vi MOVEMENTS CAUSED BY STIMULI 311 



one that corresponds with the whole evolution of the nervous 

 system, that multicellular animals act more and more by 

 reflex action the lower their position. Voluntary action 

 must in its origin be traced back to reflex action. It is a 

 question, indeed, whether the movements of the lower multi- 

 cellular animals, like the sponges, are not exclusively reflex. 1 

 The same holds for the unicellular, in which paths in the 

 protoplasm must supply the place of nerves. 



But it matters not whether we explain the power of 

 sensation as a property of all animals as distinguished from 

 plants, or as a property which first appears in the animal 

 kingdom it is even on the first supposition not a funda- 

 mental property of protoplasm, but something which has 

 been gradually acquired. 



\ But how is it to be explained? If we regard all the 

 effects of stimulation as motion, we are naturally led to 

 explain sensation as a special quality of this motion, a 

 quality indeed which' resides in certain nuclei, in the higher 

 animals in the nuclei of the ganglionic cells, an " excitation " 

 the manifestation of which constitutes consciousness, the 

 general sensation of the whole organism. 



The fundamental property of protoplasm is (I repeat it), 

 not sensation and voluntary action, but irritability (response 

 to stimulation). From that property are evolved by acquisi- 

 tion and inheritance the irritability of nerves in animals, and 

 the power of sensation and will. 



I leave it, therefore, undecided whether the latter faculties 

 belong to all animals. But in any case, a number of the 

 phenomena of movement in lower organisms, like those of 

 spermatozoa, which seem to be voluntary, are, according to 

 my investigations, nothing but the result of definitely-directed 



1 Although the actions of the free-swimming larvae of these animals might be 

 properly ascribed to volition, yet the whole activity of the adult sponge might be 

 reflex, as a consequence of degeneration of the nervous faculties, which occurs 

 often to such an extraordinary extent in other animals. 



