said about the jelly-fish, partly because the
inexpert will be puzzled at the inclusion of so active an animal, and
partly because its story admirably illustrates the principle we are
studying. The Medusa really descends from one of the plant-like animals
of the early Archaean period, but it has abandoned the ancestral stalk,
turned upside down, and developed muscular swimming organs. Its past is
betrayed in its embryonic development. As a rule the germ develops into
a stalked polyp, out of which the free-swimming Medusa is formed. This
return to active and free life must have occurred early, as we find
casts of large Medusae in the Cambrian beds. In complete harmony with
the principle we laid down, the jelly-fish has gained in nerve and
sensitiveness in proportion to its return to an active career.

But this principle is best illustrated in the other branch of the early
many-celled animals, which continued to move about in search of food.
Here, as will be expected, we have the main stem of the animal world,
and, although the successive stages of development are obscure, certain
broad lines that it followed are clear and interesting.

It is evident that in a swarming population of such animals the most
valuable qualities will be speed and perception. The sluggish Coral
needs only sensitiveness enough, and mobility enough, to shrink behind
its protecting scales at the approach of danger. In the open water the
most speedy and most sensitive will be apt to escape destruction,
and have the larger share in breeding the next generation. Imagine a
selection on this principle going on for millions of years, and the
general result can be conjectured. A very interesting analogy is found
in the evolution of the boat. From the clumsy hollowed tree of Neolithic
man natural selection, or the need of increasing speed, has developed
the elongated, evenly balanced modern boat, with its distinct stem and
stern. So in the Archaean ocean the struggle to overtake food, or escape
feeders, evolved an elongated two-sided body, with head and tail, and
with the oars (cilia) of the one-celled ancestor spread thickly along
its flanks. In other words, a body akin to that of the lower water-worms
would be the natural result; and this is, in point of fact, the next
stage we find in the hierarchy of living nature.

Probably myriads of different types of this worm-like organisation were
developed, but such animals leave no trace in the rocks, and we