172 THE MECHANISM OF LIFE 



Perhaps we had better define physical determinism, or, rather, 

 illustrate it by an example. A compass needle free to move 

 points in London in a certain direction (about 16 degrees to the 

 west of true north), and it does so because the earth's magnetic 

 force is directed along lines which are running in that direction. 

 Now we know this horizontal component of the earth's mag- 

 netism, and we can also find what would be the force exerted by 

 a feeble bar magnet placed at right angles to the magnetic 

 meridian in which the compass needle is situated and (say) a 

 foot away from the pivot of the latter. Then a physicist could 

 calculate the angle through which the needle would deviate 

 when the magnet was placed as we have indicated. This 

 deviation would always be the same provided that the earth's 

 magnetic field and that of the disturbing magnet remained con- 

 stant. Call the effect of the latter the " stimulus," and the 

 deviation of the needle the " response " ; then there is physical 

 determinism, and we can always successfully predict to a high 

 degree of approximation that a certain response will follow the 

 application of the stimulus. 



Now take a muscle-nerve preparation (see pp. 134—5) and 

 stimulate the nerve by a feeble electric current; a momentary 

 twitch of the muscle, exerting a certain pull upon a weight 

 attached to it, follows. So long as the muscle remains irritable 

 and unfatigued this response occurs, and we can predict it. There 

 is physical determinism, even although this may not be absolute. 



Next take the frog in which the brain has been destroyed, 

 leaving the spinal cord intact. When a drop of acid is placed 

 on its back it will wipe this away with one of its hind-legs, and 

 when other stimuli are applied it will respond in a mechanical, 

 automatic manner. We can, in general, predict what it will do 

 ^when certain stimuli are applied in certain ways; thus there is 

 determinism. Yet this is not absolute— for instance, the spinal 

 frog appears to prefer to use one leg rather than the other, but 

 if it is prevented from employing that leg it will use the other. 

 Generally the behaviour, in response to stimuli, of the spinal 

 animal is of this mechanical, automatic nature — it can usually 

 be predicted; and this statement is true of frogs, birds, reptiles, 

 and mammals, whatever animals have been the objects of the 

 experiments. 



It is also true of a certain number of reflex actions in the 

 normal animal — that is, it is generally true, for reflexes cannot 



