MOVEMKNTS, ETC., OF FRESH-WATER PLANA RIANS. 709 



entirely from any tliat jilanarians ever used before to attain 

 the same end, so far as we have evidence. These phenomena 

 have a considerable resemblance to such phenomena as the 

 well-known regeneration of the lens from the iris in some 

 Amphibia. It is not easy to see how such behaviour comes 

 about, and natural selection helps us very little. The matter 

 belongs apparently to the same class of phenomena as 

 morphological regulations, and probably has 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 — can 

 be obtained only with great difficulty in such decapitated 

 specimens. There is no evidence of the presence of special 

 centres in the brain. The nervous system, as a whole, 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 woi'k 

 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 t^ll 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 arc 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 jihysiologie — et memo In 

 psychologic en taut (|uc cette science est explicative — ddivenl 



