AN INTRODUCTION TO ANIMAL BIOLOGY 45 



repair are supplied by the frog's food. And in addition to 

 materials for repair, food also supplies those required for 

 growth in size, which means increase in number of cells. 



What would be the result as to the weight of the body 

 if the wasting processes were more rapid than those of repair ? 

 What are the conditions with respect to the rate of waste 

 and repair in a young frog which is growing rapidly? 



A scientific answer, then, to our question, " Why does the 

 frog need food? " is that food (1) furnishes the new substances 

 necessary for assimilation (see 19) to replace the body- 

 substances which have been wasted owing to activity ; and (2) 

 in addition to repairing, food may furnish the materials for 

 growth. Especially do young animals need food for growth. 



Energy. We may look in another way at the question, 

 "Why does the frog need food?" The activities of the 

 frog breathing, eating, moving, etc. indicate that the 

 animal is continually doing work or using energy. Energy 

 is the capacity for doing work. For example, the energy 

 of the coal may move the steam-engine, the engine may run a 

 dynamo and produce electrical energy, and the electricity 

 may run a trolley-car. 



Now, it has been discovered that energy cannot be created 

 by any machine, plant, or animal ; but energy may be trans- 

 formed. Thus the energy of the coal is transformed into 

 heat energy and the mechanical energy of the engine, the 

 mechanical energy is transformed into electrical energy, and 

 this into heat energy (electric heater), light energy (electric 

 lamp), or mechanical energy (electric motor). A steam- 

 engine does work and expends energy ; but it derives its power 

 of doing work (that is, its energy) from the coal burned in the 

 furnace. Likewise the frog, being unable to create energy 

 within itself, requires food as a source of the energy which 

 is to be used in the life-activities. The food of the frog 

 corresponds to the coal of the engine, in that each supplies 

 the energy for its respective machine. 



