INTRODUCTION. 23 



can no longer hold its store of energy, which is 

 at once liberated in its active form as heat. The 

 engine then takes the energy thus liberated, and 

 as a result of its peculiar mechanism converts it 

 into the motion of its great fly-wheel. With this 

 notion clearly in mind the question forces itself 

 to the front whether the same facts are not true 

 of the living animal organism. It, too, is fed 

 with food containing a store of energy; and 

 should we not regard it, like the steam engine, 

 simply a machine for converting this potential 

 energy into motion, heat, or some other active 

 form ? This problem of the correlation of vital 

 and physical forces is inevitably forced upon us 

 with the doctrine of the correlation of forces. 

 Plainly, however, such questions were incon- 

 ceivable before about the middle of the present 

 century. 



This mechanical conception of living activity 

 was carried even farther. Under the lead of 

 Huxley there arose in the seventh decade of the 

 century a view of life which reduced it to a pure 

 mechanism. The microscope had, at that time, 

 just disclosed the universal presence in living 

 things of that wonderful substance, protoplasm. 

 This material appeared to be a homogeneous 

 substance, and a chemical study showed it to be 

 made of chemical elements united in such a way 

 as to show close relation to albumens. It appeared 

 to be somewhat more complex than ordinary 

 albumen, but it was looked upon as a definite 

 chemical compound, or, perhaps, as a simple 

 mixture of compounds. Chemists had shown 

 that the properties of compounds vary with their 



