154 HARRY BEAL TORREY 



the source of stimulation. And it would continue to be suffi- 

 cient regardless of the agency of stimulation — whether contact 

 or chemical, light, gravity, galvanic electricity — provided only 

 that the motive for it remained the same. It has been insisted, 

 indeed, that in the orientation of the micro-organism Euglena to 

 light, its swervings from the original path are typical evidences 

 of stimulation, being regarded, in fact, as motor reflexes. 



It follows naturally from such a criterion that when organ- 

 isms such as Paramecium and Euglena, pursue unswervingly a 

 direct course, they are in an unstimulated condition. That this 

 conception, which is entirely compatible with the motive from 

 which it springs, is quite useless in physiological analysis, will 

 not be difficult to show, if it is perhaps not already apparent. 



It is characteristic of the reaction of Paramecium to a con- 

 tinuous galvanic current that it moves toward the cathode, or 

 anode, as the case may be, along a line of force, under very 

 obvious compulsion. From beginning to end of its course toward 

 the cathode, for instance, the cilia at the cathodal end of the 

 organism beat constantly in a direction the reverse of that in 

 which they beat when swimming freely in the absence of the 

 galvanic current. It is as obvious that they are being subjected 

 to constant stimulation as it is that the organism maintains a 

 constant course. How can such behavior be reconciled with the 

 criterion of stimulation established for the use of the selectionist? 



It cannot. But the difficulty may be composed by disre- 

 garding it; by asserting, in fact, that since galvanic electricity 

 has never been experienced by Paramecium in a state of nature; 

 since it has never either directly or indirectly provided material 

 for selection; the galvano-tropic response is of no serious im- 

 portance to the student of behavior who is busy with problems 

 of genesis and survival. 



Such a procedure avoids still another embarrassment, namely, 

 the radical modification of the definition of stimulus that served 

 well enough for the reactions of various organisms to sudden 

 changes in the intensity of what may be called the stimulating 

 medium. When Stent or passes from a shaded to a brightly 

 illuminated area, it executes a motor reflex at the boundary of 

 the shadow much as it would react to contact with a solid 

 object. Reactions of this type have long been recognized and 

 ascribed to a capacity of the organism commonly known as 



