Animals Before Man 



take their places, so that at no two j)eriods of 

 time were the living beings just the same. 

 Certain kinds of animals will be found in a 

 number of layers of rock and then disappear, 

 or be present in greatly reduced numbers, while 

 from time to time new plants and animals make 

 their appearance. And while these changes 

 have in the main been slow, at some periods 

 they took place much more rapidly than at 

 others, causing very marked differences between 

 the animals found in various beds of rock, and 

 these differences are used as boundary marks to 

 distinguish the divisions of geological time. 



Any well-defined stratum, or bed of rock, 

 which is shown by its structure and the fossils 

 it contains to have been the result of the unin- 

 terrupted deposit of sediment, is termed a For- 

 mation, or Stage, and it is easy to see that this 

 may vary greatly in thickness. According to 

 the extent of the resemblances between the 

 animals they contain, Formations are combined 

 in Series, the Series in Systems, while these in 

 turn are united in Groups ; such, at least, is the 

 classification and such the names adopted by 



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