CHAPTER IV. 



THE TWOFOLD CONDITIONING OF VITAL 

 PHENOMENA. IRRITABILITY. 



Appearance of internal activity of the living being— Vital 

 phenomena regarded as a reaction of the ambient world. — 

 § I. Extrinsic conditions — The optimum law. — § 2. Intrinsic 

 conditions — The structure of organs and apparatus — How 

 experiment attacks the phenomena of life. Generalization 

 of the law of inertia — Irritability. 



Instability. Mutability. The Appearance of Internal 

 Activity of the Living Being. — One of the most 

 remarkable characteristics of the Hving being is its 

 instability. It is in a state of continual change. The 

 simplest of the elementary beings, the plastid, grows 

 and goes on growing and becoming more complex, 

 until it reaches a stage at which it divides, and thus 

 rejuvenated it commences the upward march which 

 leads it once again to the same segmentation. Its 

 evolution is thus betrayed by its growth, by the 

 variations of form which correspond to it, and by 

 its division. 



If it be a question of beings higher in organiza- 

 tion than the cellular element the evolutionary 

 character of this mutability becomes more obvious. 

 The being is formed, it grows ; then in most cases, 

 after having passed through the stages of youth and 

 adult age, it grows old, declines and dies, and is 



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