150 THE SENSES OF ANIMALS 



and then add "Let me see it" as we hold out our hand for 

 it, not in fact to see it — we can do that quite well already — 

 but in order to hold it and to find out by touch what it 

 really is. When we have done that we can recognize it by 

 sight alone on a future occasion. 



I well remember driving a car on a long straight road ; as 

 I topped the crest of a hill I could see across the valley ahead 

 to the far hillside where a white goose was standing in the 

 middle of the road. I drove on down into the valley and as 

 I started climbing the far rise the goose was still there. It 

 stood quite still until I was about a hundred yards away 

 when it suddenly turned into a wet patch that had been 

 reflecting the sun. If I had turned off at the cross-road in the 

 valley I should have been ready to take any oath that there 

 was a white goose in the road, whereas the correct inference 

 would have been that a farmer had driven his herd of cows 

 across the road shortly before. Appearances certainly are 

 deceptive and sight, much as we prize it, is easily deceived ! 



It is impossible to know whether the other animals see 

 things as we do. Undoubtedly the images that fall on their 

 retinas are similar to those that fall on ours but, as we have 

 seen, eyes without experience are neither reliable nor in- 

 formative. Seeing is more than having an image in the eye, 

 it becomes useful only when there is an image in the brain, 

 and the building up of that image is so complicated a process 

 that we know little about it even in ourselves — and who 

 knows what goes on in an animal's brain? Nevertheless we 

 can, from a study of the structure of eyes, form some idea of 

 what the vertebrate animals probably see. For a start, most 

 of the mammals except the monkeys are more or less colour- 

 blind because their retinas are poor in cones but rich in rods. 

 The mammals that have eyes at the sides of their heads have 

 a much wider field of vision than we do, and some can see 

 almost all round them, but they have only limited stereo- 

 scopic vision because the fields of each eye do not overlap 

 very much. Although the picture they see is rather indistinct 

 and the detail is not very clear they have great sensitivity to 

 any movement that may occur in the field of vision. 



Most birds are not colour-blind — their brightly coloured 

 plumage used in mating display would lead us to infer this 



