82 SCIENCE IN THE LAST HALF CENTURY. 



The doctrine of the conservation of energy, which I have endeavored 

 to illustrate, is thus defined b;^' the hite Clerk Maxwell: 



"The total energy of any body or system of bodies is a quantity 

 which can neither be increased nor diminished by any mutual action of 

 such bodies, though it may be transformed into any one of the forms of 

 which energy is suscei)tible." It follows that energy, like matter, is in- 

 destructible and ingenerable in nature. The phenomenal world, so far 

 as it is material, expresses the evolution and involution of energy, its 

 passage from the kinetic to the potential condition and back again. 

 Wherever motion of matter takes place, that motion is ejected at the 

 expense of part of the total store of energy. 



Hence, as the phenomena exhibited by living beings, in so far as they 

 are material, are all molar oi molecular motions, these are included un- 

 der the general law. A living body is a machine by which energy is 

 transformed in the same sense as a steam-engine is so, and all its move- 

 ments, molar and molecular, are to be aciioanted lor by the energy which 

 is sui)plied to it. The phenomena of consciousuess which arise, along 

 with certain transformations of energy, can not be interpolated in the 

 series of these transformations, inasmuch as they are not motions to 

 which the doctrine of the conservation of energy applies. And for the 

 same reason, they do not necessitate the using up of energy ; a sensa- 

 tion has no mass and can not be conceived to be susceptible of move- 

 ment. That a particular molecular motion does give rise to a state of 

 consciousness is experimentally certain ; but the how and why of the 

 process are just as inexplicable as in the case of the communication of 

 kinetic energy by impact. 



When dealing with the doctrine of the ultimate constitution of mat- 

 ter, we found a certain resemblance between the oldest speculations and 

 the newest doctrines of physical i)hilosopliers. But there is no such 

 resemblance between the ancient and modern views of motion and its 

 causes, except in so far as the conception of attractive and repulsive 

 forces may'be regarded as the modified descendant of the Aristotelian 

 conception of forms. In fact, it is hardly too much to say that the es- 

 sential and fundamental difference between ancient and modern phys- 

 ical science lies in the ascertainment of the true laws of statics and 

 dynamics in the course of the last three centuries; and in the invention 

 of mathematical methods of dealing with all the consequences of these 

 laws. The ultimate aim of modern physical science is the deduction of 

 the phenomena exhibited by material bodies from physico-mathematical 

 first principles. Whether the human intellect is strong enough to at- 

 tain the goal set before it may be a question, but thither will it surely 

 strive. 



3. EVOLUTION. 



The third great scientific event of our time, the rehabilitation of the 

 doctrine of evolution, is part of the same tendency of increasing kuowl- 



