400 The Bird 



of a flutterer, or sealer, 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 Archseopteryx that Saururce 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 



