228 ZOOLOGICAL PHILOSOPHY 



ing them, their consecutive shrinkage and restoration throw them 

 momentarily into a condition of quivering. 



Thus a sudden contraction of the part affected, followed by an equally 

 sudden distension which restores the part to its original condition, 

 constitutes the local phenomenon of irritability. 



The production of this phenomenon does not need any special 

 organ ; for the state of the parts and the instigating cause are sufficient 

 in themselves ; it is in fact observed in the simplest of animal organisa- 

 tions : moreover the impression giving rise to the phenomenon is not 

 conveyed by any special organ to a centre of communication or nucleus 

 of activity ; the whole process is confined to the immediate site of 

 the impression ; every point of the surface of irritable parts is capable 

 of producing it and of repeating it always in the same way. This 

 phenomenon is obviously quite different from that of sensations. 



From these principles it will be clear that orgasm is the source from 

 which irritabiUty arises ; but this orgasm exhibits very different 

 degrees of intensity, according to the nature of the bodies in which it 

 is produced. 



In plants, where it is ill-defined and devoid of energy, and where it 

 works extremely slowly in causing the shrinkages and distensions of 

 the parts, it has no power to produce irritability. 



In animals on the contrary, where orgasm is highly developed on 

 account of the nature of their body-substance, it rapidly produces 

 the contractions and distensions of the parts on the stimulus of the 

 exciting causes ; in them it constitutes a marked irritability. 



Cabanis, in his work entitled : Rapports du physique et du moral 

 de I'homme, has endeavoured to prove that sensibility and irritabihty 

 are phenomena of the same nature and have a common origin (Histoire 

 des sensations, vol. i., p. 90) ; his intention no doubt was to reconcile 

 what we know of the most imperfect animals with the ancient and 

 universally received belief that all animals without exception possess 

 the faculty of feeling. 



The arguments adduced by this savant for showing the identity of 

 nature between feeling and irritability, appear to me neither clear nor 

 convincing ; hence they do not affect the following propositions by 

 which I distinguished these two faculties. 



Irritability is a phenomenon peculiar to animal organisation, 

 requiring no special organ, and continuing to subsist some time 

 after the death of the individual. This faculty may exist just the same 

 whether there are or are not any special organs, and it is therefore 

 universal for all animals. 



Sensibility on the contrary is a phenomenon peculiar to certain 

 animals ; it can only be manifested in those which have a special 



