Septembeb 6, 1907] 



SCIENCE 



317 



near the base of the flagellum, which appears 

 to be particularly sensitive to light. Like 

 Paramecium, Euglena swims in spirals, and 

 possesses a similar avoiding reaction, which 

 follows not only upon stimulation by chemical 

 and mechanical agents, but upon sudden 

 changes in the intensity of light as well. De- 

 pending on the strength of the stimulation, 

 the response may be a reduction of the speed 

 of locomotion, a total stoppage, or, rarely, a 

 reversal. Then there is a swerve toward a 

 certain structurally defined side of the organ- 

 ism so that the spiral in which it swims be- 

 comes wider than before. It thus comes into 

 a number of new positions with reference to 

 the source of the stimulus. These, according 

 to Jennings, are trial positions or orientations, 

 and which one of them may be selected for the 

 forward movement will depend upon the de- 

 gree to which it lessens the stimulation which 

 is inducing the trial movements. Continuous 

 selections, based upon a continuous series of 

 new trials, bring the organism finally into 

 such an orientation that it proceeds toward 

 the light around an axis of progression that 

 passes through the latter. 



The second case need but be mentioned. 

 The rotifer Anurcea is a very small but very 

 difPerent organism from Euglena. Neverthe- 

 less, it moves also upon a spiral path and its 

 reactions in the presence of light differ in no 

 essential respect from the reactions of Eu- 

 glena, except that the animals experimented 

 upon by Jennings moved away from rather 

 than toward the light. With this difference 

 in mind, the same figure will serve admirably 

 for both organisms (Jennings, Fig. 93, p. 137). 



My analysis of their responses, based upon 

 the figure which Jennings himself has drawn, 

 with test description, leads to quite a different 

 conclusion from his. The figure indicates 

 that Euglena is both unterschiedsempfindlich 

 and heliotropic. At a, the reversal in the 

 direction of the light which has been coming 

 from the direction in which the creature has 

 been swimming produces a sudden change in 

 intensity of stimulation, a shock which results 

 in the swerving from the previous course as 

 indicated between a and c. The organism 



recovers rapidly, only to be subjected to the 

 constant stimulus of a steady light from one 

 direction to the end of the experiment. The 

 result of the action of the constant stimulus 

 is a path, from c to 5, so perfectly in harmony 

 with the tropic schema, that, in spite of Jen- 

 nings's descriptions and elucidations, I can 

 only wonder at his running so boldly and so 

 far into the enemy's camp. It is hard for me 

 to conceive how an organism swimming of 

 necessity in a spiral course could react more 

 definitely to a moderate directive stimulus 

 than does Euglena here. 



It will be noticed that orientation by the 

 method of trial depends, according to Jen- 

 nings, upon the selection of trial orientations 

 that subject the organism to less and less 

 effective stimulation; that when the final 

 orientation is adopted, the organism is in an 

 unstimulated condition with reference to the 

 stimulus which had been acting up to this 

 point. This is clearly the application in the 

 field of light stimulation of the facts obtained 

 by the observation of the reactions of such an 

 organism as Paramecium to contacts. It is 

 assumed (1) that the locomotion of Para- 

 mecium is a necessary result of its peculiar 

 metabolism, and (2) that in the absence of 

 perturbing influences in the environment, it 

 may swim along a spiral course with a 

 straight axis. These assumptions may be 

 granted without, however, admitting thereby 

 the converse, namely, that when the axis of 

 progression is a straight line Paramecium is 

 necessarily free from the influence of external 

 stimuli. It does not appear self-evident that 

 as soon as Euglena becomes oriented so that 

 its axis of progression passes straight toward 

 the source of light it ceases to be stimulated, 

 to be again stimulated only when it chances 

 to swerve out of that course. 



For Jennings there is nothing comparable 

 to symmetrical stimulation in the field of 

 organic behavior. There is likewise nothing 

 comparable to a constant stimulus that does 

 not induce a differential movement. This is 

 as much as to say that an object which is sub- 

 jected to equal degi'ees of pressure from dia- 

 metrically opposite directions is not being 



