J54 THE MEANING OF EVOLUTION 



Place suggested that in a ring like this the material 

 could not be quite evenly distributed. While every 

 particle in the ring kept revolving around the sun, 

 those in front of the densest part were slowly held 

 back by the attraction of the thicker portion, while 

 those behind it in rotation had their speed hastened 

 until finally all the material in the ring had collected 

 at one spot and a new planet was born. La Place 

 believed that these planets formed their moons in ex- 

 actly the same way, and that Saturn was simply a 

 planet not all of whose moons had yet been formed. 

 He believed that this happy accident served to tell us 

 how the universe had been created. 



Of course, so detailed a theory concerning anything 

 of which we know so little has always had much 

 ridicule thrown upon it, and yet no truly competing 

 theory has been proposed until very recent times. 



Within a few years a Planetesimal Theory has been 

 announced, and is gaining considerable prominence, 

 although it is too early yet to say whether it will 

 supersede La Place's idea. In this theory, also, the 

 suggestion comes from the heavenly bodies. With 

 the increasing study of the nebulae, many forms of 

 these interesting bodies have been discovered. A very 

 common type consists of a great coherent central mass, 

 with two or more arms extending from opposite sides 

 in the form of a spiral. This is as if gaseous re- 



