THE WONDERS OF LIFE 



The great advance made by the comparative anatomy 

 and experimental physiology of animals and plants in 

 the first half of the nineteenth century brought to light 

 the fact that irritability or sensibility is a common 

 quality of all organisms, and that it is one of the princi- 

 pal characteristics of vital force (cf. chapter ii.). The 

 greatest merit in connection with its experimental study 

 attaches to the famous Johannes Mtiller. In his classi- 

 cal Manual of Human Physiology (1840) he established 

 his theory of the specific energy of the nerves and their 

 dependence on the sense-organs on the one hand and 

 the mental life on the other. He devoted the fifth chap- 

 ter of his book to the former and the sixth to the latter, 

 approaching particularly to Spinoza in his general psy- 

 chological views; he treated psychology as a part of 

 physiology, and thus put on a sound scientific basis that 

 naturalistic conception of the place of psychology in the 

 biological system which we now regard as the correct 

 view. At the same time he proved that sensation is 

 a function of the organism as much as movement or 

 nutrition. 



The view of sensation that prevailed in the second half 

 of the nineteenth century was very different. On the 

 one hand the experimental and comparative physiology 

 of the sense-organs and the nervous system immensely 

 enriched our exact knowledge by the invention of in- 

 genious methods of research and the use of the great 

 advance made by physics and chemistry. The famous 

 investigations of Helmholtz and Hertwig on the physics 

 of the senses, of Matteucci and Dubois-Reymond on the 

 electricity of the muscles and nerves, and the great 

 progress made in vegetal physiology by Sachs and 

 Pfeffer, and in physiological chemistry by Moleschott 

 and Bunge, enabled us to realize that even the most 

 mysterious of the wonders of life depend on physical 

 and chemical processes. By the application of the dif- 



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