xl 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 
aérial 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 forrepose. 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 gth 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- 
