The Story of Evolution 89 



ing at the time were buried, and these formed the fossils by the aid 

 of which it is possible to read the story of the past. By careful 

 piecing together of evidence the geologist is able to determine the 

 order in which the different sedimentary rocks were laid down, 

 and thus to say, for instance, that the Devonian period was the 

 time of the origin of Amphibians. In other cases the geologist 

 utilises the fossils in his attempt to work out the order of the 

 strata when these have been much disarranged. For the simpler 

 fossil forms of any type must be older than those that are more 

 complex. There is no vicious circle here, for the general succes- 

 sion of strata is clear, and it is quite certain that there were fishes 

 before there were amphibians, and amphibians before there were 

 reptiles, and reptiles before there were birds and mammals. In 

 certain cases, e.g. of fossil horses and elephants, the actual his- 

 torical succession has been clearly worked out. 



If the successive strata contained good samples of all the 

 plants and animals living at the time when the beds were formed, 

 then it would be easy to read the record of the rocks, but many 

 animals were too soft to become satisfactory fossils, many were 

 eaten or dissolved away, many were destroyed by heat and pres- 

 sure, so that the rock record is like a library very much damaged 

 by fire and looting and decay. 



The Geological Time-table 



The long history of the earth and its inhabitants is conven- 

 iently divided into eras. Thus, just as we speak of the ancient, 

 mediaeval, and modern history of mankind, so we may speak of 

 Palaeozoic, Mesozoic and Cenozoic eras in the history of the earth 

 as a whole. 



Geologists cannot tell us except in an approximate way how 

 long the process of evolution has taken. One of the methods is 

 to estimate how long has been required for the accumulation of 



