400 The Bird 
of a flutterer, or scaler, than a true flier, but as time went 
on, and birds became more and more expert on the wing, 
their wings grew stronger and their tails shorter and more 
compact. We can readily see the reason for this, if we 
imagine a ship which has been built with a rudder as long 
as its whole deck. What an awkward thing such a rudder 
would be! The waves would beat against it and great 
force would be necessary to turn it and to steer the ship. 
As long as a bird was content to climb a tree with its 
hands and feet, and then scale, like a flying squirrel, to 
the base of the next, a lizard-like tail would be all-sufficient. 
So conspicuous and so unbirdlike was the long appendage 
of the Archzopteryx that Sawrure—lizard-tailed—has 
been given as the name of the Sub-class which it occu- 
pies all to itself. 
When we look at the bones of the tail of a modern 
bird, we find that many interesting changes have taken 
place since the days of the lizard-tailed ancestors. Thus 
in the common duck, for example, we find eight free 
bones followed by a large upturned bone, which, from 
its shape, is known as the ploughshare. It is this terminal 
bone which supports all the tail-feathers of modern birds, 
and in the duck it represents ten of the lizard-tail bones 
all telescoped and fused into one. Some of the feathers 
have been lost, as there are but sixteen in this bird’s 
tail. This loss of tail-feathers is of no value in classifica- 
tion, as it may vary within narrow limits. For example, 
one species of cormorant has seven pairs of tail-feathers, 
while a closely related species has but six. Not only 
this, but the variation may be merely sexual, as in the 
