; the brief span of
human life has not been long enough to do so. The reason why we believe
the oak-tree to have passed through all these stages is, because we are
familiar with oak-trees of every gradation in size, from the seedling
up to the noble veteran. Having seen this gradation in a vast multitude
of trees, we are convinced that each individual passes through all these
stages.

It was by a similar train of reasoning that Herschel was led to adopt
the view of the origin of the stars which we have endeavoured to
describe. The astronomer's life is not long enough, the life of the
human race might not be long enough, to watch the process by which a
nebula condenses down so as to form a solid body. But by looking at one
nebula after another, the astronomer thinks he is able to detect the
various stages which connect the nebula in its original form with the
final form. He is thus led to believe that each of the nebulae passes, in
the course of ages, through these stages. And thus Herschel adopted the
opinion that stars--some, many, or all--have each originated from what
was once a glowing nebula.

Such a speculation may captivate the imagination, but it must be
carefully distinguished from the truths of astronomy, properly so
called. Remote posterity may perhaps obtain evidence on the subject
which to us is inaccessible: our knowledge of nebulae is too recent.
There has not yet been time enough to detect any appreciable changes:
for the study of nebulae can only be said to date from Messier's
Catalogue in 1771.

Since Herschel's time, no doubt, many careful drawings and observations
of the nebulae have been obtained; but still the interval has been much
too short, and the earlier observations are too imperfect, to enable any
changes in the nebulae to be investigated with sufficient accuracy. If
the human race lasts for very many centuries, and if our present
observations are preserved during that time for comparison, then
Herschel's theory may perhaps be satisfactorily tested.

A hundred years have passed since Laplace, with some diffidence, set
forth his hypothesis as to the mode of formation of the solar system. On
the whole it must be said that this "nebular hypothesis" has stood the
test of advancing science well, though some slight modifications have
become necessary in the light of more recent discoveries. Laplace (and
Herschel also) seems to have considered a primitive nebula to consist of
a "fiery mist" or