XXXVl INTRODUCTION. 



being is surrounded, there seems to be a frequent demand for 

 that relieving invention denied to those animals which are 

 solely governed by inflexible instinct. 



The velocity with which birds are able to travel in their 

 aerial element has no parallel among terrestrial animals ; and 

 this powerful capacity for progressive motion is bestowed in 

 aid of their peculiar wants and instinctive habits. The swiftest 

 horse may perhaps proceed a mile in something less than two 

 minutes ; but such exertion is unnatural, and quickly fatal. An 

 Eagle, whose stretch of wing exceeds seven feet, with ease and 

 majesty, and without any extraordinary effort, rises out of sight 

 in less than three minutes, and therefore must fly more than 

 three thousand five hundred yards in a minute, or at the rate 

 of sixty miles in an hour. At this speed a bird would easily per- 

 form a journey of six hundred miles in a day, since ten hours 

 only would be required, which would allow frequent halts, and 

 the whole of the night for repose. Swallows and other migra- 

 tory birds might therefore pass from northern Europe to the 

 equator in seven or eight days. In fact, Adanson saw, on the 

 coast of Senegal, Swallows that had arrived there on the 9th of 

 October, or eight or nine days after their departure from the 

 colder continent. A Canary Falcon, sent to the Duke of Lerma, 

 returned in sixteen hours from Andalusia to the island of Tene- 

 riffe, — a distance of seven hundred and fifty miles. The Gulls 

 of Barbadoes, according to Sir Hans Sloane, make excursions in 

 flocks to the distance of more than two hundred miles after 

 their food, and then return the same day to their rocky roosts. 



If we allow that any natural powers come in aid of the 

 instinct to migration, so powerful and uniform in birds, besides 

 their vast capacity for motion, it must be in the perfection and 

 delicacy of their vision, of which we have such striking ex- 

 amples in the rapacious tribes. It is possible that at times 

 they may be directed principally by atmospheric phenomena 

 alone ; and hence we find that their appearance is frequently 

 a concomitant of the approaching season, and the wild Petrel 

 of the ocean is not the only harbinger of storm and coming 

 change. The currents of the air, in those which make exten- 



