LIVING AND NOT LIVING. 19 



conditions, turning food into blood and thus repairing the 

 waste of matter and energy, keeping up the supply of 

 oxygen and the warmth of the body, sifting out and 

 removing waste products, and so on. 



Besides the more or less constantly recurrent activities or 

 functions, there are the processes of growth and repro- 

 duction. When income exceeds expenditure in a young 

 animal, growth goes on, and the inherited qualities of the 

 organism are more and more perfectly developed. At the 

 limit of growth, when the animal has reached " maturity," 

 it normally reproduces, that is to say, liberates parts of itself 

 which give rise to new individuals. 



Living and not living. — Although no one is wise 

 enough to tell completely what is meant by the simple word 

 alive, it is safe to say that active life involves the following 

 facts : — 



(a) The living organism grows at the expense of material 

 different from itself, while the crystal — one of the few dead 

 things which can be said to grow — increases only at the 

 expense of material chemically the same as itself. 



(b) The living organism is subject to ceaseless chemical 

 change (metabolism), and yet it has the power of retaining 

 its integrity, of remaining more or less the same for prolonged 

 periods. 



(c) The living organism resembles an engine, in being a 

 material system adapted to transform matter and energy 

 from one form to another ; but it must be granted that it is 

 a self-stoking, and, within limits, a self-repairing engine, and 

 that it is able to do what no engine can effect, namely, 

 reproduce. From a physical standpoint it differs from an 

 inanimate system in this, that the transfer of energy into 

 it is attended with effects conducive to further transfer and 

 retardative of dissipation, while the very opposite is true of 

 an inanimate system. 



{d) A living organism exhibits five everyday activities — 

 contractility (the power of movement), irritability (the 

 power of feeling in the wide sense), nutrition or utilisation of 

 food, respiration, and excretion, besides the periodic activ- 

 ities of growth and reproduction. 



Division of labour. — All the ordinary functions of life 

 are exhibited by the simple unicellular animals or Protozoa. 



