THE BOOK OF MIGRATORY BIRDS 247 



them different from other birds as to the organs of respira- 

 tion. More than this, all the animals dissected by me, of 

 the class that sleep during winter, such as lizards, frogs, 

 &c., had a very different conformation as to these organs* 

 But I am firmly of opinion that terrestrial animals cannot 

 remain any long time under water without drowning.'* 



Independently of the established principles of phy- 

 siology, the matter has been experimentally tried, and it 

 has been found that swallows kept under water, with all 

 due precautions, die in a few minutes. 



A no less fanciful, but, as it appears to many, a more 

 defensible opinion, was published in a scarce tract, pur- 

 porting to be written by "A Person of Learning and 

 Piety," who maintained with no little ingenuity that our 

 migratory birds retire to the moon. He thinks that they 

 are about two months in passing thither, and that after 

 they are arrived above the lower regions of the air into the 

 thin aether, they will have no occasion for food, as it will 

 not be so apt to prey upon the spirits as our lower air. 



Even on our earth, he argues, bears will live upon 'their 

 fat all the winter ; and hence these birds, being very succu- 

 lent and sanguine, may have their provisions laid up in 

 their bodies for the voyage; or perhaps they are thrown 

 into a state of somnolency by the motion arising from the 

 mutual attraction of the earth and moon. 



There is a preponderating list of eminent naturalists who 

 favour the idea that swallows migrate, and the gist of their 

 remarks may be very briefly summed up. "Birds certainly 

 leave our country. Without disputing that difference of 

 temperature and nourishment have much to do with it, 

 they are inclined to consider, that habit is quite as much 

 concerned; according to them the recollection of the old 

 ones, that they have made the journey, carrying the young 

 with them, and the "instinct of travel," which, at certain 

 periods, affects them with a real nostalgia, must be con- 

 sidered, especially the last, as the principal and imme- 

 diately exciting cause of these migrations. Birds of 

 passage, too, always migrate with a contrary wind, which 



