THE BEGINNINGS OF LIFE. 131 



compose water, seizing upon its hydrogen and releasing 

 its oxygen ; whilst, lastly, they abstract nitrogen either 

 directly from the atmosphere, or indirectly from the 

 nitrate of ammonia which, under particular conditions, 

 has been formed therein. Plants, therefore, are mar- 

 vellous apparatuses of reduction, working with the aid 

 of the heat and light derived from the Sun. But this is 

 not all. The carbonic acid, the water, and the nitrate 

 of ammonia are decompounded, because the carbon, the 

 hydrogen, and the nitrogen entering into their compo- 

 sition, unite with oxygen to produce the various organic 

 substances entering into the fabric of plants. Reduc- 

 tion takes place, but only that combinations of a higher 

 order may arise. Animals, on the contrary, are true 

 apparatuses of combustion: in their bodies carbon- 

 aceous matters are burnt incessantly during the per- 

 formance of animal functions, and are returned to the 

 atmosphere in the shape of carbonic acid; hydro- 

 gen burnt incessantly is returned as water; whilst 

 nitrogen is ceaselessly exhaled in the breath and thrown 

 off in the different excretions 1 . c From the animal 



1 This continual process of combustion is dependent upon the con-, 

 joint and reciprocal action of the respiratory and nutritive functions. 

 Through the process of respiration the animal is supplied with an all- 

 important element, needed for the production of such changes. Mr. 

 Spencer says : ' The inorganic substance, however, on which mainly 

 depend these metamorphoses in organic matter, is not swallowed along 

 with the solid and liquid food, but is absorbed from the surrounding 

 medium air or water, as the case may be. Whether the oxygen taken 

 in, either, as by the lowest animals, through the general surface, or, as 

 by the higher animals, through respiratory organs, is the immediate cause 



K 2 



