ting sun there would be at least thinner and denser
masses, and they must have been hurled so far into space that their
speed in travelling round the central body, perhaps seconded by the
attraction of the second star, overcame the gravitational pull back to
the centre. Recollect the force which, in the new star in Perseus, drove
masses of hydrogen for millions of miles at a speed of a thousand miles
a second.

These denser nuclei or masses would, when the eruption was over, begin
to attract to themselves all the lighter nebulous material within their
sphere of gravitational influence. Naturally, there would at first be a
vast confusion of small and large centres of condensation in the arms
of the nebula, moving in various directions, but a kind of natural
selection--and, in this case, survival of the biggest--would ensue. The
conflicting movements would be adjusted by collisions and gravitation,
the smaller bodies would be absorbed in the larger or enslaved as
their satellites, and the last state would be a family of smaller suns
circling at vast distances round the parent body. The planets, moreover,
would be caused to rotate on their axes, besides revolving round the
sun, as the particles at their inner edge (nearer the sun) would move
at a different speed from those at the outer edge. In the course of time
the smaller bodies, having less heat to lose and less (or no) atmosphere
to check the loss, would cool down, and become dark solid spheres, lit
only by the central fire.

While the first stage of this theory of development is seen in the
spiral nebula, the later stages seem to be well exemplified in the
actual condition of our planets. Following, chiefly, the latest research
of Professor Lowell and his colleagues, which marks a considerable
advance on our previous knowledge, we shall find it useful to glance at
the sister-planets before we approach the particular story of our earth.

Mercury, the innermost and smallest of the planets, measuring only some
3400 miles in diameter, is, not unexpectedly, an airless wilderness.
Small bodies are unable to retain the gases at their surface, on account
of their feebler gravitation. We find, moreover, that Mercury always
presents the same face to the sun, as it turns on its axis in the same
period (eighty-eight days) in which it makes a revolution round the sun.
While, therefore, one half of the globe is buried in eternal darkness,
the other half is eternally exposed to t