658 



RAYMOND PEAKL. 



From this close parallelism we must conclude, 1 tliiuk, that 

 iu the behaviour of planariaus the ciualitative character of a 

 stimulus is of little importance in comparison with its quanti- 

 tative relations. Or, to express it differently, to all stimuli 

 which are of low intensity the flat-worm gives the positive 

 reaction, while to stimuli which are of high intensity it gives 

 a negative response. This sort of behaviour will at once be 

 seen to be, iu the long run, purposive, and is, further, of a 

 kind which might very well have been developed by the 

 action of natural selection. In the long run the planarian's 

 reactions will take it away from injurious substances and 

 into favourable surroundings. 



These results on chemicals are interesting in connection 

 with the work so much done in recent times on the specific 



