HOW DID THE BIRDS FIRST FLY. PERHAPS? 7 
Across the front re-entering angle of a bird’s wing 
now the skin is expanded into a double membrane, 
which, when the wing is opened, is kept tight and 
spread out by a special ligament and muscle stretched 
from shoulder to wrist, to use the figure of our own 
arm. This muscle appears as if it had been stripped 
up from the region of our biceps to support the wing 
automatically when folded, but it is a distinct devel- 
opment and not stripped up. It is not at all improb- 
Sih 














Pterodacty]. 
able that the very, very early birds may have used 
this skinny expansion on the fore leg to break the 
shock of a downward leap (whether made bipedally 
or quadrupedally), or it may have been so extensive 
then as to enable them to sail downward, as the flying 
squirrels and others do now by a special development 
