TRESIDENT S ADDRESS. 7 



stimulus and reaction (on which the indirectness of the response depends) 

 must have the fullest value allowed it as a characteristic of living 

 creatures. 



For the zoological side of biology a view similar to that of Pfeffer has 

 been clearly stated by Jennings ' in his admirable studies on the beha- 

 viour of infusoria, rotifers, &c. He advances strong arguments against 

 the theories of Loeb and others, according to which the stimulus acts 

 directly on the organs of movement ; a point of view which was formerly 

 held by botanists, but has since given place to the conception of the 

 stimulation acting on the organism as a whole. Unfortunately fur 

 botanists these movements are by the zoologists called iropitims, and are 

 thus liable to be confused with the geotropism, heliotropism, &c., of plants: 

 to these movements, which are not considered by botanists to be due to 

 direct action of stimuli, Loeb's assumptions do not seem to be applicable. 



Jennings's position is that we must take into consideration what he 

 calls 'physiological state, i.e., "the varying internal physiological conditions 

 of the organism, as distinguished from permanent anatomical conditions." ' 

 Though he does not claim novelty for his view, I am not aware that it has 

 ever been so well stated. External stimuli are supposed to act by altering 

 this physiological state ; that is, the organism is temporarily transformed 

 into what, judged by its reactions, is practically a different creature. 



This may be illustrated by the behaviour of Stentor, one of the fixed 

 infusoria."^ If a fine jet of water is directed against the disc of the 

 creature, it contracts ' like a flash ' into its tube. In about half a minute 

 it expands again and the cilia resume their activity. Now we cause the 

 current to act again upon the disc. This time the Stentor does not con- 

 tract, which proves that the animal has been in some way changed by the 

 first stimulus. This is a simple example of ' physiological state.' When 

 the Stentor was at rest, before it received the first current of water, 

 it was in state 1, the stimulus changed state 1 into state 2, to which 

 contraction is the reaction. When again stimulated it passed into 

 state 3, which does not produce contraction. 



W^e cannot prove that the contraction which occurred when the 

 Stentor was first stimulated was due to a change of state. But it is a 

 fair deduction from the result of the whole experiment, for after the 

 original reaction the creature is undoubtedly in a changed state, since it 

 no longer reacts in the same way to a repetition of the original stimulus. 



Jennings points out that, as in the case of plants, spontaneous acts 

 are brought about when the physiological state is changed by unknown 

 causes, whereas in other cases we can point to an external agency by 

 which the same result is effected. 



' H. S. Jeunings, Cuntrihiitioiis to the Study of the Behavior of the Lower 

 Organisms. Carnegie Institution, 1904, p. 111. 



' Jennings, Behavior of the Lower Organisms, 1906, p. 170, 



