14 THE STORY OF THE BIRDS. 
There are some other changes, such as the change 
of the direction in the hinging of the bird wrist and 
the pitting of the bones (to better suit the attachment 
of wing quills), but we can not discuss these here. 
There is no biological evidence that wings ever 
sprouted out of birds (or any other vertebrate) as 
wings. Short, vestigial wings of some ostrich forms 
and fossil birds all show traces of having degenerated 
from a wing once good and complete, but which has 
lost its parts by disuse as a fanner. Neither can we 
say that bird wings, as we know them now, were the 
cause of flight, but rather, as we saw in the last chap- 
ter, they are the result of it. 
So likewise it is probable that the use of feathers 
in their present form was simply an incident to con- 
ditions begun long before they were useful in flight. 
But while there is no doubt that feathers are not ne- 
cessary to flight, there is every reason to believe that 
much of their present structure, when complete, is 
the result of their special adaptation to its demands, 
and that they have been a large factor in modifying a 
bird’s fore leg. 
The placing of this muscle on the chest and the 
development of the greater one that overlies it, by 
which the down stroke of the wing is made, were evi- 
dently brought about by the demands for fluttering 
up, while the fore leg began at first to be modified as 
an instrument for getting down. This great change 
in wing muscles must then have been comparatively 
later. 
Perhaps there is no better place than this to speak 
