80 THE CRITERIA OF LIVINGNESS 



from their non-living environment. What are these charac- 

 teristics ? What are the criteria of living organisms ? What 

 is essential in the admitted contrast between the living and 

 the not-living ? 



2. The Essential Characteristics of Living Organisms. 



In the most general way what we see is plain enough. 

 We see organisms acting on their environment displacing 

 it, changing it, eating it, and so on ; and again we see that 

 the environment acts upon the organism displacing it, 

 changing it, stimulating it, oxidising part of it, and so on. 

 So that living is a twofold relation between organisms and 

 their environment a twofold relation of action and reaction, 

 of thrust and parry, of doing and suffering. At one moment 

 the organism is relatively the more active, at another the 

 environment. Living is a continual adjustment between 

 these two relations. 



When we look at the facts a little more closely we see 

 that all living creatures plants as well as animals are 

 active towards two main results, their own self-maintenance 

 and the continuance of their race. Organisms have in their 

 living just two main businesses caring for themselves and 

 caring for their offspring. But all this is living rather than 

 life ; we are only hiding the problem behind the word organ- 

 ism. What are the marks of a living creature ? that is the 

 question. What is the best answer we can give for the time 

 being? Many answers have been given, but none has found 

 wide acceptance, which doubtless means that biologists have 

 not yet seen the insignia of organisms in their entirety, or 

 in proper perspective. 



One of the best statements is that of Roux, who recognises 

 five " elementary functions " : 



