IRRITABILITY AS A SIGN OF LIFE 9 



purely physical conditions, and this in spite of the fact 

 that this point of view is obviously incomplete, if not 

 fundamentally erroneous, because the source of all 

 energy of living things is chemical. The chief reason, 

 as has been stated, for the adoption of this hypothesis 

 was the fact that the most irritable tissue nervous 

 tissue had shown no sign of chemical changes when it 

 functioned, and also the fact that some other living 

 tissues, such as seeds, seemed to maintain their vitality 

 without any chemical change. The erroneous character 

 of this view will be apparent from what follows, where 

 it is shown that chemical changes are, indeed, an inva- 

 riable accompaniment of nervous activity, and of all life. 



