ea animals have very long,
stilt-like legs, and many of the sedentary types are lifted into safety
on the end of long stalks which have their bases embedded in the mud. In
adaptation to the darkness, in which there is only luminescence that
eyes could use, there is a great development of tactility. The
interesting problem of luminescence will be discussed elsewhere.

As to the origin of the deep-sea fauna, there seems no doubt that it
has arisen by many contributions from the various shore-haunts.
Following the down-drifting food, many shore-animals have in the course
of many generations reached the world of eternal night and winter, and
become adapted to its strange conditions. For the animals of the
deep-sea are as fit, beautiful, and vigorous as those elsewhere. There
are no slums in Nature.

[Illustration: THE BITTERLING (_Rhodeus Amarus_)

A Continental fish which lays its eggs by means of a long ovipositor
inside the freshwater mussel. The eggs develop inside the mollusc's
gill-plates.]

[Illustration: _Photo: W. S. Berridge._

WOOLLY OPOSSUM CARRYING HER FAMILY

One of the young ones is clinging to its mother and has its long
prehensile tail coiled round hers.]

[Illustration: SURINAM TOAD (_Pipa Americana_) WITH YOUNG ONES HATCHING
OUT OF LITTLE POCKETS ON HER BACK]

[Illustration: STORM PETREL OR MOTHER CAREY'S CHICKEN

(_Procellaria Pelagica_)

This characteristic bird of the open sea does not come to land at all
except to nest. It is the smallest web-footed bird, about four inches
long. The legs are long and often touch the water as the bird flies. The
storm petrel is at home in the Atlantic, and often nests on islands off
the west coast of Britain.]

IV. THE FRESH WATERS

Of the whole earth's surface the freshwaters form a very small fraction,
about a hundredth, but they make up for their smallness by their
variety. We think of deep lake and shallow pond, of the great river and
the purling brook, of lagoon and swamp, and more besides. There is a
striking resemblance in the animal population of widely separated
freshwater basins: and this is partly because birds carry many small
creatures on their muddy feet from one water-shed to another; partly
because some of the freshwater animals are descended from types which
make their way from the sea and the seashore through estuaries and
marshes, and only certain kinds of constitution could survive the
migration; and partly because some lakes are l