390 CONCLUSION 



gel, often fluctuating between the two conditions, with 

 particles of protein, or loose combinations of proteins 

 and fats or proteins and salts as the disperse phase, 

 and water, with various substances in solution, as the 

 continuous phase ; and it is on this definite chemical 

 nature and physical structure that the definite forms 

 of activity — ^the " vital functions " — which are dis- 

 played by all living beings, depend. So much is certain, 

 though we are still far from understanding in detail 

 exactly how all the processes which take place in the 

 protoplasmic sol or gel result in the characteristic life 

 phenomena. In some cases we can already partly 

 understand how this happens — as for instance in the 

 case of the amoeba taking certain objects into its body 

 and casting out others (p. 69) — and as the study of 

 the constitution and activity of protoplasm progresses 

 we are steadily getting to understand more and more 

 of these details. 



Perhaps the two most fundamental properties or 

 powers of protoplasm are first its power of maintaining 

 the equilibrium of its physical structure within a certain 

 range of. external conditions (temperature, vapour, 

 pressure, etc.), and secondly its power of assimilation, 

 i.e. of taking into and making part of its own structure 

 certain chemical substances that contain the elements 

 of which protoplasm is itself composed. The first of 

 these properties is the basis of the maintenance of an 

 organism, of its continued life under varying conditions, 

 provided these do not pass beyond certain limits ; the 

 second is the basis of growth and reproduction, of the 

 increase in size and number of existing organisms. 

 Neither of these powers is in itself unique in nature. 

 There are many cases in which a portion of matter 

 retains its equilibrium as a system for a longer or 



