CONTRIVANCE A NECESSITY. 171 



slight expansion of the wings, and a very slight 

 change in the axis of the body. 



And here it may be observed that the tails 

 of birds have not, as is often supposed, any 

 function analogous to the rudder of a ship. 

 Birds which have lost the tail are not thereby 

 rendered incapable of turning. If the steering 

 function had been assigned to birds' tails, the 

 vane of the tail must have been set, not, as it 

 is, horizontally, but perpendicularly to the line 

 of flight. But a bird's tail has in flight no 

 lateral motion whatever. It does, indeed, ma- 

 terially assist the bird in turning, because it 

 serves to stop the way of a bird when it rises or 

 turns in the air to take a new direction. It con- 

 tributes also largely to the general balance of 

 the body, which in itself is an important element 

 in the facility of flight. Accordingly, almost all 

 birds which depend on great ease of evolution in 

 flight — or on the power of stopping suddenly, have 

 largely developed tails. This is the case with all 

 the birds of prey — with the Kestrel in a con- 

 spicuous degree. But there are some exceptions 

 which show that great powers of flight are not 



