224 INITIATIVE IN EVOLUTION 



still allows, and gradually fall into larger groups which follow 

 somewhat indefinite paths. As they move further and further 

 towards home they are seen to follow one another in single file on 

 some score or more of clearer paths, and finally converge into one 

 well-beaten and broad path until they descend the northern slope 

 and pass out by a single roadway into which a gate opens, and so 

 reach the haven where they would be. Here one has a simple 

 picture of the common stimuli of the skin, at first indefinite and 

 ineffectual, by their cumulative action producing an individual 

 receptor and its nerve connection with the central system. 



Professor Leonard Hill 1 also gives a view of the general action 

 of the nervous system and compares it to control of the police force. 

 He supposes a murder to have been committed in a village, and 

 that the local policeman telegraphs to the local town ordering the 

 roads to be searched. The policeman is the tactile sense-organ, 

 the telegraph wire is the sensory nerve, the telegraph office in the 

 local town is the spinal cord, from this office a message is sent to 

 the town police-station by another wire and the police are set in 

 motion. The police are the muscles, the wire that sets them in 

 motion in the motor nerve. The message is also sent to neighbouring 

 towns and to London, that is to say, other local offices (parts of the 

 spinal cord) and the head office (the brain) are informed of the 

 crime or sensory impulse. The central office in London directs 

 the operations controlling the local police office. The whole order 

 of events need not be here described because it goes beyond my 

 immediate purpose, but it is enough to say that attached to the 

 head office are the cleverest detectives (higher sense-organs) and 

 in these are kept records of past crimes, lines of action of the police, 

 and success or non-success of their investigations. 



Following on this picture he speaks of the way in which con- 

 scious actions become automatic and makes a statement to the 

 effect that " There is evidence to show that the axons (or processes 

 of the nerve-cells which extend unbroken from nerve-cell to its 

 termination) become covered with a medullated coat as each new tract 

 is formed. Thus the structure, like the habit, becomes fixed " — and — 

 " It would appear as if, by repeated experiences, tracts and pathways 

 must be beaten through the nervous system V 1 (Italics not in original). 



Beside this I place a statement from Professor Graham Kerr 

 as to his view of the development of peripheral nerve-trunks. He 

 is reviewing the " outgrowth " theory of His, the " chain cell " 

 theory of Balfour, and the " Primitive Continuity " theory of 



1 Manual of Human Physiology. Leonard Hill, p. 369. 



2 Op. cit., p. 371. 



