extrusion of the relatively insignificant quantity of matter needed to
form the planets. But even then the effect of the appulse would be to
change the direction of flight, both of the sun and of its visitor,
and there is no known star in the sky which can be selected as the
sun's probable partner in their ancient pas deux. That there are
unconquered difficulties in Laplace's hypothesis no one would deny,
but in simplicity of conception it is incomparably more satisfactory,
and with proper modifications could probably be made more consonant
with existing facts in our solar system than that which is offered to
replace it. Even as an explanation of the spiral nebulæ, not as solar
systems in process of formation, but as the birthplaces of stellar
clusters, the Planetesimal Hypothesis would be open to many
objections. Granting its assumptions, it has undoubtedly a strong
mathematical framework, but the trouble is not with the mathematics
but with the assumptions. Laplace was one of the ablest mathematicians
that ever lived, but he had never seen a spiral nebula; if he had, he
might have invented a hypothesis to suit its phenomena. His actual
hypothesis was intended only for our solar system, and he left it in
the form of a ``note'' for the consideration of his successors, with
the hope that they might be able to discover the full truth, which he
confessed was hidden from him. It cannot be said that that truth has
yet been found, and when it is found the chances are that intuition
and not logic will have led to it.

The spiral nebulæ, then, remain among the greatest riddles of the
universe, while the gaseous nebulæ, like that of Orion, are no less
mysterious, although it seems impossible to doubt that both forms give
birth to stars. It is but natural to look to them for light on the
question of the origin of our planetary system; but we should not
forget that the scale of the phenomena in the two cases is vastly