periods to be 60,000, 25,000, and 

 10,000 * respectively, we get the following numbers of species as 

 occurring per 1000 feet of strata: — Palaeozoic 111, Mesozoic 435, 

 and Cainozoic 1513 ; thus showing exactly the same result almost 

 in the same proportions. 



The foregoing calculations are based on the assumption that 

 the time required for the deposition of 1000 feet of strata was 

 approximately the same in the Palaeozoic, Mesozoic, and Cainozoic 

 periods, and so far the argument is Uniformitarian ; but the 

 received interpretation of the physical conditions which prevailed 

 in those periods renders it probable that the strata were deposited 

 more slowly in the earlier than in the later periods, which, a 

 fortiori, adds considerable strength to my conclusion. 



The inorganic aspect of the subject has been discussed more 



* I purposely make this number very large, so as not to run the risk of being 

 charged with making too much of my argument from this point of view. 



