264 THE NEBULAE HYPOTHESIS. 



was necessary for the sake of stability that the planets 

 should go round the sun in the same direction and nearly 

 in one plane, they cannot thus account for the direction of 

 the axial motions. The mechanical equihbrium would not 

 have been at all interfered with, had the sun been without 

 any rotatory movement ; or had he revolved on his axis in 

 a direction opposite to that in which the planets go round 

 him ; or in a direction at right angles to the plane of their 

 orbits. With equal safety the motion of the Moon round 

 the Earth might have been the reverse of the Earth's mo- 

 tion round its axis ; or the motion of Jupiter's satellites 

 might similarly have been at variance with his axial motion • 

 or that of Saturn's satellites with his. As, however, none of 

 these alternatives have been followed, the uniformity must be 

 considered, in this case as in all others, evidence of sub- 

 ordination to some general law — implies what we call natu- 

 ral causation, as distinguished from arbitrary arrangement. 



Hence the hyj^othesis of evolution would be the only 

 probable one, even in the absence of any clue to the partic- 

 ular mode of evolution. But when we have, propounded 

 by a mathematician whose authority is second to none, a 

 definite theory of this evolution based on established me- 

 chanical laws, which accounts for these various peculiarities, 

 as well as for many minor ones, the conclusion that the So- 

 lar System teas evolved becomes almost irresistible. 



The general nature of Laplace's theory scarcely needs 

 stating. Books of popular astronomy have familiarized 

 most readers with his conceptions ; — namely, that the mat- 

 ter now condensed into the Solar System, once formed a 

 vast rotating spheroid of extreme rarity extending beyond 

 tlie orbit of Neptune ; that as this spheroid contracted, its 

 rate of rotation necessarily increased ; that by augmenting 

 centrifugal force its equatorial zone was from time to time 

 prevented from following any further the concentrating 

 mass, and so remained behind as a revolving ring ; that 



