16 



LIFE ON THE EARTH. 



the distribution of testaceous mollusca in them to be 

 as under : 



The first column gives the zones in succession 

 downwards. 



The second gives the limits of the zone in depth 

 by fathoms. 



The third, the depth in thickness of each zone in 

 fathoms. 



The fourth the number of species found in each 

 zone. 



The fifth is a column which I have calculated by 

 dividing the fourth by the third, to shew the relative 

 productiveness of each zone. (For the sake of avoid- 

 ing decimals, the quotient is multiplied by 10.) 



Thus it appears that the relative fertility of the 

 several zones decreases downwards toward zero : 

 and it is believed that at 300 fathoms life is extinct. 



If we consider that at and near the surface of the 

 sea all the influences favourable to both vegetable 



