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. 
(4) 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 gvoteids, 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 :ntegrate, 
it has a unzfied behaviour, it gives effectzve 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. 
