274 NATURE AND MAN. 



from one foot to another, without anything more than a slight 

 and transient disturbance of his equiUbrium, ceases to perceive 

 what has become monotonous by the frequency of its repetition ; 

 and it is only when his equilibrium happens to be more seriously 

 disturbed by a slip of his foot or a stumble over an unnoticed 

 obstacle, that he becomes aware of the constant control exercised 

 over his automatic movements by this delicate regulating balance. 

 All these facts distinctly point to a reflex action of the gang- 

 lionic centres of the organs of special sense, as the mechanism 

 by wliich impressions on those organs call forth and direct the 

 instinctive actions of the lower animals ; and, as we shall presently 

 see, they harmonize completely with the results of experiments 

 made upon the higher. Whether it be alone the " motion of 

 molecules" (or physical change of any kind) that excites the 

 respondent movement, or whether the sight, sound, smell, or other 

 affection of the consciousness by the object which attracts or 

 repels the insect, be a necessary link in the chain of sequences, is 

 a question which seems to me to have no essential bearing upon 

 the automatism of man ; since the appeal to our own experience 

 evokes the unhesitating response, that in him, at any rate (as pre- 

 sumably in the animals that most nearly approach him in structure), 

 the higher forms of activity can only be excited in the first instance 

 through the consciousness, though they too may become automatic 

 bv frequent repetition. The essential difference between what we 

 are accustomed to term the instinctive actions of insects, and the 

 smiply reflex movements which we have seen to be executed by 

 their headless trunks, or even by segments of those trunks, consists 

 in their greater complexity and variety, and in the special con- 

 trolling and directing power of the cephalic ganglia \ and this may 

 be equally exerted, whether the excitement of sensation (i) be a 

 necessary link in the chain of sequences ; or (2) be simply a con- 

 comitant, which must occur when the mechanism is in complete 

 working order ; or (3), as some maintain, is not really produced 

 by impressions transmitted by the afi"erent nerves to the cephalic 

 ganglia, any more than it is by the impressions which excite the 

 separated ganglia of the ventral cord to reflex action. The first 

 having been my former opinion, I was led to distinguish the actions 

 automatically excited through the cephalic ganglia as sensori-molor ; 



