28 MENTAL EVOLUTION IN ANIMALS. 



the co-ordinating power which is secured by this principle of 

 reflex action. 



Of course we may wonder how it is that the nerve-centres, 

 which preside over reflex action, not being endowed with 

 consciousness, know what to do with the stimuli which they 

 receive. The explanation of this, however, is that the ana- 

 tomical arrangement of ganglion and nerves in any particular 

 case is such as to leave no choice or alternative of action, if 

 the apparatus is called into action at all. Thus, to begin at 

 the bottom of the series, in the Medusae the simple ganglia 

 are distributed all round the margin of the animal, and 

 respond by reflex action to the stimuli which are applied at 

 any other part of the surface. This has the effect of increas- 

 ing the rate and the strength of the swimming-movements, 

 and so of enabling the animal to escape from the source of 

 danger. Now, although this is a true reflex action, and has 

 an obvious purpose to serve, it does not involve any co-ordi- 

 nation of muscular movements. For the anatomical plan of 

 a jelly-fish is so simple, that all the muscular tissue in the 

 body is spread out in the form of one continuous sheet ; so 

 that the only function which the marginal ganglia have to 

 perform when they are stimulated into reflex action, is that 

 of throwing into contraction one continuous sheet of muscular 



o 



tissue. 



Hence we may infer that in its earliest stages reflex action 

 is nothing more than a promiscuous discharge of nervous 

 energy by nerve-cells, when they are excited by a stimulus 

 passing into them from their attached nerve-fibres.* But as 

 animals become more highly organized, and distinct muscles 

 are by degrees set apart for the performance of distinct actions, 

 we can readily understand how particular nerve-centres are 

 likewise by degrees set apart to preside over these distinct 

 actions ; the nervous centres then perform the part of trig- 

 gers to the particular muscular mechanisms over which they 

 preside — triggers which can only be loosened by tlie recep- 

 tion of stimuli along their own particular lines of communi- 

 cation, or nerves. Thus, for instance, in the star- fish — animals 

 which are somewhat higher in the zoological scale than the 

 jelly-fish, and which have a more highly developed neuro- 

 muscular system — the ganglia are arranged in a ring round 



* For a full account of reflex action in Medusae, see Phil. Trans., Croonian 

 Lecture, 1875 ; also Fhil. Trans., 1877 and 1880. 



