vit FLIGHT 261 
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. 
Angle formed by backbone 
bending sideways at waist. 
Kestrel ... sity sii did ay wee W414 
Swallow... a3 i $e on ee 150° 
Common Tern oes sie Bed en Lb 
Kestrel (another specimen) ... eh a. 156° 
Domestic Duck vi oa wt we 165° 
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 
