al state of great heat and fluidity, perhaps even
from a mixture mainly consisting of gases; that such a body as the
planet Jupiter represents one of the stages through which it has passed,
that such a body as the moon represents a stage toward which it is
tending; that it has shrunk as it cooled, and as it shrank has formed
the elevations which we call mountains, and the depressions which
contain the seas and oceans; that it has been worn by the action of heat
from within and water from without, and in consequence of this action
presents the appearance when examined below the surface of successive
strata or layers; that different kinds of animal and vegetable life have
followed one another on the surface, and that some of their remains are
found in these strata now; and that all this has taken enormous periods
of time. All this is exceedingly probable, because it is the way in
which, as Laplace first pointed out, under well-established scientific
laws of matter, particularly the law of gravitation and the law of the
radiation of heat, a great fluid mass would necessarily change. And the
whole solar system may and probably did come into its present condition
in this way. It certainly could have been so formed, and there is no
reason for supposing that it was formed in any other way.

Once more, if we begin, as it were, at the other end, and trace things
backwards from the present, instead of forwards from the remote past, it
cannot be denied that Darwin's investigations have made it exceedingly
probable that the vast variety of plants and animals have sprung from a
much smaller number of original forms.

In the first place, the unity of plan which can be found pervading any
great class of animals or plants seems to point to unity of ancestry.
Why, for instance, should the vertebrate animals be formed on a common
plan, the parts of the framework being varied from species to species,
but the framework as a whole always exhibiting the same fundamental
type? If they all descended from a common ancestor, and the variations
were introduced in the course of that descent, this remarkable fact is
at once accounted for. But, in the second place, observation shows that
slight variations ARE perpetually being introduced with every
successive generation, and many of these variations are transmitted to
the generations that follow. In the course of time, therefore, from any
one parent stock would descend a very large variety of kinds.