MOVRMKNTS, ETC., OF FKESH-WATER I'LANARIANS. 709 



entirely from any tliat planarians ever nsed before to attain 

 the same end, so far as we have evidence. Tliese ]ihenomena 

 have a eonsi(U'ral)le reseml)liince to sucli phciKHiiena as tlie 

 well-known reo'eneration of the lens from the iris in some 

 Amphiljia. It is not easy to see how snch behavionr comes 

 about, and iiaturiil seh'ction helps ns very little. The matter 

 belono's apparently to the same class of ])henomcnii ;i's 

 morpliological regulations, and probably h:is ultimately the 

 same explanation. What this explanation is we do not know. 



14. Functions of the Nervous System. — The most im- 

 portant function of the brain is the preservation of the tonus 

 of the organism. After its removal the general tonus rapidly 

 diminishes, and on this account the positive reaction — which 

 depends rather closely on the physiological condition — c:in 

 be obtained only with great ditticulty in such deca])itated 

 specimens. There is no evidence of the presence of special 

 centres in the brain. The nervous system, as a Avhole, has 

 its main function in the rapid conduction of impulses. 



15. Subjective Psychic Attributes. — One of the 

 principal questions which forever recurs with regard to work 

 on animal behaviour is, does the animal possess conscious- 

 ness ? Now although it has been shown what the component 

 parts of the activities of the planarian are, yet it cannot be 

 said, as it seems to me, that the planarian does not, or, on the 

 other hand, that it does, possess consciousness. All that any 

 such an organism ever has done in the past, or ever will do 

 in the future, cannot tell us whether it was conscious in the 

 doing or not. Any " objective criterion " of consciousness 

 does not exist. Furthermore, whether consciousness is or is 

 not present in any given case is not, in any event, the greatest 

 concern of the physiologist, who rests content with the objec- 

 tive explanation of how results are brought about, regardless 

 of what the animal is thinking about the matter. On this 

 subject Claparede (: 01, p. 24), in concluding an interesting 

 and valuable discussion, has said, " A la question ; les 

 animaux sont-ils conscients ? la physiologic — et meme la 

 psychologic en tant que cette science est explicative — doivent 



