16 EVOLUTION OF LIVING ORGANISMS. 



tinually being broken down. A stream of non-living 

 matter with stored-up energy is built up into living 

 matter, and again passes out as dead matter, having 

 yielded up the energy necessary for the performance of 

 the various activities of the protoplasm. All the pheno- 

 mena of metabolism on which these activities are based 

 are strictly limited by external conditions and the pro- 

 perties of the material taking part in it. 



We cannot, therefore, speak of a special living chemical 

 substance. The full attributes of life are only possessed 

 by a mixture of substances, some very complex, others 

 more simple, which make up protoplasm as a whole. The 

 living process forms a chain every link of which is 

 essential. No single link by itself can be said to be living, 

 nor can we really draw a hard and fast line marking off 

 where the life process begins and where it ends. The 

 protoplasm itself contains many substances, such as yolk 

 of starch granules, which are merely materials for future 

 use in the building up of new protoplasm, or again 

 granules of waste products, or special products of decom- 

 position stored for future use (digestive ferments, secre- 

 tions) which may never again enter the life cycle. 



It was mentioned above that irritability, or the power 

 of responding to a stimulus, is one of the most important 

 characteristics possessed by all living matter. A mani- 

 festation of metabolism, it depends on the fact that the 

 material of protoplasm is in a state of unstable equili- 

 brium capable of being disturbed by a stimulus. Stimuli 

 are those things or conditions in the environment which 

 can bring about disturbance or response. Naturally the 

 character and amount of the stimulus has no direct re- 

 lation to the character and amount of the response. Just 

 as the pressure of a button may ring a bell, explode a 

 mine, or start an engine, so a stimulus applied to living 

 protoplasm may cause a plant to grow, an animal to 

 move, or a man to embark on a course of action. The 

 extent and nature of the response depend on the structure 



(2,031) 



