LIFE ON THE EARTH. 



61 



variety of life by the relative number of species to be 

 expected on an average in searching a given thick- 

 ness of strata in each of the great periods. Also 

 since it is known that the species are not uniformly 

 arranged through the deposits of each period, but 

 occupy many distinct stages in each, we may say that 

 on the average, they represent the rate of change in 

 the forms of marine life, or the number of different 

 species to be expected in searching successive equal 

 thicknesses in each of these systems of strata. Accord- 

 ing to Morris's estimates of thickness (p. 52), the 

 relative numbers would have been 432, 251, 59. 



Had we instead of the figures which represent 

 the thicknesses and number of species found in 

 Britain employed the data given by D 'Orbigny 1 , 

 counting all the species in all the classes of Mollusca 

 and Radiata, viz. 



we should have nearly the same result as with 

 Morris's numbers ; a nearer approximation of the 

 Mesozoic and Caenozoic ratios, but the same remark- 

 able inferiority of the Palaeozoic series. 



1 PalcBontologie et Geologic, 11. 



