20 THEORETICAL BIOLOGY 



Space is at once formed, and it is made up of the possibility 

 of movement in all directions (which possibility is the most 

 general relationship-form of the direction-signs) plus the 

 planes of direction ; the actual movements are traced out in 

 space as definite series of direction-signs. 



Thus it seems to me that the theories of one of the most 

 gifted of physiologists are really in full accord with Kant's 

 doctrine ; it is only now and then that his enthusiasm over- 

 shoots the mark. 



ILLUSORY MOVEMENTS 



When we look at the world with one eye, and at the 

 same time move the eye-ball to and fro with a finger, the 

 objects surrounding us begin to swing to and fro at the same 

 rate ; whereas, when the movements of the eye are performed 

 by our own muscles, the objects remain stationary. Passive 

 and active shifting of the eye, then, produces entirely different 

 results. From this experiment Helmholtz concluded that the 

 innervation of the eye-muscles is made known to us by special 

 qualities of a central sense. We call these qualities direction- 

 signs. The experiment tells us even more ; it shows us that 

 any displacement of the retina is alone sufficient to give us 

 information about movement. When an image of an object 

 shifts on the retina, this invariably gives an impression of a 

 movement ; and this whether it is the object that has shifted, 

 or the retina itself. The appearance of the local signs in 

 continuous series produces in the retina, just as in the skin, 

 the impression of movement in the outside world. We 

 become conscious of our own movement only through the 

 movement of our muscles. Our own movement is re- 

 vealed to us by the inner direction-signs which accompany 

 our impulses when the muscles are stimulated. The outer 



