30 THE ORIGIN OF LIFE 



One hot day would kill tliem all : but they would 

 again re-appear in cold weather, whilst if the 

 radio-activity is very small it may never have a 

 chance of asserting itself in the temperate zones. 

 There are different limits of temperature for 

 different types of life. And the fact that life 

 exists here at particular temperatures is simply 

 a result of survival of the fittest so as to suit 

 the environment. 



Simpler forms of life than those which have so 

 survived doubtless did not possess all the properties 

 of those comparatively higher types which have 

 survived, and doubtless insolubility in water at such 

 temperatures as we happen to live in has been one 

 of these properties. 



Similar remarks may apply to the actual com- 

 position of living bodies. Protoplasm may be the 

 substance which at ordinary temperatures performs 

 the processes of vitality with the greatest efficiency, 

 and thus as living substance it is itself the fittest to 

 thrive. 



Bodies may be formed which should possess a 

 definite structure, occupy a certain position in space, 

 and go through a cyclic process in time, as also 

 adjust themselves continuously to their environment 

 by a continuous interchange of relation between 

 their internal and external functions, and yet not 

 necessarily, for anything we know, consist of proto- 

 plasm. 



Similarly as to the presence of sulphur, iron, 

 aluminium, phosphorus, &c., these elements have 



