450 



NATURAL THEOLOGY. 



whence he may steal upon them unseen. This is 

 a critical distinction, and is mechanical ; but it 

 may be suggested, and, I think, not without pro- 

 bability, that it is the effect of continual habit. 



The eyes of animals which follow their prey by 

 night, as cats, owls, &c., possess a faculty not 

 given to those of other species, namely, of closing 

 the pupil entirely. The final cause of which 

 seems to be this: — It was necessary for such 

 animals to be able to descry objects with very 

 small degrees of light. This capacity depended 

 upon the superior sensibility of the retina ; that 

 is, upon its being effected by the most feeble im- 

 pulses. But that tenderness of structure, which 

 rendered the membrane thus exquisitely sensible, 

 rendered it also liable to be offended by the access 

 of stronger degrees of light. The contractile 

 range therefore of the pupil is increased in these 

 animals, so as to enable them to close the aperture 

 ^entirely, which includes the power of diminishing 



[The figure represents the iris of a Hon. B B, the straight or 

 converging fibres ; C, the fibres which circle the inner margin of 

 the iris,] 



