8 THE STORY OF THE BIRDS. 
of the skin behind the armpit and in front of the | 
flank. 
Skin-flying, as we have seen, was very fashionable 
in those days. The Pterodactyl group had true, con- 
tinuous flight by the skin of their little fingers, and 
some arboreal frogs may have been easing the jar of 
a downward leap then as now by the expansion of the 
skin between their toes. It is not likely, however, 
that birds attained true continuous flight by means of 
this membrane, which is called the patagvum, but 
that it was simply the first instrument upon which the 
forefathers of the bird first launched themselves into 
the air. Some thinkers are inclined to believe that, 
even after the coming of feathers, this membrane was 
a factor in flying. 
But doubtless in time feathers or plumous scales 
growing projectingly from the rear of the arm and 
from other regions, became similarly useful in resist- 
ing the air, and were finally, in connection with the 
wing, the exclusive instruments of flight. That 
feathers were used at first as means of sailing down 
only is strongly hinted in so complete a bird as the 
Archeopteryx. It had doubtless continuous flight, 
but the bony part of its tail was longer than its body, 
and each vertebra (or joint) had a flat feather project- 
ing from each side like the hairs on a squirrel’s tail. 
Likewise the “drumstick” of the leg was feathered 
so that, outspread as the squirrel in his leap, the an- 
cestors of this bird had likely sailed down from high 
places. 
Indeed, it is just probable that the Archwopterya 
