Mathematical investigation does not go to show that the rings, presuming they could be left behind during the contraction of the mass, would have aggregated into planetary bodies. Indeed, it rather points to the reverse. Lastly, such a discovery as that the ninth satellite of Saturn revolves in a _retrograde_ direction--that is to say, in a direction contrary to the other revolutions and rotations in our solar system--appears directly to contradict the hypothesis. Although Laplace's hypothesis seems to break down under the keen criticism to which it has been subjected, yet astronomers have not relinquished the idea that our solar system has probably had its origin from a nebulous mass. But the apparent failure of the Laplacian theory is emphasised by the fact, that _not a single example of a nebula, in the course of breaking up into concentric rings, is known to exist in the entire heaven_. Indeed, as we saw in Chapter XXIV., there seems to be no reliable example of even a "ring" nebula at all. Mr. Gore has pointed this out very succinctly in his recently published work, _Astronomical Essays_, where he says:--"To any one who still persists in maintaining the hypothesis of ring formation in nebulæ, it may be said that the whole heavens are against him." The conclusions of Keeler already alluded to, that the spiral is the normal type of nebula, has led during the past few years to a new theory by the American astronomers, Professors Chamberlin and Moulton. In the detailed account of it which they have set forth, they show that those anomalies which were stumbling-blocks to Laplace'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 nebulæ themselves in the following manner:--