Mr. Moulton concludes, from his careful examination of the subject, that 'the solar nebula was heterogeneous to a degree not heretofore considered as being probable, and that it may have been in a state' resembling that exhibited in recent photographs of spiral nebulæ.[27] But, even if all the facts do not chime in with this tempting analogy, there can be little reason to dissent from his intimated opinion that 'the Laplacian hypothesis is only partially true, and that we do not yet know the precise mode of the development of the solar system.' FOOTNOTES: [Footnote 16: _Mémoires de l'Académie de Montpellier_, tom. i.] [Footnote 17: _The Tides_, p. 327.] [Footnote 18: _Mémoires de l'Académie de Montpellier_, tom. viii.] [Footnote 19: C. Wolf, _Bulletin Astronomique_, tom. i., p. 596.] [Footnote 20: _American Journal of Science_, vol. xxxviii., p. 3.] [Footnote 21: G. H. Darwin, _Nature_, vol. xxxi., p. 506.] [Footnote 22: _Proceedings of the American Philosophical Society_, vol. xxii., p. 109.] [Footnote 23: De Vries, _Die Mutationstheorie_, Bd. II., p. 714.] [Footnote 24: _Formation Mécanique du Système du Monde._ See also _Le Problème Solaire_, by the Abbé Th. Moreux, p. 63 _et seq._] [Footnote 25: _Revue des Questions Scientifiques_, January, 1904.] [Footnote 26: _The Earth's Beginnings_, p. 247.] [Footnote 27: _Astrophysical Journal_, vol. xi., p. 130.] CHAPTER V TIDAL FRICTION AS AN AGENT IN COSMOGONY The effects of tidal friction are of almost infinite complexity. How it will act in each particular case cannot be predicted offhand; it is a matter for detailed inquiry. Mutually countervailing influences have to be taken into account, nor is the balance easy to strike. The manner of its inclination may, indeed, often depend upon qualities and relations of the bodies concerned which lie outside the range of what can be distinctly ascertained. All that may be hoped for, then, is to arrive at estimates neither misleading by their ostensible precision, nor yet so vague as to be wholly uninstructive, of the part played by tidal forces in moulding the history of connected globes.