PLANETARY EVOLUTION 



able dimensions ; Pallas having a diameter of 

 600 miles, or more than one fourth the diame- 

 ter of our moon. Most of the others are quite 

 tiny, the smallest having a surface perhaps not 

 larger than the state of Rhode Island. Not 

 only do they occupy the position which would 

 normally belong to a single planet between 

 Mars and Jupiter, but it is hardly questionable 

 that they have all originated from a single ring ; 

 for their orbits are interlaced in such a compli- 

 cated way that, if they were material rings in- 

 stead of ideal lines in space, it would be possible 

 to lift them all up by lifting any one of them. 

 Why should just one of the solar rings have 

 failed to develop into a single planet, and why 

 should such an arrest of development have oc- 

 curred in just this part of the solar system ? 



According to Olbers, the discoverer of Pallas 

 and Vesta, this is not a case of arrested devel- 

 opment, but these little bodies are merely the 

 fragments of an ancient well-developed planet, 

 which has been in some way exploded. But 

 this hypothesis, though countenanced by Mr. 

 Spencer, seems to me unsatisfactory. In Mr. 

 Spencer's essay, it is closely connected with the 

 hypothesis of a gaseous nucleus for all the 

 planets, which, though there ingeniously elabo- 

 rated, seems to me as yet too doubtful to serve 

 as a basis for further explanations. And even 

 granting the hypothesis, it would be necessary 

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