CHARACTERS OF PROTOPLASM 39I 



shorter time, and many others in which a portion of 

 matter may be said to assimilate and grow, for instance 

 the simple case of a crystal growing in a solution of 

 the salt of which it is composed by adding to its own 

 structure molecules of the salt from the solution. But 

 there is no other case in which so complicated and 

 delicately adjusted a system as a unit of protoplasm 

 (a free living cell) can do both these things indefinitely. 

 It is the combination of complexity of structure and 

 instability of its essential chemical constituents with 

 the power of maintaining equilibrium of the system as 

 a whole and of adding to the system by the assimilation 

 of fresh material from outside that makes protoplasm 

 a unique substance in nature, and what we call life 

 a unique phenomenon. 



To these two powers — of self-maintenance and of 

 assimilation and growth — we must add the power of 

 respiration, the power, that is, of oxidising organic 

 substances, principally sugars, within the protoplasmic 

 complex, and thus of setting free their potential energy, 

 which is available for the production of mass movement 

 and heat. 



Though protoplasm has everywhere the same general 

 chemical composition and physical structure, it differs 

 in certain details in every different kind of animal and 

 plant. If we accept the conclusion that all the pheno- 

 mena of life depend upon the chemical and physical 

 composition and structure of protoplasm, we must 

 believe that the differences in the manifestations of 

 Hfe shown by different kinds of organisms depend on 

 differences of composition and structure, just as the 

 essential general uniformity of life depends on the 

 essential general uniformity in the composition and 

 structure of the protoplasm of all organisms, And this 



