NATURAL THEOLOGY. 



33 



In the eel, which has to work its head through 

 sand and gravel, the roughest and harshest sub- 

 stances, there is placed before the eye, and at some 

 distance from it, a transparent, horny, convex case 

 or covering, which, without obstructing the sight, 

 defends the organ. To such an animal could any- 

 thinor be more wanted or more useful ? 



Thus, in comparing the eyes of different kinds 

 of animals, we see in their resemblances and dis- 

 tinctions one general plan laid down, and that plan 

 varied with the varying exigencies to which it^is to 

 be applied."' 



1"^ In viewing the structure of the eye as adjusted to the condition 

 of fishes, we may remark the pecuhar thickness of the sclerotic 

 coat in the whale. Although he breathes the atmosphere, and lies 

 out on the surface of the water ; to escape his enemies he will 

 plunge some hundred fathoms deep. The pressure therefore must 

 be very great upon his surface, and on the surface of the eye. If 

 a cork be knocked into the mouth of a bottle, so that it resists all 

 further pressure that we can make upon it, and if this bottle be 

 carried, by being attached to the sounding-lead, to a great depth in 

 the sea, the pressure of the water will force in the cork, and fill the 

 bottle ; for the cork is pressed with a force equal to the weight of 

 the column of water above it, of which it is the base. It is pressed 

 in all directions equally, so that a common-sized cork is reduced to 

 the size of that of a phial bottle. 



"A creature hving at the depth of 100 feet would sustain a pres- 



