ch. v.] PL A N ETA BY E VOL UTION. 371 



It was otherwise with the ancestral ring of the asteroids. 

 This thinnest and weakest of rings started on its independent 

 career at a distance of only 240,000,000 miles from Jupiter, 

 the planet whose gravitative force is more than twice that of 

 all the other planets put together. Under such circumstances it 

 would seem impossible that a planet could be formed. The 

 asteroid-ring must have been liable to rupture, not only from 

 the causes which affect all planet- forming rings alike, but 

 also from the strain exerted upon it, now in one part and 

 now in another, by Jupiter's attraction. The fragments of a 

 ring, torn asunder by such a cause, would not continue to 

 occupy the same orbit ; they would be dragged from the 

 common path in various directions and to various distances, 

 according to the ever- changing position of the disturbing 

 body. Henceforward, instead of chasing directly on each 

 other's heels, they would rush along in eccentric, continually 

 intersecting paths, and there would thus be no opportunity 

 for consolidation, except in the case of two fragments 

 meeting each other at the intersection of their orbits. As a 

 final result we should have, not one good-sized planet, but a 

 multitude of tiny planets, with intersecting orbits exhibiting 

 great differences in eccentricity. All this is true of the group 

 of asteroids. While the mean breadth of the ideal zone 

 occupied by their orbits is about 100,000,000 miles, its 

 extreme breadth reaches 250,000 000 miles. While the orbit 

 of Europa is more nearly circular than any of the orbits of 

 the true planets, on the other hand the orbit of Polyhymnia 

 attains an almost cometary eccentricity, the difference 

 between its perihelion and aphelion being nearly 200,000,000 

 miles. 



There is one other circumstance, however, which my 

 hypothesis thus far fails to explain. While the true planets 

 revolve in planes but slightly inclined to the ecliptic — the 

 orbit of Mercury showing an inclination of about seven 

 degrees as the maximum instance — the asteroids, on the con- 



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