20 bird's-eye views. 



we consider the marsupiiun as an erectile organ, we are not 

 content to understand how it erects itself, and what it ac- 

 complishes by erection ; we want to know why it is neces- 

 sary or desirable that it should do what it does. 



Putting together all that we do know of the operation of a 

 bird's eye ; and from this inferring some things that we do 

 not know, we are irresistal)ly led to the conclusion, that all 

 the essential peculiarities of a bird's eye conspire to produce 

 what we just now called its greatest physiological phenom- 

 enon — instantaneous unerring adjustment of focus. 



Study of the habits of lairds makes the necessity of some 

 such faculty as this as evident as the fact of its existence. 

 This admirable provision relates in the most direct manner 

 possible to the rapid movements of birds in the air. As 

 they dash onward in their airy course, the eye accommodates 

 itself, if not with the speed of thought, at least with the 

 speed of flight, to ever-varying distances, and surrounding 

 objects, be they far or near, all alike rush into focus. With 

 our own eyes, we see at once a book before us, and a large 

 object in the distance. Push the book away by degrees ; 

 the letters run into words, words into lines, lines into para- 

 graphs, paragraphs into a solid page of dark, surrounded 

 by a white border ; then ^the edges of the cover gain a 

 film, the outlines soften, the thing becomes a spot, and 

 finally disappears. If a bird were in our place, it would 

 still see letters long after they had disappeared from our 

 view. Its eye would change in shape, and the structures 

 within alter in position, as the book moved off, slowly, grad- 

 ually, constantly, till the limit of its power was reached ; 

 and this limit it need not be said, far exceeds ours. Walk 

 towards the large object now indistinct in the distance. 

 How long we are in approaching it : how very slowly it takes 

 form as Ave advance, until it stands forth clearly in view ! 

 Let a wild duck fly at the rate of ninety miles an hour, 

 towards the same object. How rapid must be the adjustment 

 of its eyes compared with ours ! But these are among the 



