28 THE SENSES OF ANIMALS 



mammal families, or even within a species. Among dogs, 

 greyhounds must have better eyesight than spaniels; while 

 squirrels will take alarm at something they see more quickly 

 than shrews. Moles and bats have no real need for good 

 vision, and although both have eyes which are sensitive to 

 light, it is hardly possible to say that they possess sight in the 

 ordinary meaning of the word. 



The other senses of mammals are so very delicately and 

 highly developed that it is by no means easy to say of any 

 one kind what precise use is made of sight. We know so little 

 about the world of sounds and smells by which mammalian 

 life is governed, that to dogmatize as to when one sense takes 

 over from another would be misleading and probably futile. 



The position of the eyes and the size of the eyeballs are 

 indications of habit and behaviour. Hares, with their promi- 

 nent eyes so set that they can look behind them as efficiently 

 as they can look in front, are clearly using sight as one of 

 their means of defence ; but a hedgehog, although it can see 

 well, has not the same need for all-round vision — it has its 

 own very specialized way of defending itself! In the wild, 

 rats, mice and voles have prominent eyes with good "fore 

 and aft" angles of vision, but it is worth while mentioning 

 that tame albino or partially albino fancy rats and mice 

 depend less on their eyes because albinism carries with it 

 ocular defects, and of course they do not have enemies; but 

 albino and normally coloured rats alike react to movement. 

 Albino mice and voles in the wild are rare, and this could 

 be because they are more vulnerable than their normally 

 coloured brethren and so are captured more easily. 



Nocturnal vision is well developed in mammals and some 

 clue to the possession of this ability is to be seen in the 

 elliptical pupil of those mammals that use their eyes at night 

 to a greater degree than others — the cat family comes to 

 mind at once. It must not be thought, however, that an 

 elliptical pupil is confined to mammals ; certain snakes have 

 such a pupil and our British adder is a good example. To 

 make possible confusion worse it is by no means certain that 

 adders make use of nocturnal vision any more than snakes 

 without it, and this specialized pupil in the adder requires 

 more investigation. 



