324 The Bate of Geological Change. [July, 



an affinity dependent on a divine idea"* It is necessary to bear 

 in mind these definitions of a genus in order to understand the 

 author's generalization of "Polarity" in the "development of 

 generic types," and to prevent confusion with other ideas which I 

 shall attempt to elucidate in the following pages. 



Before estimating the rate of geological change in successive 

 epochs, we must clearly understand the means by which that rate 

 is measured. Our geological chronology is divided into epochs of 

 greater or less extent, distinguished and characterized by certain 

 forms of animal and vegetable life, either peculiar to them, or pre- 

 ponderating in number and variety during their continuance. We 

 can conceive that in a previously unexplored country, far away 

 from any region whose geology is known, the explorer may meet 

 with diverse geological formations, each formation being charac- 

 terized by a sufficiently numerous and distinctive fauna. If forma- 

 tion A contains 1000 species, and is 10,000 feet thick, and forma- 

 tion B contains 5000 species, but is only 1000 feet thick, and if 

 the range of zoological rank in the 5000 species is approximately 

 the same as in the 1000 species, we should be justified in saving of 

 formation A, that, in comparison with formation B, — 



(1) It was deposited very quickly ; or, 



(2) During its deposition species changed very slowly. 



Further investigation by our hypothetical explorer might pos- 

 sibly furnish him with evidence that, during the deposition of for- 

 mation A, the conditions of climate and physical geography had 

 remained more or less stationary, and that the strata were deposited 

 slowly. On the other hand, formation B and its fossils might yield 

 evidence of great changes in climate and physical geography, and of 

 comparatively rapid deposition. Under these circumstances, he 

 would be justified in the conclusion that, during the epoch repre- 

 sented by formation A, the rate of geological change was much less 

 rapid than during the period represented by formation B. 



This hypothetical contrast will assist the reader in appreciating 

 the significance of the following synopsis of the Palaeozoic and 

 Mesozoic rocks and their fossils, and will enable him to estimate 

 how far these epochs agree with those which we have supposed to 

 be represented by formations A and B respectively. 



Professor Phillips, in his Eede lecture, delivered before the 

 University of Cambridge in 1860,t remarks that if we select among 

 the marine classes of animals those which are represented in all the 

 great periods of geology, count the number of species yet discovered 

 in them in British strata, and refer them at present to only three 

 great periods, we find that the Palaeozoic rocks contain 2729 species, 



* Loc. cit. 



f Afterwards published under the title of ' Life on the Earth.' Macmillan. 

 1860. See pp. 59-62. 



