tion. We see clearly that it was once
intensely hot! and we know from experimental researches on the cooling
of various earths that many millions of years must have been required by
the earth in cooling down from its former igneous condition. We may
doubt whether Bischoff's researches can be relied upon in details, and
so be unwilling to assign with him a period of 350 millions of years to
a single stage of the process of cooling. But that the entire process
lasted tens of millions and probably hundreds of millions of years
cannot be doubted. Recognising such enormous periods as these in the
development of one of the smallest fruits of the great solar tree of
life, we cannot but admit at least the reasonableness of believing that
the larger fruits (Jupiter, for instance, with 340 times as much matter,
and Saturn with 100 times) must require periods still vaster, probably
many times larger. Indeed, science shows not only that this view is
reasonable, but that no other view is possible. For the mighty root of
the tree of life, the great orb of the sun, containing 340 _thousand_
times as much matter as the earth, yet mightier periods would be needed.
The growth and development of these, the parts of the great system, must
of necessity require much shorter time-intervals than the growth and
development of the system regarded as a whole. The enormous period when
the germs only of the sun and planets existed as yet, when the chaotic
substance of the system had not yet blossomed into worlds, the mighty
period which is to follow the death of the last surviving member of the
system, when the whole scheme will remain as the dead trunk of a tree
remains after the last leaf has fallen, after the last movement of sap
within the trunk--these periods must be infinite compared with those
which measure the duration of even the mightiest separate members of the
system.

But all this has been left unnoticed by those who have argued in support
of the Brewsterian doctrine of a plurality of worlds. They argue as if
it had never been shown that every member of the solar system, as of
all other such systems in space, has to pass through an enormously long
period of preparation before becoming fit to be the abode of life, and
that after being fit for life (for a period very long to our
conceptions, but by comparison with the other exceedingly short) it must
for countless ages remain as an extinct world. Or else they reason as
though it had been