178 The Animal Mind 



ized "righting" reaction, given in response either to a stimu- 

 lus within, say, a statolith organ, or, as in the planarian, to 

 the absence of accustomed contact stimulation on one sur- 

 face of the body. The reaction in these cases being a 

 specialized one, it is possible that a pecuhar sensation 

 quality might be involved. 



(e) Orientation might take place through a movement 

 occurring when the position of several stimuli perceived 

 simultaneously was disturbed, and tending to restore them 

 to their original position. This is the principle involved, 

 as we shall see, in explaining the rheotropism or current 

 orientation of fishes, and the anemotropism, or orientation 

 to air currents, of insects, as due to an instinct to keep the 

 visual surroundings the same. And this form of orienta- 

 tion alone suggests a true space perception as its conscious 

 accompaniment. 



Such being the conceivable ways in which orientation 

 may be brought about, what are the observed facts ? They 

 may be considered under the heads of orientation to gravity, 

 to light, and to other forces. 



§ 48. Orientation to Gravity: Protozoa 



To this form of reaction the term "geotropism" or "geo- 

 taxis" has been appHed. In various Protozoa negative ge- 

 otropism,or a tendency to rise against the pull of gravity, 

 has been observed : first by Schwartz in two single-celled 

 organisms frequently classified as plants, Euglena and 

 Chlamydomonas (667) ; and eight years later by Ader- 

 hold, who suggested, without accepting it, the theory that 

 the orientation may be due simply to the greater weight of 

 one end of the organism's body (2) . This view was main- 

 tained by Verwom : the action of gravity, he urged, must 



