ii.] THE PLANT AND THE ANIMAL 115 



by our senses and instruments of measurement, our ex- 

 perience and science. All that the effort can do, then, is 

 to make the best of a pre-existing energy which it finds 

 at its disposal. Now, it finds only one way of succeed- 

 ing in this, namely, to secure such an accumulation of 

 potential energy from matter, that it can get, at any 

 moment, the amount of work it needs for its action, simply 

 by pulling a trigger. The effort itself possesses only 

 that power of releasing. But the work of releasing, 

 although always the same and always smaller than any 

 given quantity, will be the more effective the heavier 

 the weight it makes fall and the greater the height — or, 

 in other words, the greater the sum of potential energy 

 accumulated and disposable. As a matter of fact, the 

 principal source of energy usable on the surface of our 

 planet is the sun. So the problem was this: to obtain 

 from the sun that it should partially and provisionally 

 suspend, here and there, on the surface of the earth, its 

 continual outpour of usable energy, and store a certain 

 quantity of it, in the form of unused energy, in appropriate 

 reservoirs, whence it could be drawn at the desired moment, 

 at the desired spot, in the desired direction. The sub- 

 stances forming the food of animals are just such reservoirs. 

 Made of very complex molecules holding a considerable 

 amount of chemical energy in the potential state, they are 

 like explosives which only need a spark to set free the energy 

 stored within them. Now, it is probable that life tended 

 at the beginning to compass at one and the same time both 

 the manufacture of the explosive and the explosion by 

 which it is utilized. In this case, the same organism that 

 had directly stored the energy of the solar radiation 

 would have expended it in free movements in space. 

 And for that reason we must presume that the first living 

 beings sought on the one hand to accumulate, without 



