140 ANIMAL BEHAVIOUR 



The field is representable by " lines of force " and the numbers 

 and directions of these are specifiable in any particular experi- 

 ment. When the bar magnet is moved the whole field becomes 

 changed. The needle will '' respond " to the movements of 

 the magnet by changing its inclination to, say, the earth's 

 magnetic meridian at the place of experiment. 



But in all this there is simple physical functionality. The 

 whole system, magnet, field of force and compass needle are 

 one, just as is a stretched sheet of some flexible fabric : if one 

 corner of the fabric be pulled the whole sheet becomes distorted. 

 There is complete physical determinism. 



Let the charge of explosive in a gun be fired by a detonator 

 which *' goes off " when a momentary electric current is passed 

 through a fuse. In such a case the explosion may be called 

 the " response " to the '' stimulus " of making contact in the 

 apparatus that transmits the current. 



50fl. The Muscle-nerve Preparation. This is usually the 

 gastrocnemius muscle of the frog's hind leg, with the attached 

 sciatic nerve. When the electrodes from an induction coil are 

 laid on the nerve and when a momentary current is passed 

 through a small part of the latter a nervous impulse is initiated 

 and this is propagated along the nerve into the muscle, where 

 it initiates a momentary contraction. As often as the muscle 

 is thus stimulated it responds by a contraction. By and by the 

 muscle will fail to respond, whether it be stimulated directly, 

 or via the nerve — ^just as a flash-lamp will fail to " respond " 

 to the pressure of the button, by lighting up, when the battery 

 becomes exhausted. 



The muscle-nerve preparation, although it is organic in origin, 

 is not an organism. It would be easy to devise a mechanism of 

 artificial nerve and muscle which would do much the same things 

 (though we cannot yet, of course, elaborate a similar mechanism). 

 Such a machine would exhibit " design," but not " purpose " 

 in the sense in which we have used this term. It would be 

 something that does not naturally occur but which had been 

 fabricated, or assembled so as to do certain things. But its 

 activities would tend towards its dissipation, or inability to 

 operate, as in the case of the running-down flash-lamp. Its 

 activities would not tend to the maintenance of its normality, 

 as an organic purpose does. 



