CHAPTER VI 

 CONCLUSIONS 



Summary. While we cannot define life in a physical 

 sense, for the reason that we have no measure of the 

 psychic phenomena shown by living things, and these 

 psychic phenomena are, after all, the most important 

 of the characteristics of life, there are nevertheless certain 

 phenomena associated ahyays with the living processes 

 which are so characteristic that for the majority of 

 organisms with which we are familiar we have no diffi- 

 culty in determining whether they are living or dead. 

 Irritability is the universal sign of life, and by it living 

 matter adjusts itself to its environment. The sign 

 of this irritability is the functional power of the tissues. 

 Thus by measuring the functional power we can speak 

 of measuring the amount of irritability. The changes 

 of a physical or chemical kind which accompany this 

 functioning are very important for an understanding of 

 the living process, for when we know them completely we 

 shall probably understand the nature of irritability itself. 



In chapter ii we showed how it happened that, because 

 of the apparent exception in the case of nerves, it has 

 been generally concluded that chemical changes could 

 not be considered to be essential to all living processes. 

 Some of these changes, it appeared, must be due solely 

 to physical processes, and for this reason irritability 

 had come to be regarded as a purely physical phenome- 

 non. Various hypotheses had been made to explain how 



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