60 



LIFE ON THE EARTH. 



them at present to only three great periods, we have 

 the following result : 



The absolute number of marine species appears 

 thus to be greatest in the Palaeozoic Strata; but 

 when the thickness of the deposits which represents 

 elapsed time is taken into account, the variety of 

 forms in a given thickness or given period of time is 

 very much less. 



For if we take account of the thickness according 

 to the maximum scale for Britain proposed by Ram- 

 say, on data collected by the Geological Survey, the 

 following table may be constructed : 



The ratio thus found by dividing the number of 

 species by the maximum thickness, represents . the 



