LIFE ON THE EARTH. 17 



and animal life must have their greatest effect, here 

 being the greatest mixture of air with the water ; the 

 greatest motion of the water, to give full effect to that 

 mixture ; the greatest amount of stimulating light, 

 and the greatest change of daily temperature ; we 

 shall readily conceive that near the surface the forms 

 of life should be both more varied and more abund- 

 ant ; while, on the other hand, in the deeper and 

 calmer water, less light, less motion, less air and less 

 change of air, should correspond to fewer and less 

 varied inhabitants. Only a few of the Mollusca are, 

 like the Argonaut of the Poet, who tilts along the 

 Atlantic waves 



But if a breath of danger sound, 



With sails quick-furled she dives profound, 



And far below the tempest's path 



In coral grots defies the foe, 



That never broke, in heaviest wrath, 



The sabbath of the deeps below. 



In a Diagram, Fig. 2, we may represent the limits 

 of life above and below the level of the sea by 

 setting off on the vertical line a scale of heights, 

 and depths, and drawing at right angles to this, for 

 each selected zone of height and depth, lines repre- 

 senting the ratios of abundance of life. Thus shall 

 we have, in a general form, an expression of the 

 apparent dependence of life on elevation above and 

 depression below the general level of the surface 

 of the sea. The real dependence on the land is 

 B. L. c 



