OF NATURAL HISTORY. 481 



Two reafons feem to render this fuppofed fubmerfion of fwallows 

 impoffible. In the firft place, no land-animal can exift fo long 

 without fome degree of refpiration. The otter, the feal, and water- 

 fowls of all kinds, when confined under the ice, or entangled in 

 nets, foon perifti ; yet it is well known, that animals of this kind 

 can remain much longer under water than thofe who are deflitute 

 of that peculiar ftrudture of the heart which is neceflary for any 

 confiderable refidence beneath that penetrating element. Mr John 

 Hunter, in a letter to Mr Pennant, informs us, ' That he had dif- 



• fe£ted many fwallows, but found nothing in them different from 



• other birds as to the organs of refpiration : That all thofe animals 

 ' which he had diffeded of the clafs that fleep during winter, fuch 

 ' as lizards, frogs, &c. had a very different conformation as to thofe 

 ' organs: That all thofe animals, he believes, do breathe in their 



• torpid ftate; and, as far as his experience reaches, he knows they 



• do ; and that, therefore, he efteems it a very wild opinion, that 



• terreftrial animals can remain any long time under water without 



• drowning.' Another argument againft their fubmerfion arifes from 

 the fpecific gravity of the animals themfelves. Of all birds, the 

 fwallow tribes are perhaps the lighteft. Their plumage, and the 

 comparative fmallnefs of their weight, indicate that Nature deftined 

 them to be almoft perpetually on the wing in queft of food. From 

 this fpecific lightnefs, the fubmerfion of fwallows, and their conti- 

 nuing for months under water, amount to a phyfical impoffibilhy. 

 Even water- fowls, when they wifh to dive, are obliged to rife and 

 plunge with confiderable exertion, in order to overcome the refiftence 

 of the water. Klein's idea of fwallows employing reeds and ftraws as 

 means of fubmerfion is rather ludicrous ; for thefe light fubflances, 

 inftead of being proper inftruments for affifting them to reach the 

 bottom, would infallibly contribute to fupport them on the furface, 

 and prevent the very objed; of their intention, Befides, admitting 

 the pofTibility of their reaching the bottom of lakes and feas, and 



-j- 3 P fuppofing 



