much more closely analogous to the real
or mythical conditions of the toad-in-a-hole. That curious West African
mud-fish, the Lepidosiren (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 contingency, 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 aestivates, for months together, in
a torpid condition. The surrounding mud then hardens into a dry ball;
and these balls 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 zoological
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 proceed 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 hibernation is.
Everybody knows nowadays, I suppose, that there is a very close analogy
between an animal and a steam-engine. Food is the fuel that makes the
animal engine go; and this food acts almost exactly as coal does in the
artificial machine. But coal alone will not drive an engine; a free
draught of open air is also required in order to produce combustion.
Just in like manner the food we eat cannot be utilised to