XIV.] LIVING MATTER AND ITS EFFECTS. 227 



forms of graduall)' increasing complexity. Finally, cells 

 are given off and extruded from the body as eggs; 

 each of which is competent to run anew through the 

 series, characteristic of that form of living matter known 

 as a pigeon. 



Hence, there is a very close analogy between the animal 

 and the vegetable forms of life at present under considera- 

 tion, but the differences are no less striking. The pigeon 

 cannot live on a watery solution of ammoniacal and mineral 

 salts, however much fresh air and sunshine may be added to 

 this diet. It has np power of manufacturing the protein 

 compounds, or the fatty or saccharine matter of its body 

 out of simpler substances ; but, directly or indirectly, it 

 is dependent upon the plant for all the most important 

 constituents of its body. 



Like all other animals, the pigeon is a consumer, not a 

 producer. The complex substances, which it obtains from 

 the peas on which it feeds, are assimilated to its own sub- 

 stance, and are then slowly burned by the oxygen which it 

 obtains by the process of respiration. The animal is in 

 fact a machine, fed by the materials it derives from the vege- 

 table world, as a steam-engine is fed with fuel Like the 

 steam-engine, it derives its motor power from combustion ; 

 and, as in the case of the steam-engine, the products of the 

 combustion are incessantly removed from the machine. 

 The smoke and ashes of the animal are the carbonic acid 

 evolved in expiration, and the faecal and urinary excreta. 

 The latter are returned to the earth in a more or less fluid, 

 or, at any rate, soluble form ; the former is diffused through 

 the air. 



When the bird dies, the soft parts rapidly putrefy, and pass 

 off, as gaseous and fluid products, into the air and the water. 

 The dense bones resist decay longer; but, sooner or later, 



Q 2 



