10 OUTLINES OF E VOLUTION AKY BIOLOGY 



accompanied by the liberation of energy and resulting in the 

 formation of waste products, substances which, in falling down 

 one side of the triangle to a lower level, to a greater or 

 less extent lose their potential energy. On the other side of 

 the triangle, however, complex food material, containing fresh 

 supplies of potential energy, is supposed to be taken in, or, in 

 the case of green plants, actually built up from simple con- 

 stituents by the energy of the sun's rays, and used in repairing 

 the waste of the protoplasmic body. 



If the constructive processes proceed more vigorously and 

 rapidly than the destructive, if the food supply is abundant and 

 the expenditure of energy comparatively low, the body may 

 grow, though, as we shall see presently, only within certain 

 limits. If the reverse is the case and the expenditure exceeds 

 the income, the body may dwindle away and finally die. If it 

 is to remain in a condition of healthy equilibrium a just balance 

 must be maintained between the two sides of the account. 



Perhaps the most characteristic property of living things is, 

 as we have already suggested, the power of reproduction. This is 

 the last resort of the organism in the struggle for existence. The 

 individual, which owes its very life to the perishable nature of 

 its body, always succumbs to the destructive influences of the 

 environment sooner or later, but before yielding to the inevitable 

 it will, under normal conditions, have produced offspring which 

 will carry on the struggle for another generation. 



The phenomenon of reproduction is intimately associated 

 with that of growth, and may be traced back to the division of 

 a simple ancestral mass of protoplasm into two parts whenever 

 its size increased to such an extent that the ratio of surface to 

 volume became too small for the necessary intercourse between 

 the organism and its environment. With this division of the 

 protoplasmic body the proper proportion is restored, and hence 

 reproduction by multiplication of protoplasmic units may be 

 looked upon as primarily the direct consequence of super- 

 abundant nutrition. 



We have now learnt to look upon an animal or a plant as a 

 complex and extremely delicate piece of mechanism, constantly 

 employed in collecting energy directly or indirectly from the 

 sun's rays and in using that energy to maintain an incessant 

 struggle against the destructive forces of its environment. This 

 incessant getting and spending, winning and losing, constitutes 



