304 BIOLOGY 



alent to the amount of food consumed in his body during 

 this given time. The difficulties of carrying on such an experi- 

 ment have been great, but they have been surmounted satis- 

 factorily, and the results are always the same. There is an 

 exact equivalence between the income and the outgo of a liv- 

 ing animal, both as to force and matter. The amount of 

 excretion from the individual is exactly equal to the amount 

 of food consumed; and the amount of energy developed is 

 the exact equivalent of the energy contained in the food that 

 he uses during the same experiment. The general conclusion 

 is that the income and the outgo of an animal balance, and that 

 the living machine, like other machines, simply transforms one 

 form of energy into another without creating or destroying it. 

 In this statement no account is made of the energy of the 

 action of the nervous system, which does not show itself in 

 such experiments, the probable reason being that the record- 

 ing apparatus is too coarse to show an amount of energy so 

 slight as that exhibited by the nervous svstem. 



DETAILS OF THE ACTION OF THE MACHINE 



In the running of an ordinary machine, like a steam engine, 

 we understand fairly well the details of its action. We can 

 understand how the forces of chemical affinity break up .the 

 chemical compounds in coal; how the heat thus liberated 

 vaporizes the water; how the water under pressure acts on the 

 piston in the cylinder, and how this produces the revolution 

 of the flywheel. It is true that we do not understand the 

 forces of chemical affinity by which coal burns, but, apart from 

 this, there is nothing mysterious in the fact that the engine 

 converts the stored-up energy contained in the coal into the 

 motion of the flywheel. Is a similar intelligible explanation 

 possible of the activities that go on in the living organism? 

 In other words, do chemical and physical forces suffice to ex- 

 plain the activity of the living machine, just as they do the 

 activity of the non-living machine? 



