10 APPLICATION OF THE ARGUMENT. 



casionally changed, and a most artificial apparatus provid- 

 ed to produce that change. This is far beyond the com- 

 mon regulator of a watch, which requires the touch of a 

 foreign hand to set it ; but is not altogether unlike Harri- 

 son's contrivance for making a watch regulate itself, by in- 

 serting within it a machinery, which, by the artful use ot 

 the different expansion of metals, preserves the equability 

 of the motion under all the various temperatures of heat 

 and cold in which the instrument may happen to be placed. 

 The ingenuity of this last contrivance has been justly prais- 

 ed. Shall, therefore, a structure which differs from it 

 chiefly by surpassing it, be accounted no contrivance at all? 

 or, if it be a contrivance, that it is without a contriver ? 



But this, though much, is not the whole : by different 

 species of animals the faculty we are describing is possess- 

 ed, in degrees suited to the different range of vision which 

 their mode of life, and of procuring their food, requires, 

 Birds, for instance, in general, procure their food by means 

 of their beak ; and the distance between the eye and the 

 point of the beak being small, it becomes necessary that 

 they should have the power of seeing very near objects dis- 

 tinctly. On the other hand, from being often elevated much 

 above the ground, living in air, and moving through it with 

 great velocity, they require for their safety, as well as for 

 assisting them in descrying their prey, a power of seeing 

 at great distance ; a power, of which, in birds of rapine, 

 surprising examples are given. The fact accordingly is^ 

 that two peculiarities are found in the eyes of birds, both 

 tending to facilitate the change upon which the adjustment 

 of the eye to different distances depends. The one is a 

 bony, yet, in most species, a flexible rim or hoop, [Plate III. 

 fig. 1, 2.] surrounding the broadest part of the eye; which, 

 confining the action of the muscles to that 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, call- 

 ed the marsupiura, [Plate III. fig. 3,4, 6.] to draw, upon 

 occasion, the crystalline lens back, and so 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 adjustment, with more ease and readiness 

 than the eyes of other animals. 



The eyes of fishes also, compared with those of terres- 

 trial animals, exhibit certain distinctions of structure, 

 adapted to their state and element. We have already ob- 



