ORIENTATION IN SPACE 461 



animal under any circiinistanccs. Take, for example, the trielad 

 Leptoplana variabilis, whieh is cathodally fjaKaiiotropic and 

 negatively phototropie, and place it in a black rubber plu^tographic 

 developing dish. The dish is furnished with cotton electrodes at 

 either short end and a potential difference of about 0-4 volt estab- 

 lished between them. Three opal electric lamps placed at one side 

 of the dish supply the light. When the light is off, the planaria 

 move straight to the negative electrode, and, when the current is 

 off, they move straight away from the source of light. By varying 

 the intensity of either form of stimulation, it has been fovmd that 

 the trielad moves half-way between these extreme paths, i.e. with 

 an angle of deviation of 45° C. when the current intensity is pro- 

 portional to the logarithm of the intensity of the light (Crozier and 

 Steer) (see also Adaptation, Chap. XXXIV.). 



Sufficient has been said to show the nature and indicate the 

 mechanism of those actions termed tropisms. In principle they 

 depend on unilateral stimulation of a symmetrical animal. How 

 far they can be accepted as explanations of all the instinctive 

 actions of the lower organisms or of any of the actions of the 

 higher animals remains an open and debatable question. 



