156 LIFE AND DEATH. 



The community of the phenomena of vitality in 

 animals and plants being thus placed beyond a doubt, 

 we must now discover the reason why. This reason 

 is to be found in their anatomical and in their 

 chemical unity. The fundamental phenomena are 

 common because the composition is common, and 

 because the universal anatomical basis, the cell, 

 possesses in all cases a sum total of identical 

 properties. 



If we appeal to physiology for the characteristics 

 common to living beings, it will generally give us the 

 following : — A structure, or organization ; a certain 

 chemical composition which is that of living matter ; 

 a specific form; an evolution which in the earliest 

 stage occasions the being to grow and develop until 

 it is divided, and which in the highest stage includes 

 one or more evolutive cycles with growth, the adult 

 stage, senility, and death ; a property of increase or 

 nutrition, with its consequence — namely, a relation of 

 material exchanges with the ambient medium ; — and 

 finally, a property of reproduction. It is important 

 to pass them rapidly in review. 



