Evolution of the Corals
Again we must refrain from following in detail the development of
this new world of life which branches off in the Archaean ocean.
The Evolution alone would be a lengthy and
interesting story. But a word must be 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.
|
