THE CRITERIA OF LIFE 15 



All life is accompanied by the liberation of a certain 

 amount of energy which implies the breaking up of the 

 complex protoplasm by oxidation into simpler but more 

 highly oxidised compounds. There is, therefore, a con- 

 tinual waste going on in living things, and this would 

 ultimately lead to the total destruction of the proto- 

 plasm were there not within the living substance a 

 power to make good this waste. This power to make 

 good the continual waste is one of the most characteristic 

 features of living substances, and is carried on by a 

 process of intussusception ; that is, the continual absorp- 

 tion of new material from without and its transformation 

 into the substance of the body. So that we may say the 

 living organism is a centre of continual waste and repair, 

 of nicely balanced destructive and constructive processes. 



3. Its tendency to undergo cyclical changes. 



The cyclical changes are, for the most part, incidental 

 to re-integration. The living organism coming into 

 being as the result of previous living matter (for we have 

 no evidence of non-living matter giving rise to living), 

 proceeds to increase its own amount of living substance, 

 and to dispose of the excess thus formed in setting it 

 free as new individuals, or, in other words, in repro- 

 ducing its kind. This power of reproducing its kind is 

 a necessary feature of living organisms, for it is the 

 heritage of all living organisms that a stage is reached 

 when the constructive processes are no longer able to 

 outweigh or even to balance the destructive processes, 

 and what we call death e&sues. But in spite of this 

 ceasing to live, the original organism has been able to 

 pass on its peculiar properties through its periodic or 

 cyclical function of reproduction. 



