CH. I.]' INTRODUCTORY 7 



4. Power of reproduction ; this is a variety of growth. 



5. Power to excrete ; to give out waste materials, the products of 

 other activities. 



It should, however, be recognised that certain of these five char- 

 acteristics may be absent or latent, and yet the object may be living. 

 For instance, power of movement is absent in many vegetable struc- 

 tures ; certain seeds and spores can be dried and kept for many years 

 in an apparently dead condition, and yet will sprout and grow when 

 placed in appropriate surroundings. 



Of all the signs of life, those numbered 2 and 5 in the foregoing 

 table are the most essential. Living material is in a continual state 

 of unstable chemical equilibrium, building itself up on the one hand, 

 breaking down on the other ; the term used for the sum total of these 

 intra-molecular rearrangements is metabolism. The chemical sub- 

 stances in the protoplasm which are the most important from this 

 point of view are the complex nitrogenous compounds called Proteids. 

 So far as is at present known, proteid material is never absent from 

 living substance, and is never present in any thing else but that 

 which is alive or has been formed by the agency of living cells. It 

 may therefore be stated that Proteid Metabolism is the most essential 

 characteristic of vitality. 



