in ORGANIC ADAPTABILITY 33 



In these and no doubt in many other ways that might 

 be specified the machine is made to perform functions 

 which in this or that feature recall the behaviour of a living 

 organism. Such examples are instructive both in their 

 success and in their failure. For the condition of success 

 is a combination of mechanical forces expressly designed 

 to meet the end in view, and this suggests that whatever 

 the creative force that brings the organism into being, the 

 organic structure is a combination of parts determined in a 

 general way by the functions which they will have to per- 

 form in the structure as a whole. They are so arranged 

 as to maintain a certain balance, to regain it from within 

 tolerably wide limits of variation, and even to readjust 

 the principle of the balance itself (/.., to modify the 

 organic structure), if this is demanded by the environment. 

 Any single characteristic in this self-maintaining process 

 can probably be paralleled from the inorganic world ; but 

 whether the process as a whole can be paralleled is quite 

 another question. It seems rather from the illustra- 

 :ions I have given that the thing that resembles an organ- 

 ism in one respect differs in another, and that where this 

 difference is overcome a new one breaks out. 



5. In the most fundamental feature of self-mainten- 

 ance, it may be doubted whether any inanimate substance 

 can be fairly matched against the living organism. It is by 

 suitable response to stimulus in the main that organisms 

 preserve themselves. Now the excitability of living tissue 

 differs from the response of an elastic spring to pressure 

 in that the energy evolved in the response bears no definite 

 relation to the energy of the stimulus. It is interesting 

 to see how Dr. Verworn deals with this point. 



" It can be said in general that irritability is the capacity of a 

 body to react to an external influence by some kind of change in 

 its condition, in which the extent of the reaction stands in no 

 definite proportion to the extent of the influence. As a matter 

 of fact, irritability, or excitability, is a property of all living sub- 

 stance, whether the organism responds to the external influence 

 by the production of definite substances, as with secreting gland- 

 cells, or definite forms of energy, as with muscle-cells, phosphor- 

 escent cells, and electric cells, or whether it responds by depression 

 or even standstill of its vital activities. But irritability is not the 



