INTRODUCTION. 
xiv 
ly a mile in one minute, but that was only for the fmall fpace 
of a fecond of time.* In this and fimilar inftances we find, that 
an uncommon degree of exertion was attended with its ufual 
confequences, debility, and a total want of power to continue it 
to the fame extent ; but the cafe is very different with birds, 
their motions are not impeded by the fame caufes, they glide 
through the air with a quicknefs fuperior to that of the fwiftefl: 
quadruped, and they can continue on the wing with the fame 
fpeed for a confiderable length of time. Now, if we can fup- 
pofe a bird to go at the rate of only half a mile in a minute, for 
the fpace of twenty-four hours, it will have gone over, in that 
time, an extent of more than feven hundred miles, which is fuf- 
ficient to account for almoft the longed: migration ; but if aid- 
ed by a favourable current of air, there is reafon to fuppofe that 
the fame journey may be performed in a much fhorter fpace of 
time. To thefe obfervations we may add, that the fight of 
birds is peculiarly quick and piercing ; and from the advantage 
they poflefs in being railed to confiderable heights in the air, 
which is well known to be the cafe with the Stork, Bittern, 
and other kinds of birds, they are enabled, with a fagacity pe- 
culiar to inflin&ive knowledge, to difcover the route they are to 
take, from the appearance of the atmofphere, the clouds, the 
direction of the winds, and other caufes ; fo that, without having 
recourfe to improbable modes, it is eafy to conceive, from the 
velocity of their fpeed alone, that mod: birds may tranfport 
themfelves to countries laying at great diftances, and acrofs vaft 
tradls of ocean. 
The following obfervations from Catefby are very applicable, 
and will conclude our remarks on this head : “ The manner of 
their journeyings may vary according as the ftrufture of their 
bodies, enables them to fupport themfelves in the air. Birds 
“ with fhort wings, fuch as the Redttart, Black-cap, &c. may 
<c pafs by gradual and flower movements ; and there feems no 
i6 necefiity for a precipitate pafiage, as every day affords an in- 
* Sec Hiftory of Quadrupeds, page 6, 3d edition. 
