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. 
