ce's theory do not
contradict theirs. To deal at length with this theory, to which the name
of "Planetesimal Hypothesis" has been given, would not be possible in a
book of this kind. But it may be of interest to mention that the authors
of the theory in question remount the stream of time still further than
did Laplace, and seek to explain the _origin_ of the spiral nebulae
themselves in the following manner:--

Having begun by assuming that the stars are moving apparently in every
direction with great velocities, they proceed to point out that sooner
or later, although the lapse of time may be extraordinarily long,
collisions or near approaches between stars are bound to occur. In the
case of collisions the chances are against the bodies striking together
centrally, it being very much more likely that they will hit each other
rather towards the side. The nebulous mass formed as a result of the
disintegration of the bodies through their furious impact would thus
come into being with a spinning movement, and a spiral would ensue.
Again, the stars may not actually collide, but merely approach near to
each other. If very close, the interaction of gravitation will give rise
to intense strains, or tides, which will entirely disintegrate the
bodies, and a spiral nebula will similarly result. As happens upon our
earth, two such tides would rise opposite to each other; and,
consequently, it is a noticeable fact that spiral nebulae have almost
invariably two opposite branches (see Plate XXII., p 314). Even if not
so close, the gravitational strains set up would produce tremendous
eruptions of matter; and in this case, a spiral movement would also be
generated. On such an assumption the various bodies of the solar system
may be regarded as having been ejected from parent masses.

The acceptance of the Planetesimal Hypothesis in the place of the
Hypothesis of Laplace will not, as we have seen, by any means do away
with the probability that our solar system, and similar systems, have
originated from a nebulous mass. On the contrary it puts that idea on a
firmer footing than before. The spiral nebulae which we see in the
heavens are on a vast scale, and may represent the formation of stellar
systems and globular clusters. Our solar system may have arisen from a
small spiral.

We will close these speculations concerning the origin of things with a
short sketch of certain investigations made in recent years by Sir
George H. Darwin,