SEVEN- YEAE SLEEPEES 77 



siren (familiar to all readers of evolutionary literature as 

 one of the most singular existing links between fish and 

 amphibians), lives among the shallow pools and broads of 

 the Gambia, which are dried up during the greater part of 

 the tropical summer. To provide against this annual con- 

 tingency, the mud-fish retires into the soft clay at the 

 bottom of the pools, where it forms itself a sort of nest, 

 and there hibernates, or rather sestivates, for months 

 together, in a torpid condition. The surrounding mud 

 then hardens into a dry ball ; and these baUs are dug out 

 of the soil of the rice-fields by the natives, with the fish 

 inside them, by which means many specimens of lepidosiren 

 have been sent alive to Europe, embedded in their natural 

 covering. Here the strange fish is chiefly prized as a zoo- 

 logical curiosity for aquariums, because of its possessing 

 gills and lungs together, to fit it for its double existence ; 

 but the unsophisticated West Africans grub it up on their 

 own account as a delicacy, regardless of its claims to 

 scientific consideration as the earliest known ancestor of 

 all existing terrestrial animals. Now, the torpid state of 

 the mud-fish in his hardened ball of clay closely resembles 

 the real or supposed condition of the toad-in-a-hole ; but 

 with one important exception. The mud -fish leaves a 

 small canal or pipe open in his cell at either end to admit 

 the air for breathing, though he breathes (as I shall pro- 

 ceed to explain) in a very slight degree during his 

 aestivation ; whereas every proper toad-in-a-hole ought by 

 all accounts to live entirely without either feeding or 

 breathing in any way. However, this is a mere detail ; 

 and indeed, if toads-in-a-hole do really exist at all, we must 

 in all probability ultimately admit that they breathe to 

 some extent, though perhaps very slightly, during their 

 long immurement. 



And this leads us on to consider what in reality hiber- 



