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BIRD’S-EYE VIEWS. 581 
we consider the marsupium 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 irresistably 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 birds 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 eyer-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; 
ad 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 we advance, until it stands forth clearly in view! 
Tet 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 t 
