3/11 THE ORGANISM AS MACHINE 



tial in the analytic study of the behaviour of a mechanism, 

 whether animate or inanimate. But its usefulness in the labora- 

 tory should not obscure the fact that the free-living animal is not 

 subject to these constraints. 



Sometimes systems which seem at first sight to be one-way 

 prove on closer examination to have feedback. Walking on a 

 smooth pavement, for instance, seems to involve so little reference 

 to the structures outside the body that the nervous system might 

 seem to be producing its actions without reference to their effects. 

 Tabes dorsalis, however, prevents incoming sensory impulses from 

 reaching the brain while leaving the outgoing motor impulses un- 

 affected. If walking were due simply to the outgoing motor 

 impulses, the disease would cause no disturbance to walking. In 

 fact, it upsets the action severely, and demonstrates that the 

 incoming sensory impulses are really playing an essential, though 

 hidden, part in the normal action. 



Another example showing the influence of feedback occurs when 

 we try to place a point accurately (e.g. by trying to pass a wire 

 through a small hole in a board) w r hen we cannot see by how 

 much we are in error (e.g. when we have to pass the wire through 

 from the far side, towards us). The difficulty we encounter is 

 precisely due to the fact that while we can affect the movements 

 of the wire, its movements and its relations to the hole can no 

 longer be communicated back to us. 



Sometimes the feedback can be demonstrated only with diffi- 

 culty. Thus, Lloyd Morgan raised some ducklings in an incubator. 



The ducklings thoroughly enjoyed a dip. Each morning, 



at nine o'clock, a large black tray was placed in their pen, 



and on it a flat tin containing water. To this they eagerly 



ran, drinking and washing in it. On the sixth morning the 



tray and tin were given them in the usual way, but without 



any water. They ran to it, scooped at the bottom and made 



all the motions of the beak as if drinking. They squatted 



in it, dipping their heads, and waggling their tails as usual. 



For some ten minutes they continued to wash in non-existent 



water . . .' 



Their behaviour might suggest that the stimuli of tray and tin 



were compelling the production of certain activities and that the 



results of these activities were having no back-effect. But further 



experiment showed that some effect was occurring: 



'The next day the experiment was repeated with the dry tin. 



39 



