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Why do cats' eyes contract to a vertical slit?: Cats evolved to be nocturnal hunters, and they can see well in very
dim light. Because their eyes are so sensitive to light, cats need
precise control over the amount of light reaching their eyes.
Being able to reduce the pupils to slits rather than tiny circles
gives the cat greater and more accurate control of how much light
enters their eyes; this ability is particularly important in bright
sunlight.
Vertical slits also have an advantage over horizontal slits. Because
the cat's eyelids close at right angles to the vertical pupil,
the cat can reduce the amount of light even further by bringing
its eyelids closer and closer together. This combination of the
vertical slits of the pupils and the horizontal slits of the eyelids,
allows the cat to make the most delicate adjustments of the light
reaching its eye compared to any other animal.
The pupils of the lion are an interesting comparison. The lion
hunts by day, not by night as the cat does. The lion does not have
the same sensitivity to light as the cat. And the lion's eyes
contract -- like ours do -- to tiny circles, not vertical slits.
Source: Catwatching, by Desmond Morris, Crown Publishers,
Inc., New York, 1986, p. 85.
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