ANIMALS. 21 



intercept their view : it must therefore only be, that they go on 

 ns they continue to perceive the atmosphere more suitable to their 

 present wants and dispositions.* 



All this seems to be pretty plain -. but there is a circumstance 

 attending the migration of swallows which wraps this subject in 

 great obscurity. It is agreed on all hands, that they are seen 

 migrating into warmer climates, and that in amazing numbers, at 

 the approach of the European winter. Their return into Europe 

 is also as well attested about the beginning of summer ; but we 

 have another account, which serves to prove that numbers of 

 them continue toipid here during the winter, and, like bats, make 

 their retreat into old walls, the hollow of trees, or even sink into 

 the deepest lakes, and find security for the winter season by 

 remaining there in clusters at the bottom. However this latter 

 circumstance may be, their retreat into old walls is too well 

 authenticated to remain a doubt at present. The difficulty, 

 therefore, is to account for this difference in these animals thus 

 variously preparing to encounter the winter. It was supposed 

 that in some of them the blood might lose its motion by the cold, 

 and that thus they were rendered torpid by the severity of the 

 season ; but Mr Buffon having placed many of this tribe in an 

 ice-house, found that the same cold by which their blood was 

 congealed was fatal to the animal ; it remains, therefore, a doubt 

 to this hour, whether there may not be a species of swallows to 

 all external appearance like the rest, but differently farmed with- 

 in, so as to fit them for a state of insensibility during the winter 

 here. It was suggested, indeed, that the swallows found thus 

 torpid, were such only as were too weak to undertake the mi- 

 gration, or were hatched too late to join the general convoy; but 

 it was upon these that Mr Buffon tried his experiment ; it was 

 these that died under the operation, f 



* By attaching- a silken tliread to their leg, it has hoen well ascertained 

 tliat swallows return to their former haunts. 



f The analof;^ between birds of passage, and animals which remain iu a 

 state of torpidity during the winter, does not appear to be accurately drawn 

 by our author : the following ai'e the objections to the supposed constitu- 

 tional connexion. Tliose quadrupeds, reptiles, and insects, which pass the 

 winter in a state of insensibility, may be recalled to sensation and action at 

 jdeasure, by the application of a gentle degree of warmth. Philosophers 

 have been induced, from this constitutional singularity of these animals, to 

 conclude unanimously, that the return of spring' rouses them from their 



