ANTAGONISM BETWEEN VEGETABLE AND ANIMAL LIFE. 17 



All organised beings absolutely require oxygen for their 

 existence, and receive it from the inexhaustible sources of the 

 atmosphere. When we reflect on the countless millions of 

 animals which are constantly inhaling and consuming this prime 

 necessary of life, and as constantly evolving carbonic acid, a gas 

 destructive to life, we well may wonder how, in spite of this 

 enormous consumption and perpetual pollution, the composition 

 of the atmosphere still remains unchanged from age to age. 



This immutability in the midst of eternal disturbance, this 

 constancy where so many changes are perpetually at work, can 

 only be the result of a wonderful order, of a masterly balance 

 between conflicting influences. 



The opposite wants of animal and vegetable life are the chief 

 means which Providence uses for maintaining the purity of the 

 atmosphere. Animals consume oxygen and exhale carbonic acid, 

 while in the economy of plants the inverse operation takes 

 place. Thus, without the plants, the animals would soon decline 

 and perish, in consequence of the increasing impurity of the at- 

 mosphere ; and, on the other hand, the plants could not exist 

 without the carbonic acid, which the vital process of animals is 

 constantly imparting to the air. 



Even in the narrow space of an aquarium we are able to per- 

 ceive the beneficial effects of this opposition between vegetable 

 and animal respiration. For if we enclose marine animals alone 

 mollusks, annelides, star-fishes, crustaceans in one of these re- 

 servoirs, they soon perish, in consequence of the want of oxygen 

 and the pollution of the water; but by adding a few plants 

 ulvse or confervas the equilibrium maintains itself, and while 

 the latter enjoy a vigorous growth, the former are able for a 

 long time to preserve an unimpaired health. 



Yet, in spite of this admirable antagonism between the vege- 

 table and animal kingdoms, the purity of the air would have 

 been but imperfectly maintained if the atmosphere had not been 

 kept in a state of constant motion by the magnificent system of 

 the winds, which force the air to wander in perpetual currents 

 from the equator to the pole, and from the pole to the equator. 



The unequal influence of the heat of the sun upon the atmo- 

 sphere between the tropics and in the higher latitudes is the 

 first grand cause of this immense aerial circulation. In those 

 favoured regions where the sun darts his vertical rays upon the 



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