CH. I.] INTRODUCTORY. 7 



In the case of the higher animals and plants, they are always 

 unicellular to start with, but on dividing and subdividing the 

 resulting cells stick together and subsequently become differen- 

 tiated and altered in the manner already indicated. In spite 

 of these changes, the variety of which produces the great com- 

 plexity of the adult organism, there are certain cells which still 



Fig. 5. Human colourless blood-corpuscle, showing its successive cha 

 within ten minutes when kept moist on a warm stage. (Schi 



retain their primitive structure ; notable among these are the 

 white corpuscles of the blood. 



It would appear at first sight an easy problem to distinguish 

 between a living thing, and one which is not living. The principal 

 signs of life are the following : 



1 . Power of movement ; this is seen in amoeboid movement, 

 ciliary movement, muscular movement, etc. 



2. Power of assimilation, that is, ability to convert into proto- 

 plasm the nutriment material or food which is ingested. 



3. Power of growth ; this is a natural consequence of the 

 power of assimilation. 



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 

 characteristics may be absent or latent, and yet the object may 

 be living. For instance, power of movement is absent in many 

 vegetable structures ; 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. 



< >f 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 substances in the protoplasm which are the most 

 important from this point of view are the complex nitrogenous 

 compounds called Proteuh. 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. 



