Septembeb 18, 1908] 



SCIENCE 



361 



an infusorian a case of short-circuiting 

 precisely like the case which has been 

 quoted from Herbert Spencer as illustra- 

 ting association. But Jennings's case has 

 the advantage of being based on actual ob- 

 servation. He generalizes 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 phenomena which we commonly 

 call memory, association, habit-formation, 

 and learning." 



In spite of this evidence of mnemic 

 power in the simplest of organisms, objec- 

 tions will no doubt be made to the state- 

 ment that association of engrams can occur 

 in plants. 



Pfeffer, whose authority none can ques- 

 tion, accounts for the behavior 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 fol- 

 lowed by a reversal— a swing of the physi- 

 ological 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- 

 hoiirs ' rhythm it would be a different mat- 

 ter. But Pfeffer has shown that a rhythm 

 of six hours can equally well be built up. 

 And the experiments of Miss Pertz and 

 myselP^ show that a half-hourly or quarter- 



^* See Pfeffer, Aihandl. K. Sachs. Ges., Bd. 

 XXX., 1907. It is impossible to do justice to 

 Pfeffer's point of view in the above brief state- 

 ment. 



=* Annals of Botany, 1892 and 1903. 



hourly rhythm can be produced by alter- 

 nate geotropic stimulation. 



We are indebted to Keeble^" for an inter- 

 esting case of apparent habit among the 

 lower animals. Convoluta roscoffensis, a 

 minute wormlike creature found on the 

 coast of Brittany, leads a life dependent on 

 the ebb 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 re- 

 markable fact is that when kept in an 

 aquarium, and therefore removed from 

 tidal action, they continue for a short time 

 to perform rhythmic movements in time 

 with the tide. 



Let us take a human habit, for instance 

 that of a man who goes a walk every day 

 and turns back at a given mile-post. This 

 becomes habitual, so that he reverses his 

 walk automatically when the limit is 

 reached. It is no explanation of the fact 

 that the stimulus which makes him start 

 from home includes his return— that he 

 has a mental return-ticket. Such explana- 

 tion does not account for the point at which 

 he turns, which as a matter of fact is the 

 result of association. In the same way a 

 man who goes to sleep will ultimately wake ; 

 but the fact that he wakes at four in the 

 morning depends on a habit built up by his 

 being compelled to rise daily at that time. 

 Even those who will deny that anything 

 like association can occur in plants can not 

 deny that in the continuance of the nycti- 

 tropic rhythm in constant conditions we 

 have, in plants, something which has gen- 

 eral character of habit, i. e., a rhythmic 

 action depending on a rhythmic stimulus 

 that has ceased to exist. 



On the other hand, many will object 

 that even the simplest form of association 

 implies a nervous system. With regard to 



^' Gamble and Keeble, Quar. Jour. Micros. Sci., 

 XL VII., p. 401. 



