1 8 BIOLOGY AND HUMAN LIFE 



way. Yet we now know enough to say rather positively that 

 the chemical elements of which human beings and other organ- 

 isms consist are exactly the same as those found in the air, in 

 the waters, and in the soil and rocks that make up our world 

 (see Fig. 3). At the same time, plants and animals contain 

 compounds, or combinations of these elements, that occur 

 naturally only in living things. Examples of such are starches 

 and sugars, fats and oils, proteins and amino acids, and chloro- 

 phyl and other pigments. All such substances have accordingly 

 been called organic, to distinguish them from water, salts and 

 other minerals, metals, etc., which are called inorganic. Or- 

 ganic compounds are present in all plants and animals. 



Within the last hundred years chemists have been able to reproduce a 

 large number of substances that ordinarily originate only in the bodies 

 of plants and animals ; and they have made up new compounds along 

 similar lines, that are never found in nature. 



14. Activities of animals. As we watch our grasshopper and 

 our plant we are at once reminded of a striking difference be- 

 tween them ; that is, the liveliness, or movement, of the animal 

 as contrasted with the quietness of the plant. Since we are 

 looking for similarities, we are tempted to set this point aside 

 and search for other features ; but let us keep the facts before 

 us without prejudice, and let us complete our record of the 

 grasshopper's activities before making up our minds which are 

 important for our purpose. Very well, then, the grasshopper 

 moves. He not only moves from place to place, but he moves 

 parts of his body in relation to one another, as the antennae and 

 the parts about the mouth. These movements at once suggest 

 other activities ; the mouth movements suggest eating, and the 

 movements of the antennae suggest feeling. 



From our experience with food we already know that it is 

 related to growing ; and while the grasshopper does not increase 

 in size under our eyes, we know that he must have grown, for 

 he was not born full size. And that suggests another thing that 

 animals do— they reproduce. 



