172 THE REIGN OF LAW. 



always dependent on the possession of a large tail 

 — as, for example, the Swift. 



Another explanation has been given of the 

 means by which birds are able to turn in flight, 

 which is a curious example how preconceived 

 theories founded on false analogies will vitiate 

 our observation of the commonest facts in nature. 

 I do not know of any modern work which gives 

 any account of the theory of flight, which is even 

 tolerably correct. But in most points an admir- 

 able account is to be found in the celebrated work 

 of Borelli, " De Motu Animalium." On the ques- 

 tion, however, of steerage in flight, he gives a 

 solution which the most ordinary observation is 

 sufficient to contradict. Borelli is quite aware that 

 the tail in birds has no such function as that 

 which is usually assigned to it, and he points out 

 the true theoretical objection to the possibility of 

 its having any guiding power — viz., its horizontal 

 position, and its immobility in the lateral direc- 

 tion. But the theory which he himself propounds 

 is equally erroneous. It is this, — that birds deflect 

 their course to the right or to the left, as rowers 

 turn a row-boat — by striking more quickly and 



