HISTORICAL REVIEW 47 



acterized by this phrase is very precisely determined by 

 structural and other internal conditions; indeed, its dis- 

 tinguishing feature is the fact that it is thus determined by 

 such conditions, rather than exclusively by the external 

 conditions." 



Jennings places particular emphasis on the idea that 

 " activity does not require present external stimulation." 

 This is an idea of which Darwin made much in his work on 

 movement in plants. To explain orientation, Darwin said, 

 we do not need to account for movement; it is only neces- 

 sary to account for change in the direction of movement. 

 Jennings applies this idea to the orientation of animals. 

 The animals are in motion; the question is, how is the 

 direction of motion regulated so as to result in orientation? 

 He says that in many of the infusoria it is regulated by 

 means of the avoiding reaction. "This reaction" (1906, 

 p. 79) "consists in successively 'trying' not only different 

 directions of locomotion, but also different positions of the 

 body axis. As soon therefore as a position is reached in 

 which the disturbance causing the reaction no longer exists, 

 the reaction of course stops; the animal therefore retains 

 this axial position." 



It will thus be seen that orientation in these forms is, 

 according to Jennings, not brought about by a direct turn- 

 ing of the anterior end of the body toward or away from 

 the source of stimulation. It is not due to unequal stimu- 

 lation of points symmetrically situated on the body; the 

 external agent does not act constantly as a directive stimu- 

 lus. " The position of orientation is not one in which a 

 median plane of symmetry takes up a definite position 

 with reference to the external agent." Not all reactions 

 resulting in orientation are however of this sort. Many 

 organisms have the power of turning directly toward or 

 aw.ay from the side stimulated; in these orientation may 

 take place directly, as Jennings clearly states in the follow- 

 ing words (1906, p. 271), " In the symmetrical Metazoa 

 we of course find many cases in which the animal turns 



