records of geology. He then continues:

"According to Sir Norman Lockyer's Meteoritic Hypothesis, nebulae,
comets, and many so-called stars consist of swarms of meteorites which,
though normally cold and dark, are heated by repeated collisions, and so
become luminous. They may even be volatilised into glowing meteoric
vapour; but in time this heat is dissipated, and the force of gravity
condenses a meteoritic swarm into a single globe. 'Some of the swarms
are,' says Lockyer, 'truly members of the solar system,' and some of
these travel round the sun in nearly circular orbits, like planets. They
may be regarded as infinitesimal planets, and so Chamberlain calls them
'planetismals.'

"The planetismal theory is a development of the meteoritic theory, and
presents it in an especially attractive guise. It regards meteorites as
very sparsely distributed through space, and gravity as powerless to
collect them into dense groups. So it assigns the parentage of the solar
system to a spiral nebula composed of planetismals, and the planets as
formed from knots in the nebula, where many planetismals had been
concentrated near the intersections of their orbits. These groups of
meteorites, already as dense as a swarm of bees, were then packed closer
by the influence of gravity, and the contracting mass was heated by the
pressure, even above the normal melting-point of the material, which was
kept rigid by the weight of the overlying layers."

Now, adopting this theory as the last word of science upon the subject
of the origin of planets, we see that it affords immense scope for
diversity in results depending on the total _amount_ of matter available
within the range of attraction of an incipient planetary mass, and the
_rates_ at which this matter becomes available. By a special combination
of these two quantities (which have almost certainly been different for
each planet) I think we may be able to throw some light upon the
structure and physical features of Mars.

_The Probable Mode of Origin of Mars._

This planet, lying between two of much greater mass, has evidently had
less material from which to be formed by aggregation; and if we
assume--as in the absence of evidence to the contrary we have a right to
do--that its beginnings were not much later (or earlier) than those of
the earth, then its smaller size shows that it has in all probability
aggregated very much more slowly. But the internal heat acquired by a
planet while