184 THE STORY OF THE LIVING MACHINE. 



not as yet be measured, and have not yet been 

 shown to be correlated with physical energy. In 

 other words, it has not yet been proved that 

 mental force is energy at all ; and if it is not 

 energy, then of course it can not be included in 

 the laws which govern the physical energy of 

 the universe. Although a close relation exists 

 between physical changes in the brain cells and 

 mental phenomena, no further connection has yet 

 been drawn between mental power and physical 

 force. All other secondary phenomena, however, 

 are intelligently explained by the action of natu- 

 ral forces in the machinery of the living organ- 

 ism. 



While we have thus found that the secondary 

 phenomena of life are intelligible as the result of 

 the structure of the machine, certain other fun- 

 damental phenomena have been constantly forc- 

 ing themselves upon our attention as a founda- 

 tion of these secondary activities. The power of 

 contraction, the power of causing certain kinds 

 of chemical change to occur which result in me- 

 tabolism, the property of sensibility, the property 

 of reproduction these are fundamental to all liv- 

 ing activity, and are, after all, the real phenom- 

 ena which we wish to explain. But these are not 

 peculiar to the complicated machines. We can 

 discard all the apparent machinery of the animal 

 or plant and find these properties still developed 

 in the simplest bit of living matter. To learn 

 their significance, therefore, we have turned to 

 the study of the simplest form of matter in which 

 these fundamental properties are manifested. 

 This led us at once to the study of the so-called 

 protoplasm, for protoplasm is the simplest known 

 form of matter that is alive. Protoplasm itself 



