190 LIFE AND DEATH. 



action of these two elements that all vital phenomena 

 inevitably result." The environment furnishes the 

 living being with three things: its matter, its 

 energy, and the exciting forces of its vitality. All 

 vital manifestation results from the conflict of two 

 factors : the extrinsic factor which provokes its 

 appearance ; the intrinsic factor, the very organiza- 

 tion of the living body, which determines its form. 

 Bichat and Cuvier saw in the phenomena of life the 

 exclusive intervention of a principle of action entirely 

 internal, checked rather than aided by the universal 

 forces of nature. The exact opposite is true. The 

 protozoan finds the stimuli of its vitality in the 

 aquatic medium which is its habitat. The really 

 living particles of the metazoan that is to say, its 

 cells, its anatomical elements meet these stimuli in 

 the lymph, in the interstitial liquids which bathe 

 them and which form their real external environ- 

 ment. 



Auguste Comte thoroughly understood this truth, 

 and has clearly expressed it in the passage we have 

 just quoted. Claude Bernard has fully developed it 

 and given it its classical form. 



In order to manifest the phenomena of vitality, 

 the elementary being, the protoplasmic being, re- 

 quires from the external world certain favourable 

 conditions ; these it finds there, and they may be 

 called the stimuli, or extrinsic conditions of vitality. 

 This being possesses no initiative or spontaneity in 

 itself, it has only a faculty of entering into action 

 when an external stimulus provokes it. This sub- 

 jtection of the living matter is called irritability. 

 The term expresses that life is not solely an internal 

 attribute, but an internal pjrinc|ple of action. 



