Address to Section C, Geology. 413 
a long period with a universal tropical climate, under which coral 
reefs grew where flow our polar seas, and palms flourished on what 
are now the Arctic shores. Still further cooling had established our 
climatic zones; and it was predicted that in time the polar cold would 
creep outward, driving all living beings toward the equator, until at 
length the whole earth, like the moon, would become lifeless through 
cold, as it had once been uninhabitable through heat. This theory 
has permanently impressed itself on geological terminology; and its 
corollaries, secular refrigeration and the contortion of the shrinking 
erust, once dominated discussions concerning climatic history and the 
formation of mountain chains. This nebular hypothesis, however, we 
are now told, is mathematically improbable, or even impossible ; and 
it is only consistent with the facts of geology on the assumption that, 
in proportion to the age of the world, the whole of geological time is 
so insignificant that the secular refrigeration during it is quite 
inappreciable; hence geology can no more confirm or correct the 
theory than a stockbreeder could refute evolution by failing to breed 
kangaroos into cows in a single lifetime. 
The theory of the gaseous nebula has been probably of more 
hindrance than help to geologists; its successors, the meteoritic 
hypothesis of Lockyer and the planetismal theory of Chamberlin, are 
of far more practical use to us, and they give a history of the 
world consistent with the actual records of geology. According to 
Sir Norman Lockyer’s meteoritic hypothesis, nebule 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 them travel around the sun in nearly circular orbits, like 
planets. They may be regarded as infinitesimal planets, and so 
Chamberlin 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 solid 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. 
This theory has the recommendation of being consistent with the 
history of the earth as interpreted by geology. For whereas the 
nebular hypothesis represents the earth as having been originally 
intensely hot, and having persistently cooled, yet geological records 
show that an extensive low-level glaciation occurred in Cambrian 
times in low latitudes in South Australia’; indeed, it seems probable 
1 As shown by the work of Professor Howchin, of Adelaide, 
