that the fore-wing, when at rest, is folded longitudinally back 
upon itself. 
Finally, turn to the flies. Herein it will be seen that there 
is but a single pair of wings, the hind pair having become reduced 
to mere stumps, known as “ balancers.” 
Much, very much more, might have been said of these wings: 
but our conversation is of birds. We cannot, however, properly 
appreciate either the essential characters of their wings, or their 
flight, without some such standards of comparison as is afforded 
by the wings of other creatures. 
II 
