n
the throat, the mouth was improved by the formation of jaws, and--the
worm culminated in the shark.

Some experts think, however, that the fish developed directly from a
Crustacean, and hold that the Ostracoderms are the connecting link. A
close discussion of the anatomical details would be out of place here,
[*] and the question remains open for the present. Directly or
indirectly, the fish is a descendant of some Archaean Annelid. It is
most probable that the shark was the first true fish-type. There are
unrecognisable fragments of fishes in the Ordovician and Silurian rocks,
but the first complete skeletons (Lanarkia, etc.) are of small shark-
like creatures, and the low organisation of the group to which the shark
belongs, the Elasmobranchs, makes it probable that they are the most
primitive. Other remains (Palaeospondylus) show that the fish-like
lampreys had already developed.

* See, especially, Dr. Gaskell's "Origin of Vertebrates"
(1908).

Two groups were developed from the primitive fish, which have great
interest for us. Our next step, in fact, is to trace the passage of the
fish from the water to the land, one of the most momentous chapters in
the story of life. To that incident or accident of primitive life we
owe our own existence and the whole development of the higher types of
animals. The advance of natural history in modern times has made this
passage to the land easy to understand. Not only does every frog reenact
it in the course of its development, but we know many fishes that
can live out of water. There is an Indian perch--called the "climbing
perch," but it has only once been seen by a European to climb a
tree--which crosses the fields in search of another pool, when its own
pool is evaporating. An Indian marine fish (Periophthalmus) remains
hunting on the shore when the tide goes out. More important still,
several fishes have lungs as well as gills. The Ceratodus of certain
Queensland rivers has one lung; though, I was told by the experts in
Queensland, it is not a "mud-fish," and never lives in dry mud. However,
the Protopterus of Africa and the Lepidosiren of South America have two
lungs, as well as gills, and can live either in water or, in the dry
season, on land.

When the skeletons of fishes of the Ceratodus type were discovered in
the Devonian rocks, it was felt that we had found the fish-ancestor of
the land Vertebrates, but a closer anatomical examination has made thi