3o6 THEORETICAL BIOLOGY 



animal corresponds to the food-ball, and the alimentary canal 

 to the surrounding-world. This holds good, not only for 

 animals that move along through cavities and passages in 

 the earth, but for all locomotor animals whatsoever. If we 

 J follow the track of some selected animal, we can re-create its 

 surrounding-world around that track, by setting up again 

 the indicators with which it came into contact. The sur- 

 rounding-world of the animal thus becomes a tunnel. 



In this indication-tunnel the animal moves forward, in 

 virtue of the antagonistically arranged muscles of its locomotor 

 organs. This is true of progress in the water and in the air, 

 as well as on the dry land. Each forward movement causes 

 an indicator to disappear and a new one to arise. Whereupon 

 a new function-circle begins. The course of the function in 

 each new circle means a new action, and so here also one action 

 connects up with the others into a chain, which winds itself 

 through the whole life. 



Now it is true that each member of the chain — i.e. each 

 function-circle — is an independent action on the part of the 

 animal ; but the chain itself — i.e. the rhythmic sequence of 

 the function-circles — is a creation of the external world, 

 because the order in which the indicators appear depends on 

 associations that are independent of the animal. 



Here for the first time we meet with the idea of an external 

 rhythm which enters into competition with the internal rhythm 

 of the animal. 



If the several function-circles are fixed as reflexes, the 

 whole life-course of an animal may give the impression of 

 being a process that unrolls automatically. An indicator, 

 such as the prey, attracts the animal, is devoured, and 

 disappears. The indicator " enemy " appears, and repels 

 the animal, whose flight results in the vanishing of this in- 

 dicator also. This led Loeb to consider the life of an animal 

 purely from the standpoint of physics, as a chain of tropisms. 



