12 PRESIDENTS ADDRESS. 



rcactiuii. As the carmine stimulus is continued or repeated, state 13 is pw- 

 duced, to which the Stentur reacts by bending to one side. After several 

 repetitions of the .stimulus, state C is produced, to which the animal 

 responds by reversing its ciliary movement, and C hnally passes into D, 

 which results in the Stentor contracting into its tube. The important 

 thing is that after many repetitions of the above treatment the organism 

 ' contracts at once as soon as the carmine comes in contact with it.' In 

 other words, states B and C are apparently omitted, and A passes directly 

 into D, i.e., into the state which gives contraction as a reaction. Thus we 

 have in an infusorian a case of short-circuiting precisely like the case 

 which has been quoted from Herbert Spencer as illustrating associa- 

 tion. But Jennings' case has the advantage of being based on actual 

 observation. He generalises the result as the ' law of the resolution 

 of physiological states ' in the following words : ' The resolution of one 

 physiological state into another becomes easier and more rapid after it 

 has taken place a number of times.' He goes on to point out that the 

 operation of this law is seen in the higher organisms, ' in the pheno- 

 mena which we commonly call memory, association, habit-formation, and 

 learning.' 



In spite of this evidence of mnemic power in the simplest of organisms, 

 objections will no doubt be made to the statement that association of 

 engrams can occur in plants. 



PfefFer, whose authority none can question, accounts for the behaviour 

 of sleeping plants principally on the more general ground that when any 

 movement occurs in a plant there is a tendency for it to be followed by a 

 reversal— a swing of the physiological pendulum in the other direction. 

 Pfeffer ^ compares it to a released spring which makes several alternate 

 movements before it settles down to equilibrium. But the fact that the 

 return movements occur at the same time-intervals as the stimuli is 

 obviously the striking feature of the case. If the pendulum-like swing 

 always tended to occur naturally in a twelve hours' rhythm it would be a 

 different matter. But Pfeffer has shown that a rhythm of six hours can 

 equally well be built up. And the experiments of Miss Pertz and myself * 

 show that a half-hourly or quarter-hourly rhythm can be produced by 

 alternate geotropic stimulation. 



We are indebted to Keeble^ for an interesting case of apparent habit 

 among the lower animals. ConvohUa roscoffensis, a minute wormlike 

 creature found on the coast of Brittany, leads a life dependent on the eblj 

 and flow of the sea. When the tide is out the Convoluta come to the 

 surface, showing themselves in large green patches. As the rising tide 

 begins to cover them they sink down into safer quarters. The remarkalile 

 fact is that when kept in an aquarium, and therefore removed from tidal 



' See Pfeffer, Ahhandl, K. Sticks. Ge^., Bd. xxx. 1907. It is impossible to do 

 justice to Pfeffer's point of view in tlie above brief statement. 

 2 Annals of Botany, 1892 and 1903, 

 =" Gamble and Keeble, Q. J. Mic. Science, xlvii. p. 401. 



