The-Wbrfd'o'r]filKe^ 



whit as much as do land animals, only in a 

 different mode. 



Large quantities of oxygen are ever present, 

 dissolved, in the sea. It is not the wa^er 

 flowing through the fish's gills, but the oxygen 

 dissolved in that water, which carries on com- 

 bustion in the body, and so keeps it in life 

 and health. 



The amount of oxygen thus obtained is small ; 

 therefore the combustion is slow ; therefore the 

 blood is cold. But if, for a very short time, all 

 oxygen could be withdrawn from the ocean, the 

 whole multitude of fishes therein would fast die 

 of suffocation. Sea-water alone, without oxygen, 

 could not keep them alive. 



The gills are, however, so constructed that, 

 while they can use small supplies of oxygen 

 dissolved in water, they cannot use large sup- 

 plies of oxygen forming part of the free air. 

 So when a fish out of water draws in atmo- 

 spheric air, its gills become dry and useless, and 

 it dies of suffocation. 



A world of rapid living and dying is this which 

 we have now in view — of killing and being killed 

 — of an incessant struggle for existence. 



Many of its inhabitants have no means of 

 defence, save by flight ; and these feed only on 



233 



