proved that the relatively short life-bearing periods
in the existence of the several planets must of necessity synchronise,
instead of all the probabilities lying overwhelmingly the other way.

While this has been (in my judgment) a defect in what may be called the
Brewsterian theory of other worlds, a defect not altogether dissimilar
has characterised the opposite or Whewellite theory. Very useful service
was rendered to astronomy by Whewell's treatise upon, or rather against,
the plurality of worlds, calling attention as it did to the utter
feebleness of the arguments on which men had been content to accept the
belief that other planets and other systems are inhabited. But some
among the most powerfully urged arguments against that belief tacitly
relied on the assumption of a similarity of general condition among the
members of the solar system. For instance, the small mean density of
Jupiter and Saturn had, on the Brewsterian theory, been explained as
probably due to vast hollow spaces in those planets' interiors--an
explanation which (if it could be admitted) would leave us free to
believe that Jupiter and Saturn may be made of the same materials as our
own earth. With this was pleasantly intermixed the conception that the
inhabitant of these planets may have his 'home in subterranean cities
warmed by central fires, or in crystal caves cooled by ocean tides, or
may float with the Nereids upon the deep, or mount upon wings as eagles,
or rise upon the pinions of the dove, that he may flee away and be at
rest,' with much more in the same fanciful vein. We now know that there
can be no cavities more than a few miles below the crust of a planet,
simply because, under the enormous pressures which would exist, the most
solid matter would be perfectly plastic. But while Whewell's general
objection to the theory that Jupiter or Saturn is in the same condition
as our earth thus acquires new force, the particular explanation which
he gave of the planet's small density is open to precisely the same
general objection. For he assumes that, because the planet's mean
density is little greater than that of water, the planet is probably a
world of water and ice with a cindery nucleus, or in fact just such a
world as would be formed if a sufficient quantity of water in the same
condition as the water of our seas were placed at Jupiter's greater
distance from the sun, around a nucleus of earthy or cindery matter
large enough to make t