266 BEHAVIOR OF THE LOWER ORGANISMS 



therefore the axial orientation of the cell body, and the central point in 

 the mechanism of this phenomenon, lies in the explanation of this axial 

 position" (Verworn, "General Physiology," 1899^.480). After the 

 animal has thus become oriented it may move forward in the usual 

 way. If it does so, it will of course incidentally move toward or away 

 from the source of stimulation, but this approach or retreat is not an 

 essential or determining part of the reaction. "The really fundamental 

 phenomenon which characterizes these directed movements is always 

 not so much the forward movement as such, as rather a process which 

 may be called a movement of orientation. The organism places its 

 axis in a definite localized relation to the stimulus, which may be photic, 

 thermic, chemical, etc. That is, it places its axis either in the direction 

 of the stimulation or perpendicular to it (diatropism). In the former 

 case the 'anterior' end may be directed 'positively,' toward the source of 

 stimulation, or 'negatively,' away from it. It now appears a matter of 

 course that if forward motion takes place after such orientation, its 

 direction will correspond to the direction of the stimulus" (Driesch, 

 1903, p. 5, translation). 



(2) This orientation is produced, according to this tropism theory, 

 by the direct action of the stimulating agent on the motor organs of that 

 side of the body on which it impinges. A stimulus striking one side of 

 the body causes the motor organs of that side to contract or extend or 

 to move more or less strongly. This, of course, turns the body, till the 

 stimulus affects both sides equally ; then there is no occasion for further 

 turning, and the animal is oriented. "These tropisms are identical 

 for animals and plants. The explanation of them depends first on the 

 specific irritability of certain elements of the body surface, and, second, 

 upon the relations of symmetry of the body. Symmetrical elements at 

 the surface of the body have the same irritability; unsymmetrical ele- 

 ments have a different irritability. Those nearer the oral pole possess 

 an irritability greater than that of those near the aboral pole. These 

 circumstances force an animal to orient itself toward a source of stimu- 

 lation in such a way that symmetrical points on the surface of the body 

 are stimulated equally. In this way the animals are led without will of 

 their own either toward the source of stimulus or away from it" (Loeb, 

 1900, p. 7). Holt and Lee (1901, pp. 479-480) bring out this point in 

 the prevailing theory, as applied to light, as follows : "The light operates, 

 naturally, on the part of the animal which it reaches. The intensity of 

 the light determines the sense of the response whether contractile or 

 expansive, and the place of the response, the part of the body stimulated, 

 determines the ultimate orientation of the animal." 



How the orientation is brought about according to this theory may 



