indeed, now reject Laplace's hypothesis in toto, preferring to think
that even our solar system originated from a spiral nebula. Since the
spiral type prevails among the existing nebulæ, we must make any
mechanical theory of the development of stars and planetary systems
from them accord with the requirements which that form imposes. A
glance at the extraordinary variations upon the spiral which Professor
Keeler's photographs reveal is sufficient to convince one of the
difficulty of the task of basing a general theory upon them. In truth,
it is much easier to criticize Laplace's hypothesis than to invent a
satisfactory substitute for it. If the spiral nebulæ seem to oppose it
there are other nebulæ which appear to support it, and it may be that
no one fixed theory can account for all the forms of stellar evolution
in the universe. Our particular planetary system may have originated
very much as the great French mathematician supposed, while others
have undergone, or are now undergoing, a different process of
development. There is always a too strong tendency to regard an
important new discovery and the theories and speculations based upon
it as revolutionizing knowledge, and displacing or overthrowing
everything that went before. Upon the plea that ``Laplace only made a
guess'' more recent guesses have been driven to extremes and treated
by injudicious exponents as ``the solid facts at last.''

Before considering more recent theories than Laplace's, let us see
what the nature of the photographic revelations is. The vast celestial
maelstrom discovered by Lord Rosse in the ``Hunting Dogs'' may be
taken as the leading type of the spiral nebulæ, although there are
less conspicuous objects of the kind which, perhaps, better illustrate
some of their peculiarities. Lord Rosse's nebula appears far more
wonderful in the photographs than in his drawings made with the aid of
his giant reflecting telescope at Parsonstown, for the photographic