156 The Animal Mind 



The theory cannot explain why an animal at rest should be 

 oriented. Another argument that tells against it is offered 

 by experiments showing that animals placed in solutions 

 of the same density as their own bodies, in which, therefore, 

 they have no weight, still display negative geotropism, and 

 that the direction of the response is not reversed when the 

 fluid is made heavier than the animals (255). Lyon's own 

 theory, accepted by Jennings, is that the stimulus for geo- 

 tropism is furnished by the action of gravity within the body 

 of the organism, upon substances of different weight which 

 exert varying pressures and take up different positions ac- 

 cording to the position of the body (255). 



It has been shown that the reactions of Paramecium to 

 gravity are modified by a variety of conditions. Negative 

 geotropism, in a sense their normal condition, is favored by 

 plentiful food supply and by an increase in temperature 

 within certain limits; positive geotropism, movement down- 

 ward, may be brought about temporarily by mechanical 

 shock, by salts and alkalies, by temperature changes (278, 

 388) to which, however, the animals may adapt themselves; 

 with less constancy by increase in the density of the fluid 

 containing them, and with lasting effect by lack of food. 

 It has been suggested that the downward movement under 

 these circumstances is protective, since it shields the animals 

 from surface agitation of the water, from surface ice, and 

 from failure of the surface food supply (278). We shall 

 see that similar conditions often change the direction of an 

 animal's response to light. 



57. Orientation to Gravity: Ccdenterates 

 Among the ccelenterates, geotropism is shown by certain 

 hydroids, whose stems have a tendency to curve upward 

 and their " roots " a tendency to grow vertically downward 



