4 OUTLINES OF EVOLUTIONARY BIOLOGY 



action of the environment ; hard and rigid metals which will 

 bear heavy strains, and as far as possible such as will be proof 

 against the chemical action of the atmosphere ; and we do our 

 best by means of oil and paint to protect even these from injurious 

 influences. 



A living body may also have its hard protective structures, as 

 the shell of the oyster and the scales of the fish, or its rigid 

 levers, as the bones in our own limbs, but the really essential 

 part of the organism is built up of just those materials which are 

 most liable to destruction by chemical and physical agencies of 

 that almost liquid and extremely unstable substance which we 

 have already referred to as protoplasm, and of its various 

 derivatives. The experience of every day teaches us how 

 rapidly the bodies of animals and plants decay when they are 

 left exposed to the atmosphere after life has become extinct ; and 

 this decay is simply the destruction caused by the disintegrating 

 forces of the environment. A disabled steamboat may lie 

 rusting on the shore for many years without undergoing much 

 change, but the dead body of a stranded jelly-fish thrown up 

 beside it will become disorganized and disappear in the course of 

 a few hours, and yet the jelly-fish when alive was undoubtedly 

 the more complex and perfect piece of apparatus. 



The body of an organism, moreover, undergoes destruction 

 not only after death ; it is always undergoing destruction, and 

 its very life depends upon its destruction, just as the flame of a 

 candle depends upon the destruction of the candle. But as it is 

 destroyed it is constantly built up again; new protoplasm is 

 formed and new tissues take the place of those which are worn 

 out. The life of the organism is, in fact, the outcome of the 

 constant struggle between destructive^ and constructive forces, 

 and the keener the struggle the more vigorous will be the life- 

 just as the flame will be brighter or hotter in proportion to the 

 activity of the combustion to which it owes its existence. 



Life, like the flame, is a manifestation of energy, and tke 

 living body is, like the steam-engine, a machine for transforming 

 one kind of energy into another. Moreover, the ultimate source 

 of the energy is the faame in both cases. That of the steam- 

 engine is derived from the combustion or oxidation of coal, whioii 

 contains stores of energy derived millions of years ago from the 

 light and heat of the sun by the green plants which flourished 

 in the vast forests of the Carboniferous epoch. The green plants 



