184 



THE PAST AND FUTUEE OF GEOLOGY. 



823 against 1,820 recent species. Birds are still worse represented, as 

 only 18 fossil species occur against 354 recent species. 

 But the multiplicity of British fossils, however surprising as a whole, 



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383-96 Khizopods and Protozoa'. 



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has to be viewed in another and different light. The large total repre- 

 sents, not as the recent species do, the life of one period, but the sum of 

 those of all the geological periods. Geological periods, as we construct 

 them, are necessarily arbitrary. The whole geological series consists of 

 subdivisions, each one of which is marked by a certain number of char- 

 acteristic species, but each having a large proportion of species com- 

 mon to the subdivisions above and below it. These various subdivis- 

 ions are again massed into groups or stages, having certain features and 

 certain species peculiar to them and common throughout, and which 

 groups are separated from the groups above and below by greater breaks 



