2 THE MECHANISM OF LIFE 



Some modern physiologists regard sensibility, others 

 irritability, as the characteristic of life, and define life as the 

 faculty of responding, by some sort of change, to an external 

 stimulus. As in the case of movement, we have found by 

 more attentive observation that this faculty also is universal 

 in nature. There is no action without reaction; an elastic 

 body repels the body that strikes it. Every object in nature 

 dilates with heat, contracts with cold, and is modified by the 

 light which it absorbs. Everything in nature responds to 

 exterior action by a change, and hence this faculty cannot be 

 the characteristic of life. 



A distinguished professor of physiology was accustomed to 

 teach that the disproportion between action and reaction was 

 the characteristic of life. " Allow a gramme weight to fall on 

 a nerve, and the muscle will raise a weight of ten grammes. 

 This disproportion is the characteristic of life." Hut there is a 

 much greater disproportion between action and reaction when 

 the friction of a match blows up a powder factory, or the 

 turning of a switch lights the lamps and animates the tram- 

 ways and the motors of a great city. The disproportion 

 between action and reaction is therefore no characteristic 

 of life. 



The essential characteristic of life is often said to be 

 nutrition — the phenomenon by which a living organism 

 absorbs matter from its environment, subjects it to chemical 

 metamorphosis, assimilates it, and finally ejects the destructive 

 products of metamorphosis into the surrounding medium. 

 Hut this characteristic is also common to a great number of 

 ordinary chemical reactions, so that we cannot call it peculiar 

 to life. Consider, for instance, a fragment of calcium 

 chloride immersed in a solution of sodium carbonate. It 

 absorbs the carbonic ion, incorporates it into a molecule of 

 calcium carbonate, and ejects the chlorine ion into the 

 surrounding medium. 



It may be argued that this is merely a chemical process, 

 since the substance which determines the reaction is also 

 modified, the chloride of calcium changing into carbonate of 

 calcium. But every living thing is also changing its chemical 



