26 THE SENSES OF ANIMALS 



such kinds as trout, grayling, chub and many others which 

 can be tempted by the artificial fly have a very wide area of 

 vision. To approach these fish with any hope of success the 

 fisherman must keep well out of sight — a position which is 

 almost directly behind the fish being necessary. 



The acuity of a fish's vision can also be observed by 

 watching a feeding fish that may be poised just above mid- 

 water (which itself may be quite turbulent) yet the fish will 

 "rise" and take a minute insect, such as a gnat, that is in- 

 visible to the angler ; the circle on the surface of the water 

 made when the fish comes up being the only clue the angler 

 may have to its presence and position. 



In some kinds of sea fish, too, sight plays a great part in 

 hunting or seeking food, though naturally it is not so easy 

 to make observations of marine fish as it is in rivers and 

 lakes. 



It will pay the really keen student of the senses of fishes to 

 keep some kind of aquarium where observation is much easier 

 than in the natural habitat; and it is worth pointing out 

 here that in an aquarium it is possible to create natural con- 

 ditions more easily than is the case with birds or mammals. 

 But while it is true that the arrangement of conditions most 

 closely resembling the normal habitat will add value to the 

 observations made, it is not, in my opinion, correct to say — as 

 many authorities maintain — that observations on behaviour 

 are valueless if the animal studied is in captivity. 



That Birds can see colours should be well known even to 

 the less experienced, for if colour vision played no part in 

 the lives of birds as a whole it is unlikely that so many would 

 have bright and distinctively coloured plumage. But the fact 

 that birds' eyes are capable of seeing colours must be quali- 

 fied to some extent in relation to the type of life various birds 

 have to live. A nocturnal bird will clearly have less use for 

 colour vision than a diurnal one ; and the eye will be adapted 

 in structure to meet the necessary needs. In brief, a bird of 

 the dusk or dark must be able to make the most of whatever 

 light there may be (no bird can see in complete darkness), 

 and will, therefore, have little or no use for an ability to see 

 colours. A bird that is active in daylight only, will normally 

 have no need for extra light-receiving mechanisms, but 



