lds. Throughout this
immeasurable ocean of ether the particles of matter are driven together
and form bodies. These bodies swarm throughout space, like fish in the
sea; travelling singly (the "shooting star"), or in great close shoals
(the nucleus of a comet), or lying scattered in vast clouds. But the
inexorable pressure urges them still, until billions of tons of
material are gathered together. Then, either from the sheer heat of the
compression, or from the formation of large and unstable atomic systems
(radium, etc.), or both, the great mass becomes a cauldron of fire,
mantled in its own vapours, and the story of a star is run. It dies out
in one part of space to begin afresh in another. We see nothing in
the nature of a beginning or an end for the totality of worlds, the
universe. The life of all living things on the earth, from the formation
of the primitive microbes to the last struggles of the superman, is a
small episode of that stupendous drama, a fraction of a single scene.
But our ampler knowledge of it, and our personal interest in it, magnify
that episode, and we turn from the cosmic picture to study the formation
of the earth and the rise of its living population.

CHAPTER IV. THE PREPARATION OF THE EARTH

The story of the evolution of our solar system is, it will now be seen,
a local instance of the great cosmic process we have studied in the last
chapter. We may take one of the small spiral nebulae that abound in the
heavens as an illustration of the first stage. If a still earlier stage
is demanded, we may suppose that some previous sun collided with, or
approached too closely, another mighty body, and belched out a large
part of its contents in mighty volcanic outpours. Mathematical reasoning
can show that this erupted material would gather into a spiral nebula;
but, as mathematical calculations cannot be given here, and are less
safe than astronomical facts, we will be content to see the early shape
of our solar system in a relatively small spiral nebula, its outermost
arm stretching far beyond the present orbit of Neptune, and its great
nucleus being our present sun in more diffused form.

We need not now attempt to follow the shrinking of the central part of
the nebula until it becomes a rounded fiery sun. That has been done in
tracing the evolution of a star. Here we have to learn how the planets
were formed from the spiral arms of the nebula. The principle of their
formation is already clea