The Story of Evolution 77 



Reflex Actions 



Among simple multicellular animals, such as sea-anemones, 

 we find the beginnings of reflex actions, and a considerable part 

 of the behaviour of the lower animals is reflex. That is to say, 

 there are laid down in the animal in the course of its development 

 certain prearrangements of nerve-cells and muscle-cells which 

 secure that a fit and proper answer is given to a frequently 

 recurrent stimulus. An earthworm half out of its burrow 

 becomes aware of the light tread of a thrush's foot, and jerks 

 itself back into its hole before anyone can say "reflex action." 

 What is it that happens? 



Certain sensory nerve-cells in the earthworm's skin are 

 stimulated by vibrations in the earth; the message travels down 

 a sensory nerve-fibre from each of the stimulated cells and enters 

 the nerve-cord. The sensory fibres come into vital connection with 

 branches of intermediary, associative, or communicating cells, 

 which are likewise connected with motor nerve-cells. To these 

 the message is thus shunted. From the motor nerve-cells an 

 impulse or command travels by motor nerve-fibres, one from each 

 cell, to the muscles, which contract. If this took as long to 

 happen as it takes to describe, even in outline, it would not be of 

 much use to the earthworm. But the motor answer follows the 

 sensory stimulus almost instantaneously. The great advantage of 

 establishing or enregistering these reflex chains is that the 

 answers are practically ready-made or inborn, not requiring to 

 be learned. It is not necessary that the brain should be stimu- 

 lated if there is a brain ; nor does the animal will to act, though in 

 certain cases it may by means of higher controlling nerve-centres 

 keep the natural reflex response from being given, as happens, 

 for instance, when we control a cough or a sneeze on some solemn 

 occasion. The evolutionary method, if we may use the expression, 

 has been to enregister ready-made responses ; and as we ascend the 

 animal kingdom, we find reflex actions becoming complicated and 

 often linked together, so that the occurrence of one pulls the 



