SIO MENTAL FACULTIES ' sec. 



important facts, to assume, as the botanists do, that irritability 

 and sensation are equivalent (identical) ; the very fact that 

 in animals a special nervous system, nerve-fibres and nerve- 

 cells, have arisen, which are absent in plants, proves of itself 

 that the motions which constitute the reception and conduc- 

 tion of stimuli in animals must be quite different to those in 

 plants, for the stimuli are, as we shall see, the causes of the 

 origin of the nervous system. IN'ervous substance is something 

 peculiar to animals. It is true there are animals which possess 

 the power of voluntary action, and therefore sensation — the 

 latter is a necessary antecedent of the former — which yet 

 possess no morphologically demonstrable nerves, e.g. the lowest 

 multicellular and the unicellular forms. But here, as was 

 previously argued, portions of the protoplasm must act, with 

 regard to the stimuli they receive or conduct, as nervous 

 substance — only these parts are not yet externally recognis- 

 able as nervous paths. For the single reason, then, that the 

 substance which reacts to stimuli in animals, which in any 

 case in the higher animals is the instrument of sensation and 

 voluntary action, is quite peculiar to animals, it is allowable 

 to infer that the two latter faculties belong to animals only. 

 We might thus in a certain sense recur to the old aphorism 

 of Linnaeus : " Lapides crescunt, plantte crescunt et vivunt, 

 animalia crescunt, vivunt et sentiunt." 



In fact, the division of the organic world, according to the 

 evidence of the capacity for receiving "nerve-stimuli" and 

 acting upon them, is still that which should be first estab- 

 lished. The Linn^ean proposition, interpreted as meaning that 

 all animals, in contrast to plants, are capable of sensation 

 and the exercise of will, would therefore certainly be more 

 justified than the doctrine of the will and sensation of atoms. 

 But I attribute to animals merely the special faculty of 

 reacting to nerve stimuli. This does not imply that they all 

 are capable of sensation and will. It is an evident fact, and 



