APPLIED BIOLOGY 



such as soil, water, and air ; and hence animals and plants 

 cannot be distinguished from lifeless things by the substances 

 entering into their composition. What, then, does distin- 

 guish the living from the lifeless ? Let us first try to answer 

 this question by examining a living animal (e.g., a frog) 

 and, if possible, determine just how the living frog is different 

 in behavior from lifeless objects, such as a stone or a dead 

 frog. 



LIFE-ACTIVITIES OF AN ANIMAL 



18. The living frog moves automatically or spontaneously. 

 By this we mean that within the animal there is machinery for 

 producing motion. A stone or other lifeless thing can 

 move only through the action of some external agency ; 

 it may fall (gravitation), or be moved by swiftly flowing 

 water, by a violent wind, or by some animal. Not only 

 does the frog have the power of locomotion, but also there 

 are internal movements, such as breathing and the beating of 

 the heart, which go on continuously as long as the frog lives. 



The statement that the living frog has machinery for 

 producing motion reminds us of the steam-engine, but a 

 little study shows that the engine is not automatic, as is 

 the body of a living animal. For example, the engine re- 

 quires the attendance of an engineer to supply it with water 

 and fuel (that is, to feed it) ; but the frog is able to obtain 

 its own food (fuel) and water to feed itself. Careful study 

 of all machines invented by man shows that the movement 

 which at first may appear to be as automatic as that of an 

 animal is really dependent upon regular human attendance. 

 Among common objects there is nothing lifeless which seems 

 to have automatic movement, and in the vast majority of 

 cases it is easy to decide whether an animal is dead or alive 

 by simply testing for evidences of movement. 



19. The living frog requires food if it is to continue to 

 live and move and grow. This is a fact so well known that 



