THE WONDERS OF LIFE 



Since we regard the whole of organic life as, in the 

 ultimate analysis, merely a very elaborate chemical 

 process, we shall quite expect that chemical stimuli are 

 the most important factors in sensation. And this is so 

 in point of fact; from the simplest moneron up to the 

 most highly differentiated cell and on to the flower in 

 the plant and the mental life of man, the vital processes 

 are dominated by chemical forces and conversions of 

 energy, which are set in play by external or internal 

 chemical stimuli. The excitation which they produce 

 is called, in a general way, "sensation of matter" or 

 chemaesthesis ; the basis of it is the mutual relation of 

 the chemical elements which we describe as chemical 

 affinity. In this affinity we have the play of attractive 

 forces which lie in the nature of the elements them- 

 selves, especially in the peculiar properties of their con- 

 stituent atoms; and this cannot be explained unless 

 we ascribe unconscious sensation (in the widest sense) 

 to the atoms, an inherent feeling of pleasure and the 

 reverse, which they experience in the contact of other 

 atoms (the "loves and hatreds of the elements" of 

 Empedocles). 



The numbers of different stimuli that act chemically 

 on the plasm and excite its "sensation of matter" may 

 be divided into two groups — external and internal 

 stimuli. The latter lie within the organism itself, and 

 cause the internal "organic sensations"; the former are 

 in the outer world, and are felt as taste, smell, sex- 

 impulse, etc. In the higher animals special chemical 

 sense-organs have been developed for these chemical 

 stimuli. As these are well known to us from our own 

 human experience, and comparative physiology shows 

 us the same structures in the higher animals, we will 

 deal first with them. In general the same law holds for 

 these external chemical stimuli as for optical and 

 thermic stimuli; we can recognize a maximum limit of 



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