SIGHT FIELDWORK AND EXPERIMENTS 67 



poorer sight than others, and some with virtually no effective 

 vision at all. Among the larger species the rhinoceros stands 

 out as having such short sight that until it is nearly on top 

 of some object — particularly a static one — it probably has 

 no precise picture. Its other senses, however, more than make 

 up for this deficiency. At the other end of the scale of size 

 we have such mammals as moles, shrews and bats which, 

 though they have eyes, do not rely on them any more than 

 as indicators of light and dark. 



Moles and shrews can be easily tested in this respect ; and 

 even bats may be experimented with, though their feeding 

 in captivity is much more difficult than with moles and 

 shrews. Both the latter can be kept under captive conditions 

 for long enough to enable experiments to be made; but as 

 they consume something in the region of their own weight 

 per day in food, it is clear that keeping them for any length 

 of time is an arduous task which, for the purposes of testing 

 their eyesight, is not in any way necessary. 



Both these insectivorous mammals can be housed in an 

 old aquarium tank for a day or two. A sprinkling of peat moss 

 litter on the floor and a flower-pot or artificial tunnel so that 

 they can hide away at will is all that is required, except for a 

 dish of water and an adequate supply of earthworms. The 

 idea of the experiment is to satisfy yourself that moles and 

 shrews can find their food in complete darkness. 



Only one individual mole or shrew must be used at any 

 one time, and all that has to be done is to place in the tank 

 a known number of worms and then either cover the tank 

 in such a way that light is completely excluded, or do your 

 experiment at night in a pitch-dark room. The length of time 

 required for the completion of the test will, of course, depend 

 upon the food already taken before you start; but as these 

 creatures require to be almost continuously feeding, you 

 won't have to wait long. 



An hour should be sufficient once you have placed the 

 worms in the tank and excluded all light. After this interval 

 inspect the tank when, unless you are most unlucky, you 

 will find that the worms have vanished. A word of warning 

 is necessary if you decide to adopt the method in which a 

 cover for the tank is used. Moles and shrews may be able 



