LIVING AND NOT LIVING. 21 



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 either parts 

 of itself or special germ-cells 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 not- 

 living 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. The physical basis of life invariably includes com- 

 plex compounds known as proteids^ built up chiefly of 

 Carbon, Hydrogen, Oxygen, and Nitrogen, and these are 

 continually being broken down and made anew. 



(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 is a self-stoking, and, 

 within limits, a self-repairing engine, and 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 creature is a more or less perfect integrate^ 

 it has a unified behaviour, it gives effective response to 

 external stimuli. 



(e) 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 

 activities of growth and reproduction. 



