154 The Animal Mind 



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. 



56. Orientation to Gravity: Protozoa 



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

 taxis " has been applied. In various Protozoa negative geo- 

 tropism, 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 

 (378); and eight years later by Aderholcl, who suggested, 

 without accepting it, the theory that the orientation may be 

 due simply to the greater weight of one end of the organ- 

 ism's body (2). This view was maintained by Verworn : the 

 action of gravity, he urged, must be purely passive. It can- 

 not operate as a stimulus to active response on the animal's 

 part, for a stimulus is always a change in environment, and 

 gravity is a constant force (416). This ignores the fact that 

 the animal's relations to gravity may change though 

 gravity does not. According to Verworn's theory, the 

 geotropic orientation of a single- celled organism takes place 

 through a series of "little falls" whereby the heavier end is 

 directed downward. Massart opposed this view on the 

 basis of observations which showed that the actual move- 



