NATURAL HISTORY. 



-OO&iOO- 



ANIMAL LIFE. 



THERE is a marked difference between living and non-living matter ; 

 and, while we do not yet know what life is, we are familiar with 

 many of its phenomena. "We find, for instance, in all living bodies a 

 peculiar chemical compound, or group of chemical compounds, known as 

 jjroteids, the principal constituents of which are carbon, hydrogen, oxygen, 

 and nitrogen ; and these compounds occur nowhere except as a result of 

 life. A simple, undifferentiated form of these proteids is protoplasm, — 

 a substance about which much has been written, while in reality we know 

 but little about it. 



All living bodies have the power of growth ; and this takes place, 

 not, as in non-living bodies, by additions to the exterior, but in each and 

 every part of the substance. They have the power of taking other 

 substances, either living or non-living, and of working them over and 

 adapting them into such new forms as are needed for nutrition. In other 

 words, they have the power of digestion. 



A part of the material thus added goes to make an increase in size ; 

 another portion is required to make good the constant waste, for to a 

 certain extent we can compare a living body to a steam-engine. The 

 coal under the boiler is burned, and the heat thus obtained is converted 

 into molar motion by the steam produced acting on a piston. So with 

 living bodies : there is a constant chemical alteration going on within 

 them, and the result is the production of what are known as vital forces. 

 The food represents the coal, the body itself the boiler and engine. If 

 food or fuel is lacking, the body dies — the engine stops; as the engine 

 wears out, just so after a certain time a wearing out occurs in living 

 bodies, and death ensues. 



Life is dependent on certain external conditions besides food. Thus 

 it needs a certain amount of moisture, while if the temperature rise too 

 high or fall too low, life ceases. These extremes of temperature between 

 which life is possible vary, of course, with different forms ; but if we 

 except some spores of the lower plants, we may say that a heat of 208° 



