vn FLIGHT 251 



is made, the whole operation is like a flash, and 

 your opportunity has gone before it has come. If the 

 inequality were great, photographs would of course 

 supply irrefragable evidence ; but for or against such 

 a minute difference as the one in question their 

 testimony is not worth much. In this dearth of 

 evidence we can only say that, when other animals 

 have freedom of movement, it would be strange that 

 birds should have their wings tied to one another, and 

 that nothing but an unequal stroke can account for 

 the rapidity with which a Swallow dashes from right 

 to left. But, though I lean to this view, I believe that 

 a bird has other means of altering its balance for 

 steering purposes, which are very likely the sole 

 means employed in the slower turns and co-operate 

 in effecting the most rapid. I have already mentioned 

 that a bird's back is at this point by no means the 

 rigid rod that it is said to be. The waist is capable of 

 considerable movement not only up and down but 

 from side to side. I give what measurements I have 

 to show the amount of pliability, regretting that the 

 evidence is so meagre. 



A black-headed Gull and a Sand-martin both showed 

 great flexibility,but I have no measurements. From the 

 figures it appears that a Kestrel, the first specimen 

 measured, had a more flexible waist than any of the 



