86 THE MECHANISM OF LIFE 



Falling upon tlie sea, it evaporates water, whicli rises up into the 

 air as vapour, falls down as rain and snow, and returns to the 

 sea in rivers, having done nothing of itself but wear away the 

 land and generate heat by friction. The heat is radiated away 

 into space, and is for ever lost to the earth. Falling upon rocks, 

 stones, gravel, sand, soil, etc., and upon the atmosphere, the 

 solar energy heats up all these substances to a slight degree ; but 

 again this heat is radiated away, and is therefore irretrievably 

 lost. Apart from vegetable life, there is therefore a continual 

 dissipation of solar energy. 



But the green plants intervene. They absorb the solar radia- 

 tion, utilising this to recombine the water, carbonic acid, and 

 nitrate which result from animal metabolism. Apart from the 

 activity of the plants, this energy would be dissipated and the 

 products of animal life would remain unavailable for further life. 

 They are, however, recombined by the plants, and the solar 

 energy which would otherwise be wasted is thus fixed. The 

 plants are producers. 



In the past plant life has, on the whole, been more active than 

 animal life, and the result is the accumulations of coal (and 

 perhaps oil) upon which modern industrial civilisation is based. 

 This civilisation we must, however, regard as merely an episode 

 in the history of terrestrial life, for the enormous increase of 

 human population during the last few centuries has only been 

 possible by the utilisation of the materials produced by a former 

 surplus of vegetable life. With the depletion of this surplus the 

 balance will be restored. It would, of course, be very rash to 

 regard the future reduction of the human population of the earth 

 as inevitable, for there may be other immense accumulations of 

 energy in that now bound in the atoms of some material sub- 

 stances. Also some supply of energy may be obtained from the 

 tides, winds, and rivers. But, again, it would be very foolish to 

 count upon this possibility, for all that we know so far about 

 radio-active transformations suggests that the process is one 

 which we cannot initiate or control, and that the energy of the 

 atoms, all but a very insignificant fraction, is bound. Also, the 

 practical difficulties in the way of utilising tidal energy are prob- 

 ably enormous. Now coal and oil are diminishing rather rapidly, 

 and there are no other sources of energy so far available. Therefore 

 it seems safe to assume a return to an agricultural and pastoral 

 civilisation, and with that a great reduction of population. 



