142 Journal of Comparative Neurology and Psychology. 



various paths crossing at all sorts of angles ; the only require- 

 ment is that the path shall on the whole lead away from the 

 greatest (or least) intensity of the stimulating agent. Hence 

 when many specimens react in this way their paths need 

 not be parallel, and as a rule no marked orientation results. 

 When a common orientation of many individuals does occur, it 

 is produced through "exclusion," — through the fact that under 

 the given conditions movement in any direction but one causes 

 the changes which act as effective stimuli, so that all are forced 

 to move in that direction. The behavior of Oxytricha in react- 

 ing to heat, as shown in Fig. 7 of the author's paper on Reac- 

 tion to Heat and Cold* will serve as a type for this reaction 

 method. It is, so far as the infusoria are concerned, the basis 

 for apobatic taxis (Rothert), phobotaxis (Pfeffer), and -pathy 

 (Davenport), and might farther be classed, from certain points 

 of view, as kinesis or -metry. 



The second class of reactions includes those in which the 

 changes that act as stimuli are brought about by a swerving to- 

 ward one side or the other, while the movements in the axis of 

 progression have no such effect. In the infusoria movements 

 from side to side are of course a regular part of the locomotion. 

 In the reactions to the effects of water currents, of centrifugal 

 force, of gravity, and of light rays coming from one side, the 

 lateral movements of unoriented animals induce marked changes. 

 In a water current or under the action of a centrifugal force, or 

 of gravity, lateral movements meet with less resistance in one 

 direction, greater resistance in another direction, and these 

 changes of resistance act as stimuli. In light coming from one 

 side the sensitive anterior end is more illuminated as the organ- 

 ism swerves towards one side; less illuminated as it swerves to- 

 ward the other, and these changes act as stimuli. The reactioa 

 is the same as in the first class ; the organism changes its course 

 by a series of trial movements. It continues these movements, 

 till it comes into a position in which it is no longer subjected to 

 the changes that act as stimuli. Such a position is found only 

 when the axis of the course coincides with the direction of ac- 



'Carnegie Institution of Washington, Publication i6, 1904, p. 16. 



