ANIMALISATION OP THE EAETH. 



crust, by the internal movements of the fluid nucleus of the 

 globe, caused various changes in the distribution of land and 

 water, so that parts of the globe which at one time were raised 

 above the waters, and inhabited by terrestrial tribes, were sub- 

 sequently submerged ; while other parts, being elevated, emerged 

 from the waters and formed new continents or islands. Indeed, 

 changes which are in actual progress, and which will be presently 

 noticed more fully, show that such phenomena are still produced, 

 though probably on a much smaller scale, than at the earlier 

 stages of the growth of the earth when its crust, having less 

 thickness and strength, offered less resistance to the internal 

 movements of its fluid nucleus. 



52. Since the strata were deposited during a succession of 

 periods of long duration, each receiving the remains of the 

 organised tribes which inhabited the earth at the period of its 

 deposition, it follows that the organic contents of these successive 

 strata may be regarded as so many museums presenting to us 

 specimens of the zoology and botany of the globe at the successive 

 periods of their deposition. By examining, therefore, these re- 

 mains, we shall be able to compare with each other, and with the 

 existing tribes, the living inhabitants of the globe at the several 

 periods of the formation of these strata. 



53. The first and most obvious inference suggested by such an 

 analysis is, that the number and variety of organised beings has 

 rapidly increased, from the period at which the earth became 

 habitable to the present epoch. This is rendered evident by a 

 comparison of the number of species found in a given thickness of 

 strata, proceeding downwards from the surface, which has been 

 estimated by Professor Phillips as follows : 



Number of Species in 

 1000 feet thickness. 



Tertiary .... 1410 

 Cretaceous . . . . 707 

 Oolitic . . . .456 

 Triassic 1 g 2 



Permian j 



Carboniferous ... 47 

 Silurian . . . . 27 



These numbers represent the relative proportion of marine spe- 

 cies, by far the most numerous of the organic remains, more 

 especially in the lower strata. This calculation was based upon 

 the results of observations made about 1834, and consequently the 

 numbers given are considerably below what would be obtained 

 from more recent observation ; but their proportion, which is all 

 that concerns us here, would probably not be altered by subse- 

 quent results. 



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