284 COSMIC PHILOSOPHY. [vt. ii. 



the planets formed from liim must have kept up a revolution, 

 and acquired a rotation, in the same direction. 



Such is tlie grand theory of nebular genesis, first elabo- 

 rated with rare scientific acumen by Kant in 1755, and after- 

 wards independently worked out by Laplace in 1796. The 

 claims of this theory to be regarded as a legitimate scientific 

 deduction have been ably stated by Mr. Mill, in his " System 

 of Logic," book iii. chapter xiv. As we are there reminded, 

 " there is in this theory no unknown substance introduced on 

 supposition, nor any unknown property or law ascribed to a 

 known substance." Once grant that the sun and planets are 

 cooling bodies, the inference is unavoidable that the matter 

 which composes them was formerly much more rare and dif- 

 fused than at present. If we are to infer the sun's past con- 

 dition from its present condition, we must necessarily sup- 

 pose that its constituent matter once occupied much more 

 space than at present, " and we are entitled to suppose that 

 it extended as far as we can trace effects such as it might 

 naturally leave behind it on retiring ; and such the planets 

 are." The abandonment of successive equatorial zones by 

 the shrinking solar nebula follows from known mechanical 

 laws ; and the subsequent breaking up of each zone, and the 

 consolidation of its fragments into a planet, are processes 

 which similarly involve none but established dynamical prin- 

 ciples. It equally follows, from elementary laws of mecha- 

 nics, that the planets thus formed would revolve and rotate 

 both in the directions and in the planes in which they are 

 actually observed to revolve and to rotate. There is thus, 

 observes Mr. Mill, nothing gratuitous in Laplace's specula- 

 tion : " it is an example of legitimate reasoning from a 

 present effect to a possible past cause, according to the known 

 laws of that cause." 



But the evidence in favour of the theory of nebular genesis 

 is not restricted to these general coincidences between obser- 

 vation and deduction. Many striking minor details in the 



