8 THE LESSON OF EVOLUTION 



the whole of the meteorites would travel nearly in 

 the mean plane of their aggregate motions. The 

 larger of the meteorites would tend to settle towards 

 the centre, while other aggregations might easily 

 occur at different distances from the centre. And 

 of these the outer planets would be larger than the 

 inner ones, because in the more distant regions, 

 where the attraction of the central sun was less, the 

 movements of the meteorites would be slower, and 

 there would be a greater tendency to agglomeration 

 than where the movements were more rapid. As 

 meteorites contain but little oxygen, hydrogen, 

 carbon, silicon and alkalies substances which are 

 all abundant on the surface of the earth large 

 numbers must have been fused together to form the 

 earth, and the lighter substances must have collected 

 near the surface. Consequently, the collisions 

 between these meteorites must have occurred with 

 sufficient rapidity to melt the whole mass. For, 

 after a solid crust had been formed, all the meteorites 

 which fell on the earth would remain on the surface, 

 as they do now. 



Evolution of the Earth. As with the Solar 

 System, so also in the Earth itself, we can trace 

 distinctly a physical evolution. The discovery of 

 tidal friction gave an independent proof that the 

 Earth had had a beginning not infinitely remote ; 

 for, if that had been the case, the tidal friction would 

 have reduced the time of the Earth's rotation on 

 its axis to that of the moon. Also we have sufficient 

 geological evidence to show that not more than one 

 hundred millions of years ago the earth was in a 



