ustration: PHOTOGRAPH OF THE ENTIRE SHELL OF THE PEARLY NAUTILUS

The headquarters of the Nautilus are in the Indian and Pacific Oceans.
They sometimes swim at the surface of the sea, but they usually creep
slowly about on the floor of comparatively shallow water.]

[Illustration: NAUTILUS

A section through the Pearly Nautilus, _Nautilus pompilius_, common from
Malay to Fiji. The shell is often about 9 inches long. The animal lives
in the last chamber only, but a tube (S) runs through the empty
chambers, perforating the partitions (SE). The bulk of the animal is
marked VM; the eye is shown at E; a hood is marked H; round the mouth
there are numerous lobes (L) bearing protrusible tentacles, some of
which are shown. When the animal is swimming near the surface the
tentacles radiate out in all directions, and it has been described as "a
shell with something like a cauliflower sticking out of it." The Pearly
Nautilus is a good example of a conservative type, for it began in the
Triassic Era. But the family of Nautiloids to which it belongs
illustrates very vividly what is meant by a dwindling race. The
Nautiloids began in the Cambrian, reached their golden age in the
Silurian, and began to decline markedly in the Carboniferous. There are
2,500 extinct or fossil species of Nautiloids, and only 4 living
to-day.]

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

SHOEBILL

A bird of a savage nature, never mixing with other marsh birds.
According to Dr. Chalmers Mitchell, it shows affinities to herons,
storks, pelicans, and gannets, and is a representative of a type equal
to both herons and storks and falling between the two.]

But it is very important to realise that among plant and animals
likewise, _Evolution is going on_.

The Fountain of Change: Variability

On an ordinary big clock we do not readily see that even the minute hand
is moving, and if the clock struck only once in a hundred years we can
conceive of people arguing whether the hands did really move at all. So
it often is with the changes that go on from generation to generation in
living creatures. The flux is so slow, like the flowing of a glacier,
that some people fail to be convinced of its reality. And it must, of
course, be admitted that some kinds of living creatures, like the
Lamp-shell _Ligula_ or the Pearly Nautilus, hardly change from age to
age, whereas others, like some of the birds and butterflies, are always
giving rise to something new. The E