CHAPTER XX T. 
THE WAY OF A BIRD IN THE AIR. 
We have seen that perhaps the first flight of the 
bird was sailing down, and that it likely first devel- 
oped a wing adapted solely to this sort of flight. But 
when flight came in perfection, and birds began to 
depend upon it as a means of motion and escape, the 
wing must be shaped for getting wp also, and getting 
up quickly, to avoid an enemy. 
There can be no doubt, therefore, that the wing 
has undergone many modifications, being shortened 
or lengthened, widened or narrowed, concaved or flat- 
tened (beneath) and variously outlined to suit the 
habits. Here, again, the task has shaped the tool. 
We have seen in the last chapter that all birds ex- 
cept penguins show evidences of having once been 
terrestrial, using the lee so much that the wing set 
up a tendency to degenerate or shorten. This resulted 
among some fossil forms in complete loss of wing and 
in various grades of degeneration. Some doubtless 
retained the use of the wing to a small extent in true 
flight. Out of these latter have likely come the low 
short-winged birds which we now find so near the 
ostrich forms, as the fowls on the dry land side and 
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