28 NATURAL THEOLOGY. 



part, increases the effect of their lateral pressure upon the 

 orb, by which pressure its axis is elongated for the purpose 

 of looking at very near objects. The other is an additional 

 muscle called the marsupium, to draw, on occasion, the 

 crystalline lens back, and to fit the same eye for the viewing 

 of very distant objects. By these means, the eyes of birds 

 can pass from one extreme to another of their scale of adjust- 

 ment, with more ease and readiness than the eyes of other 

 aiiimais. 



The eyes oi fishes also, compared with those of terrestrial 

 animals, exhibit certain distinctions of structure adapted to 

 their state and element. We have already observed upon 

 the figure of the crystalhne compensating by its roundness 

 the density of the medium through which their light passes. 

 To which we have to add, that the eyes of fish, in their nat- 

 ural and indolent state, appear to be adjusted to near ob- 

 jects, in this respect differing from the human eye, as well 

 is those of quadrupeds and birds. The ordinary shape of 

 the fish's eye being in a much higher degree convex than 

 that of land animals, a corresponding difference attends its 

 muscular conformation, namely, that it is throughout calcu- 

 lated iox flatteiiing the eye. 



The iris also in the eyes of fish does not admit of con- 

 traction. This is a great difference, of which the probable 

 reason is, that the diminished light m water is never too 

 strong for the retina. 



In the eel, which has to work its head through sand and 

 gravel, the roughest and harshest substances, 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 

 thing be more wanted or more useful ? 



Thus, in comparing the eyes of different kinds of animals, 

 we see in their resemblances and distinctions one general 

 plan laid down, and that plan varied with the varying exi 

 gencies to which it is to be applied. 



