496 Dil MURRAY ON THE DEEP AND SHALLOW-WATER MARINE FAUNA 



solar system at present. This cold matter fallins; together under the intiuence of mutual 

 gravitation developed such an enormous quantity of heat that eveu its most refractory 

 constituents were at once dissipated into gas. Lord Kelvin says : — " The vapour or gas 

 thus generated will fly outwards, and after several hundreds or thousands of years of 

 outward and inward oscillatory motion, may settle into an oblate rotating nebula extend- 

 ing its equatorial radius far bej'ond the orbit of Neptune, and with moment of momentum 

 equal to or exceeding the moment of momentum of the solar system. This is just the 

 beginning postulated by Laplace for his nebular theory of the evolution of the solar 

 system, which, founded on the natural history of the stellar universe as observed by 

 the elder Herschel and completed in details by the profound dynamical judgment and 

 imaginative genius of Laplace, seems converted by thermodynamics into a necessary 

 truth, if we make no other uncertain assumption than that the materials at present 

 constituting the dead matter of the solar system have existed under the laws of dead 

 matter for a hundred million years. Thus there may in reality be nothing more of 

 mystery or of difficulty in the automatic progress of the solar system from cold matter 

 difiused through space, to its present manifest order and beauty, lighted and warmed 

 by its brilliant sun, than there is in the winding-up of a clock and letting it go till 

 it stops."^ 



The mass of rotating hot gas would at once begin to lose heat by radiation into 

 space ; but this loss of heat would cause contraction, which would generate an amount 

 of energy more than sufficient to account for the loss by radiation. In due time the 

 various planets would (either successively, in order of their distances as held by Laplace,* 

 or either simultaneously or irregularly as held by Kirkwood and Newcomb'') become 

 separated from the gaseous mass as more or less nebulous annuli. These annuli would 

 ultimately condense into planets, which, on account of their small size, would cool down 

 comparatively rapidly. C. Wolf says : — " La periode geologique de la Terre, masse de 

 peu d'importance et par suite rapidement refroidie, a done pu commencer bien avant la 

 formation du Soleil actucl, et lorsque la nebuleuse n'avait peut-etre pas encore donnt^ 

 naissance a Venus ni a Mercure. Les geologues pourront trouver, dans le diametre con- 

 siderable de la masse solaire k ces epoques, I'explication de I'tigalite de climat dont parait 

 avoir joui la terre jusqu'au commencement de I'epoque actuelle."^ 



A sun, the diameter of which was equal to that of the orbit of Venus, that is about 

 137,400,000 miles, would subtend an arc of over 95° in the heavens, and at the nodes, 

 instead of barely illuminating the poles, the rays would pass over both poles and strike 

 the earth as. far as the forty-third parallel. The shortest daj' would be at the equator, 

 and even there it would be of about 18 hours duration. Under such a sun the poles 



• L(j1(\ Kelvin, Popular Lectures and Addresses, ed. 2, vol. i. pp. 421-2, London, 1891. 



'' Laplace, Exjrosition du Systenie du Monde, ed. 6, 1836. 



^ Kirkwood, " On Certain Harmonies of the Solar System," Amer. Journ. Science, eer. 2. vol. xxxviii. p. .5 ; 

 S. Kewcome, Poindar Astronomy, p. 513. 



■* Les Hypothhes Cosmogoniques, p. 32, Paris, 188G ; see also Blandet, Bull. Soc. (j^ol. de France, sdr. 2, t. xxv. 

 p. 777, 1808. 



