Page:Is Mars habitable - Wallace 1907.djvu/104

VII.] give us a history of the world consistent with the actual 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,