INTRODUCTION xxxv 



particle of well authenticated fact supporting the theory of 

 hibernation. 



The explanation is not difficult for anyone who carefully 

 follows the movements of the swallows and martins before 

 they leave us in the autumn, and then turns to White's 

 letters on the subject of migration, giving them his close 

 attention. These letters are X., XII., XXIII. and XXXVIII. 

 to Pennant, and IX., XII., XVIIL, LI. and LV. to Bar- 

 rington. 



Swallows and martins between mid-September and mid- 

 October travel leisurely in parties towards and along our 

 south coast. At first, especially if they are following the 

 coast line, these parties consist of large numbers, but they 

 gradually grow smaller and smaller, until only a few stragglers 

 pass at long intervals. But it is by no means easy to discover 

 that these parties are travelling. A stationary observer sees 

 them dallying round his garden, or collecting on his house- 

 roof, like those that have been with him all the summer, and 

 very naturally fails to notice that they are successive parties, 

 which remain for a short while and then go on their way, 

 giving place to others. It may need several hours of close 

 attention to the flocks to convince one that this is so, and 

 the observer should be in a commanding position, and should 

 have the necessary leisure at a time when the migration is 

 in full flow, and the parties numerous and full. White, 

 though he moved about a good deal, never seems to have 

 discovered this movement in parties : his rides along the 

 South Downs never seem to have taken place just at the right 

 time. And thus he also failed to discover the fact that as 

 October advances the parties become smaller and fewer, with 

 long intervals between them, until at last, it may be after an 

 interval of some days, a dozen belated stragglers may appear 

 for a few hours and then vanish, while still later on two or 

 three may possibly be seen for a day or so. These stragglers 

 will probably be late broods from the north of England or 

 Ireland, which for one reason or another have been slow in 

 making their way south. 



Now, if the observer have a preconception, as White had, 

 that swallows can go into hiding for the winter, what is more 



