r out to sea, to where the old margin of the once larger
continent of Europe slopes down to the great abysses, from the Hebrides
southwards. Eels seem to spawn in the deep dark water; but the just
liberated eggs have not yet been found. The young fry rises to near the
surface and becomes a knife-blade-like larva, transparent all but its
eye. It lives for many months in this state, growing to be about three
inches long, rising and sinking in the water, and swimming gently.
These open-sea young eels are known as Leptocephali, a name given to
them before their real nature was proved. They gradually become shorter,
and the shape changes from knife-blade-like to cylindrical. During this
change they fast, and the weight of their delicate body decreases. They
turn into glass-eels, about 2-1/2 inches long, like a knitting-needle in
girth. They begin to move towards the distant shores and rivers, and
they may be a year and a half old before they reach their destination
and go up-stream as elvers. Those that ascend the rivers of the Eastern
Baltic must have journeyed three thousand miles. It is certain that no
eel ever matures or spawns in fresh water. It is practically certain
that all the young eels ascending the rivers of North Europe have come
in from the Atlantic, some of them perhaps from the Azores or further
out still. It is interesting to inquire how the young eels circumvent
the Falls of the Rhine and get into Lake Constance, or how their kindred
on the other side of the Atlantic overcome the obstacle of Niagara; but
it is more important to lay emphasis on the variety of habitats which
this fish is trying--the deep waters, the open sea, the shore, the
river, the pond, and even, it may be, a little taste of solid earth. It
seems highly probable that the common eel is a deep-water marine fish
which has learned to colonise the freshwaters. It has been adventurous
and it has succeeded. The only shadow on the story of achievement is
that there seems to be no return from the spawning. There is little
doubt that death is the nemesis of their reproduction. In any case, no
adult eel ever comes back from the deep sea. We are minded of Goethe's
hard saying: "Death is Nature's expert advice to get plenty of life."

Sec. 4

Forming New Habits

There is a well-known mudfish of Australia, Neoceratodus by name, which
has turned its swim-bladder into a lung and comes to the surface to
spout. It expels vitiated air with considerable