1875.) The History of our Earth. 3II 
Poisson supposes that the earth may have passed, in its 
earlier history, through regions of space respectively hotter 
and colder than the part where she now revolves. On this 
Mr. Croll remarks, that though there may be a difference in 
the quantity of force passing through different parts of space 
in the form of heat, yet we cannot imagine space in itself to 
be either cold or hot. During the hot periods, therefore, 
the earth must, on this supposition, have passed near to or 
among other sources of heat and light in addition to the 
sun. But masses capable of thus seriously affecting our 
climate could not do other than greatly perturb the me- 
chanism of the solar system. The orbits of the planets 
might therefore be expected to give some evidence of such 
an occurrence. 
Others have assumed that the sun is a variable star, its 
energies waxing and waning at regular or irregular periods. 
Against this it may be urged—which, indeed, applies also to 
the hypothesis of Poisson—that a general reduction of tem- 
perature over the whole globe could not produce the charac- 
teristic phenomena of glaciation. 
A variation in the obliquity of the ecliptic is sometimes 
put forward in explanation of former fluctuations in the 
earth’s temperature. But Mr. Croll declares that “it can 
be shown from celestial mechanics that the variations in the 
obliquity of the ecliptic must always have been so small 
that they could not materially affect the climatic conditions 
of the globe; and even admitting that the obliquity could 
vary to an indefinite extent, it can be shown that no increase 
or decrease, however great, could possibly account for either 
the Glacial epoch or a warm temperate condition of climate 
in polar regions.” —(Page 8). On this point Mr. Croll is at 
issue with Mr. Belt, who maintains ,(“‘ Quarterly Journal of 
Science,” October, 1874) that the Glacial epoch resulted 
from a great increase in the obliquity of the ecliptic. 
“There exist,” he says, ‘‘ glacial conditions at present 
round the poles, due primarily to the obliquity of the 
ecliptic.” ‘*‘ Were the earth’s axis perpendicular to the 
plane of its orbit, spring would reign round the arétic 
circle,” and “‘under such circumstances the piling up of 
snow, or even its production at the sea-level, would be im- 
possible, excepting, perhaps, in the immediate neighbourhood 
of the poles, where the rays of the sun would have but little 
heating power, from its small altitude.” In reply, Mr. Croll 
remarks that, were the earth’s axis to become perpendicular 
to the plane of its orbit, the quantity of heat received by 
the polar regions would be far less than itis now. It is 
VOL. V. (N.S.) 2R 
