128 THEORETICAL BIOLOGY 



ground, there must be some connection established with one 

 or other of these media. 



The ground and all fixed bodies always constitute obstruc- 

 tions in the medium, and consequently act as indications. 

 So most animals are furnished with organs of locomotion for 

 getting over these obstacles, while only a small proportion 

 have organs serving for movement in the free media of air 

 or water. 



In many cases, the medium is spatially restricted by 

 special indications which fix the animal to a definite habitat. 

 Thus for gastropods and crustaceans, and also for insects, 

 dark and light surfaces give signs which influence them in 

 their course. In addition, there is also in the bilateral animals 

 an unequal development of the two sides, which favours 

 movement in a circle. 



In addition to the function-circle of the medium, we can 

 distinguish the function-circles of food and of the enemy. 

 In both cases, the animal receives a stimulus proceeding 

 from the indications of the food (be it of animal or vegetable 

 nature), or from those of the enemy, which latter is always, 

 with almost negligible exceptions, an animal. Following on 

 the food stimulus, movement is turned in the direction of the 

 food, and then, when contact is established, fresh indications, 

 tactile or chemical, come in, which seem to guide the masti- 

 catory apparatus. At this point a number of circles often 

 appear, which belong to the food circle. Following on the 

 enemy stimulus, the organs of locomotion are directed to lead 

 the animal away from the enemy, or else the organs of defence 

 are directed to drive it away. In both cases the enemy- 

 indications disappear. 



In many cases, as among the crustaceans, the function- 

 circle of the enemy passes through the eyes, and the function- 

 circle of food through the olfactory organs. 



As a fourth function-circle there is the sexual, which 



