MIGRATION. 87 



out early in the morning : at first there was a vast 

 fog, but, by the time that I was got seven or eight 

 miles from home towards the coast, the sun broke 

 out into a delicate warm day. We were then on a 

 large heath, or common, and I could discern, as the 

 mist began to break away, great numbers of swallows 

 (hirundines rusticce) clustering on the stunted shrubs 

 and bushes, as if they had roosted there all night. 

 As soon as the air became clear and pleasant, they 

 all were on the wing at once ; and, by a placid and 

 easy flight, proceeded on southward, towards the 

 sea : after this I did not see any more flocks, only 

 now and then a straggler. 



I cannot agree with those persons who assert, 

 that the swallow kind disappear some and some, 

 gradually, as they come ; for the bulk of them seem 

 to withdraw at once ; only some stragglers stay 

 behind a long while, and do never, there is the 

 greatest reason to believe, leave this island. Swal- 

 lows seem to lay themselves up, and to come forth 

 in a warm day, as bats do continually of a warm 

 evening, after they have disappeared for weeks. 

 For a very respectable gentleman assured me, that, 



The causes influencing the migration of birds appear more 

 difficult to solve, than the possibility of the execution of it. 

 They seem to be influenced by an innate law, which we do 

 not, and cannot, comprehend, though in some measure depen- 

 dent on the want of food or climate congenial to the system 

 of each, and which acts almost without the will of the indi- 

 vidual. Neither this, however, nor the duties incumbent on 

 incubation, can be the only exciting causes, as we may judge 

 by the partial migrations of some to different parts of the same 

 country, where food and the conveniences for breeding are 

 alike ; by the partial migration only, of a species from one 

 country to another, differing decidedly in temperature, and 

 where the visiting species thrives equally with the resident one; 

 and by the males of some species migrating, while the females 

 remain. W. J. 



