350 THEORETICAL BIOLOGY 



tion of the actions pertaining to these. For both these tasks 

 discipline is above all necessary. For the rest, the super- 

 mechanical principle of the community is expressed in those 

 plastic actions of its members endowed with protoplasm, 

 which form the community and maintain it. 



THE LIFE-ENERGY 



The comparison of the organisation of the community 

 with that of the body of the living creature has revealed a 

 pervading property which is determinative for all living 

 organisms — the presence of functioning individualities or 

 " indivisible units ", having their own irritability, their own 

 conduction of excitation (or steering), and producing their 

 independent effect. 



Each individual well deserves this name, for only so long 

 as it contains the undivided, tripartite chain, can it fill its 

 place as living member of an organism. 



An individual may consist of one cell, or of an association 

 of cells. In the latter case, the single cells have so far fitted 

 themselves into a whole of a higher order that they have 

 specially developed a portion of their function-chain in the 

 interests of the whole, without, however, surrendering the 

 other portions. The sensory cells have had to develop 

 especially their receptor part, the nerve-cells their conducting 

 or steering part, and the muscle-cells and gland-cells their 

 effector part, in order that, for instance, a reflex-arc may be 

 formed. The nerve-fibres, which conduct the excitation 

 further in one direction, have the simplest kind of steering, 

 which permits the excitation to proceed always along the 

 same routes ; while the nerve-cells of the centre are able to 

 conduct it now into this outgrowth and now into that. It 

 is immaterial whether we speak of steering as a complicated 

 conduction, or of conduction as a simplified steering. Both 



