706 RAYMOND PEARL. 



reference to the location of the stimuhis. Again, the Infu- 

 soria do not show qualitatively different reactions to differing 

 intensities of stimuli, as does PI an aria to a marked degree. 

 We thus see that Planaria stands considerably higher in 

 the psychological scale than the Infusoria, and that the 

 development is taking place along two main lines : (a) the 

 higher organism reacts differentially with reference to the 

 location and intensity of the stimulus ; and (b) the physio- 

 logical balance in the higher organism is much more deli- 

 cately adjusted than in the lower, and as a consequence we 

 see much more variation in the physiological condition. 

 These variations in the ]3hysiological condition bring about 

 variability in the reactions. 



In the case of the ctenophore Mnemiopsis Leidyi we 

 have an intermediate stage between the Infusoria and 

 Planaria, Here the animal reacts with reference to the 

 position, but not the intensity of the stimulus. This condi- 

 tion, in which an organism reacts with relation to the position 

 of a stimidus, and not to its intensity, must be for the indi- 

 vidual a precarious one, because the animal must either go 

 towards or away from all stimuli alike, whether good or 

 harmful. Chances are theoretically equal that after each 

 stimulus it may get a toothsome morsel of food, or, on the 

 contrary, serve in that capacity itself. Further development 

 beyond the point in the behaviour series where Planaria 

 stands must be in the line of further differential reactions 

 with reference to quality of stimulus. A beginning along 

 this line is made by the planarian, and the process is carried 

 a step farther in the case of G onion em us, as recently 

 described by Yerkes (loc. cit.). 



12, llelation of Behaviour and Structure. — The reac- 

 tions of organisms are evidently, in any case, very closely de- 

 pendent on the structural relations of the given organism, and 

 on the conditions under which it lives, i, e, its environment 

 in the broadest sense. Thus we find the asymmetrical Infu- 

 soria, which live freely in the water and move about by means 

 of cilia, all reacting in the same way, and the determinative 



