402 CONCLUSION 



moist climates, hot dry climates, cold climates, and 

 so on ; and again to seashores, marshes, forests, as 

 weeds of cultivated land, and to many other special 

 habitats. They continue to grow in these places and 

 no others, because their economy is adjusted to these 

 particular conditions of life. 



In all this we see varied special cases of the great 

 universal law of equilibrium, which governs all the 

 processes of which we have any knowledge, from the 

 movements of the planets to those of molecules, atoms 

 and electrons, from the activity of protoplasm to the 

 vagaries of the human mind. All things which exist 

 are constantly tending towards positions of balance or 

 equilibrium, i.e. of relative repose, and all activity, all 

 motion, represents some phase of this universal process. 

 The universe consists of the most varied kinds of 

 systems in relatively stable or unstable equilibrium, 

 and every fresh disturbance of equilibrium from out- 

 side any system leads to fresh activity in the system 

 which tends towards the establishment of a new 

 equilibrium. 



A free-living unit of protoplasm represents a very 

 striking special case of a system of particles which can 

 maintain a moving equilibrium, by assimilation of fresh 

 particles of special kinds of matter from without, and 

 the breaking down (katabolism, respiration) of certain 

 substances within. This double process it can only 

 maintain within a certain range of external conditions. 

 Outside these the equilibrium is destroyed and the 

 organism dies, the matter of which it is composed 

 breaking up into simpler systems of dead matter which 

 return to simpler states of equilibrium. By assimila- 

 ting faster than it katabolises the protoplasmic unit 



