180 CHARLES DARWIN 



nebula, which once spread its faint and cloud-like mass 

 with inconceivable tenuity, at least as far from its 

 centre, now occupied by the sun's body, as the furthest 

 point in the orbit of Neptune, the outermost of the yet 

 known planets. From this remote and immense peri- 

 phery it has gradually gathered itself in, growing 

 denser and denser all the time, towards its common 

 core, and has left behind, at irregular intervals, con- 

 centric rings or belts of nebulous matter, which, after 

 rupturing at their weakest point, have hardened and 

 concentrated round their own centre of gravity into 

 Jupiter, Saturn, the Earth, or Venus. The main central 

 body of all, retreating ever within as it dropped in its 

 course the raw material of the planetary masses, has 

 formed, at last, the sun, the great ruler and luminary 

 of our system. Much as this primitive evolutionary 

 concept of the development and history of the solar 

 system has been modified and altered of late years by 

 recent researches into the nature of comets and meteors 

 and of the sun's surface, it still remains for all practical 

 purposes of popular exposition the best and simplest 

 mental picture of the general type of astronomical 

 evolution. For the essential point which it impresses 

 upon the mind is the idea of the planets in their several 

 orbits and with their attendant satellites as due, not to 

 external design and special creation, in the exact 

 order in which we now see them, but to the slow and 

 regular working out of preordained physical laws, in 

 accordance with which they have each naturally assumed, 

 by pure force of circumstances, their existing size, and 

 weight, and orbit, and position. 



Geology has applied a similar conception to the 



