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 force and takes fresh
gulps. At the same time, like an ordinary fish, it has gills which allow
the usual interchange of gases between the blood and the water. Now this
Australian mudfish or double-breather (Dipnoan), which may be a long way
over a yard in length, is a direct and little-changed descendant of an
ancient extinct fish, Ceratodus, which lived in Mesozoic times, as far
back as the Jurassic, which probably means over five millions of years
ago. The Queensland mudfish is an antiquity, and there has not been much
change in its lineage for millions of years. We might take it as an
illustration of the inertia of evolution. And yet, though its structure
has changed but little, the fish probably illustrates evolution in
process, for it is a fish that is learning to breathe dry air. It cannot
leave the water; but it can live comfortably in pools which are foul
with decomposing animal and vegetable matter. In partially dried-up and
foul waterholes, full of dead fishes of various kinds, Neoceratodus has
been found vigorous and lively. Unless we take the view, which is
_possible_, that the swim-bladder of fishes was originally a lung, the
mud-fishes are learning to breathe dry air. They illustrate evolution
agoing.
[Illustration: DIAGRAM OF THE LIFE HISTORY OF THE COMMON EEL (_Anguilla
Vulgalis_)
1. The transparent open-sea knife-blade-like larva called a
Leptocephalus.
2 and 3. The gradual change of shape from knife-blade-like to
cylindrical. The body becomes shorter and loses weight.
4. The young elver, at least a year old, which makes its way from the
open sea to the estuaries and rivers. It is 2/3 inches long and almost
cylindrical.