-bladder has become porous and cellular, so as to be modified into a
pair of true and serviceable lungs. In fact, the lungs themselves in all
the higher animals are merely the swim-bladders of fish, slightly
altered so as to perform a new but closely allied office. The mud-fish
is common enough in all the larger English aquariums, owing to a
convenient habit in which it indulges, and which permits it to be
readily conveyed to all parts of the globe on the same principle as the
vans for furniture. When the dry season comes on and the rice-fields are
reduced to banks of baking mud, the mud-fish retire to the bottom of
their pools, where they form for themselves a sort of cocoon of hardened
clay, lined with mucus, and with a hole at each end to admit the air;
and in this snug retreat they remain torpid till the return of wet
weather. As the fish usually reach a length of three or four feet, the
cocoons are of course by no means easy to transport entire. Nevertheless
the natives manage to dig them up whole, fish and all; and if the
capsules are not broken, the unconscious inmates can be sent across by
steamer to Europe with perfect safety. Their astonishment when they
finally wake up after their long slumber, and find themselves inspecting
the British public, as introduced to them by Mr. Farini, through a sheet
of plate-glass, must be profound and interesting.

In England itself, on the other hand, we have at least one kind of fish
which exemplifies the opposite or migratory solution of the dry pond
problem, and that is our familiar friend the common eel. The ways of
eels are indeed mysterious, for nobody has ever yet succeeded in
discovering where, when, or how they manage to spawn; nobody has ever
yet seen an eel's egg, or caught a female eel in the spawning condition,
or even observed a really adult male or female specimen of perfect
development. All the eels ever found in fresh water are immature and
undeveloped creatures. But eels do certainly spawn somewhere or other in
the deep sea, and every year, in the course of the summer, flocks of
young ones, known as elvers, ascend the rivers in enormous quantities,
like a vast army under numberless leaders. At each tributary or
affluent, be it river, brook, stream, or ditch, a proportionate
detachment of the main body is given off to explore the various
branches, while the central force wriggles its way up the chief channel,
regardless of obstacles, with undiminished vigour. W