78 NATURAL HISTORY OF SELBORNE. 



in England,* and what I have never been able yet to procure) retires 

 or migrates very early in the summer ; it also ranges very high for 

 its food, feeding in a different region of the air ; and that is the 

 reason I never could procure one. Now this is exactly the case 

 with the swifts ; for they take their food in a more exalted region 

 than the other species, and are very seldom seen hawking for flies 

 near the ground, or over the surface of the water. From hence I 

 would conclude that these hiruudines and the larger bats are sup- 

 ported by some sorts of high-flying gnats, scarabs, or phal(zn<z, that 

 are of short continuance ; and that the short stay of these strangers 

 is regulated by the defect of their food. 



By my journal it appears that curlews clamoured on to October 

 the thirty-first ; since which I have not seen or heard any. Swal- 

 lows were observed on to November the third. 



* See also Letters XXII., XXXVI., and note. 



