SOUKCE OF ENEEGY OF OKGANISMS 5 



of to-day still derive supplies of energy from the same source 

 and lock up in their leaves and stems what they do not them- 

 selves expend, while the animals in turn derive their energy 

 from the green plants upon which they directly or indirectly 

 feed. Hence both plants and animals are ultimately dependent 

 upon the sun for their existence. 



Even the most superficial examination is sufficient to demon- 

 strate the fact that the body of any of the more familiar animals 

 or plants is, as we have already indicated, an extremely complicated 

 thing. Whatever may be the degree of complexity, however, and 

 however much one organism may differ from another in details 

 of structure, whether it be a microscopic alga or an oak tree, an 

 Amoeba or a man, there are always certain things which have to 

 be done, certain actions or functions which have to be performed, 

 in order that its life may be maintained. 



In the first place, the organism must safeguard itself as far as 

 possible from the destructive influences of its environment. It 

 must not only be able to protect itself from such physical agents 

 as heat and cold, mechanical impact and friction, but it must be 

 able to select a situation where life is possible, and to escape 

 from other organisms by which it is liable to be attacked. All 

 this involves the expenditure of energy in some form or another ; 

 it may be in the manufacture or secretion of protective envelopes 

 or shells, such as we find even in some of the simplest Protozoa, 

 or it may be in actively moving away from the source of danger. 

 Thus it appears that the very first thing necessary for the 

 maintenance of life is the expenditure of energy. This 

 energy, though ultimately derived from the sun, is, as we have 

 already seen, derived immediately from the combustion of 

 fuel, very much as in the case of a steam-engine, but with the 

 important difference that in the living organism the fuel is, at 

 any rate to a large extent, the actual substance of which the 

 body is composed. In this respect the comparison with a candle 

 is especially apt, for it is the combustion of the actual substance 

 of which the candle is composed that liberates the energy 

 manifested in the light and heat of the flame. 



Now combustion is, of course, simply another name for what 

 chemists term oxidation, or combination with the element 

 oxygen, a process which is often accompanied by the liberation 

 of a considerable amount of energy in the form of heat and 

 light, though this is by no means always the case. In the 



