6 OUTLINES OF EVOLUTIONARY BIOLOGY 



higher animals sufficient heat is evolved to maintain the 

 temperature of the body at a level considerably above that of the 

 surrounding atmosphere, and such animals are accordingly 

 termed " warm-blooded " ; in plants, on the other hand, and in 

 the " cold-blooded " lower animals, the amount of heat evolved 

 is not as a rule sufficient to raise the body temperature to any 

 great extent, if at all. Heat, however, is only one form in which 

 energy may be manifested, and in living organisms it is, as a 

 matter of fact, much more conspicuously manifested in the form 

 of motion, especially in animals, while in not a few cases even a 

 low temperature combustion may liberate energy in the form of 

 light, as in the glow-worm and numerous other luminous 

 animals and plants. 



When a piece of charcoal is burnt in the air it enters into 

 combination with the elementary oxygen gas of the atmosphere, 

 and another invisible gas which we term carbon dioxide, or 

 carbonic acid, is produced, in accordance with the equation 



C + 2 C0 2 



(Carbon) (Oxygen) (Carbon Dioxide). 



In this process energy is set free in the form both of heat and 

 light. We must now inquire a little more carefully whence this 

 energy really comes, for although this is a question primarily 

 for the chemist and physicist it is also clearly one which the 

 biologist cannot afford to leave unanswered. 



In accordance with the principles of the conservation of 

 energy and the indestructibility of matter we believe that the 

 quantities of energy and matter which exist in the universe 

 are fixed and constant. Neither energy nor matter can be 

 created and neither can be destroyed, though each may express 

 itself in a great variety of ways and change more or less readily 

 from one mode of expression to another. Thus, as we have 

 already seen, the energy of the sun's rays may be utilized in 

 building up the bodies of green plants, and locked up, as it were, 

 in the substances of which these are composed. 



We also know that different chemical elements have a very 

 strong " affinity " for one another, their atoms tending to 

 unite and form compound molecules when they are brought 

 within the sphere of each other's attraction. Once united 

 they can only be separated again by the expenditure of energy, 

 and when they unite a corresponding amount of energy is 



