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 drive our muscles and other organs unless it is supplied with oxygen from the air to burn it slowly inside our bodies.  This oxygen is taken into the system, in all higher animals, by means of lungs or gills.  Now, when we are working at all hard, we require a great deal of oxygen, as most of us have familiarly discovered (especially if we are somewhat stout) in the act of climbing hills or running to catch a train.  But when we are doing very little work indeed, as in our sleeping hours, during which muscular movement is suspended, and only the general