THE WAY OF A BIRD IN THE AIR. wal 
state of perfection—in fact, but little beyond that 
of the Archwopteryx. The peculiar pocket found 
among some of them, for hatching the egg in, hints 
that they may have diverged from the bird stem at 
an early period. 
These suggestions are given to add interest to our 
study of the shape of wings in this chapter, and the 
deductions might not be sustained if all the facts were 
presented, nor is it likely that they will be indorsed 
by all the students of bird flight. 
Whatever the origin may be, we find ground- 
haunting birds now usually with wings that are short 
and round, so that they may be fluttered rapidly, and 
that are broad and concave beneath, so as to resist 
the air greatly. The hollow side of a curved disk 
resists the air more than if it were flat, and very much 
more than the convex side. 
Such birds as persist in ground-haunting, skulking 
and hiding habits need such wings for suddenly hurl- 
ing themselves up when discovered or chased; and 
short flight only is desired that they may again hide, 
run, etc. Hence, there is nothing in their habits to 
develop a better wing. In fact, there is no better 
wing for their purposes. But such birds as wish to 
sustain themselves in long continuous flight need a 
wing that is a little longer and which does not require 
so much effort, at least such rapid strokes, to keep 
the flyer up and going. The longer the flight the 
longer the wing usually. We may find all grades of 
length also according to habits. 
In birds that are given to long continuous flights 
