62 SCIENCE PROGRESS 



the original nuclei must have had nearly their present masses. 

 The original eccentricities of the orbits of both planets and 

 satellites would be considerably reduced ; the inclination to 

 the plane of the ecliptic would be small at the commencement, 

 and would remain so ; if the medium revolved the effect on 

 the major axes of the orbit, and hence on the periods would 

 probably be small. Direct satellites would approach their 

 primaries, and retrograde ones would ultimately be left on the 

 outskirts of their sub-systems . Given suitable initial conditions, 

 then, a system might be developed that would bear a strong 

 resemblance to the existing solar system. The resisting medium 

 itself would gradually degenerate and approach the sun on 

 account of its internal friction ; the zodiacal light may be the 

 last remnant of it. It may, however, be regarded as certain that 

 there has been no large amount of resisting matter near the 

 earth's orbit for a very long time ; there has probably been 

 ample time for the evolution of the earth and moon to take 

 place from the state that Darwin traced them back to. The 

 moon was then probably formed from the earth by the dis- 

 ruptive action of the solar tides ; but, as this would be a reson- 

 ance effect, increasing in amplitude over thousands of vibrations, 

 whereas the formation of a system of nuclei in the way sug- 

 gested by Moulton would take place at once, there need be no 

 surprise that the former event led to a single satellite of ^ of 

 the mass of the primary, while the latter formed several, the 

 largest having a mass of t^tt of its primary. 



The unsymmetrical nebula here considered might have been 

 produced in the manner described in the last section. A 

 symmetrical nebula becoming gravitationally unstable would 

 lead to an unsymmetrical one, as was proved by Jeans, but it 

 is difficult to see how the phenomenon of retrograde and direct 

 motions occurring to the same subsystem could occur on this 

 hypothesis. On the whole, then, the most plausible hypothesis 

 seems to be that a gaseous nebula with a system of secondary 

 and tertiary nuclei was formed round the sun by tidal disruption 

 owing to the close passage of another star, and that this has 

 been subsequently modified by gaseous viscosity, and at a later 

 stage by tidal friction. The moon was probably formed from 

 the earth by solar tidal disruption, this method being abnormal 

 in the system, and the later evolution of the earth and moon 

 has been dominated by bodily tidal friction. 



