THE SCOPE OF BIOLOGY 3 



in proper conditions, we conclude that the seed is not alive. 

 Hence the best criterion that we have for separating the living 

 from the non-living is to determine whether or not the body 

 in question either shows any signs of independent activity or, 

 when put under proper conditions, may be made to show any 

 signs of such activity. 



Automatic activity. The simple fact of showing activity is, 

 however, not enough to serve as a criterion of life. Other 

 things besides living beings have the power of activity. A 

 watch, or a locomotive, or a steam engine certainly shows 

 activity, and yet none of these is alive. There is, however, 

 one distinction between the activity of such machines and 

 the activity of a living organism. Machines show activity 

 only when they are started into action by some outside in- 

 fluence; while a living organism develops activity from its 

 own internal, independent power. With this modification, 

 the first criterion that we have for distinguishing the living 

 from the non-living is the power of developing automatic 

 activity, and only objects possessing this power do we speak 

 of as being alive. 



2. Death. The fact that living things show automatic activ- 

 ity has a converse side. This activity may cease, the object- 

 losing its power of showing spontaneous activity. This consti- 

 tutes the phenomenon spoken of as death. To define either 

 life or death has proved a puzzle to both science and philosophy. 

 For our purpose, however, they can be fairly well defined as 

 follows: By life, we mean the possession of the power of show- 

 ing spontaneous, automatic activity; by death, we mean the 

 disappearance of this power. Why an animal or plant, when it 

 dies, loses this power, we do not know. In some cases it is 

 undoubtedly because the complicated machinery which com- 

 poses the body is injured and consequently cannot work 

 properly. This we find true also in the case of ordinary ma- 

 chines. If a locomotive should burst its cylinders, it would no 

 longer be able to run. If a watch has its mainspring broken, 



