SENSES AND NERVES I35 



of the Stimuli produced by their sense organs in the same 

 way that we are. The reaction to a stimulus may be almost 

 automatic, as when we jump if we sit on a drawing-pin — it 

 is only after reacting that we look to see what has hurt us. 

 On the other hand, when your dog spots you coming down 

 the street a hundred yards away he obviously sees you in 

 much the same way that you see him. As we heard in 

 Chapter I, frogs and similar creatures will not seize their 

 food unless it is moving ; yet frogs have eyes of complicated 

 structure that throw a perfectly good image of what they 

 are looking at on the screen at the back of the eye. Why does 

 the image of food not produce any reaction until it moves? 

 We must look for the reason not in the frog's eye, in the 

 sense organ, but in its brain where the messages from the 

 eye are sent. If the brain is not capable of building up and 

 interpreting a mental image of what the sense organ tells it, 

 no signal for action is sent on to the muscles. Compared 

 with the brain of a man the brain of a frog is much less 

 complex; in particular it lacks the enormous expansion in 

 front that makes up the largest part of the human brain 

 and gives the facility of building up complicated mental 

 images. So although a frog may look at a fly it does not see 

 it until there is movement, and the difference is not the fault 

 of the sense organ but of the brain to which it is reporting. 

 Similar things happen in the human brain in spite of its 

 enormous advantages over less complicated ones. The whole 

 art of camouflage depends upon getting the eye to send 

 messages to the brain that will build up a meaningless mental 

 image — to make the beholder look, but to prevent him 

 seeing. A similar process is very common in nature ; we all 

 know how difficult it is to find the nests of some ground- 

 nesting birds such as lapwings, ringed plover, or oyster- 

 catchers, even when the eggs are completely exposed — we 

 can look straight at them less than a yard away without 

 seeing them. The mental image we build up from the message 

 sent from the eyes means nothing — or at least it does not 

 mean "eggs" — although the image of the eggs is perfectly 

 clear at the back of the eye we are no better off' than the 

 frog looking at a stationary fly. It is strange that we, and 

 other animals, should have such nearly perfect sense organs, 



