78 GEOLOGY. 



is dependent on the eccentricity of its own orbit, and on the eccen- 

 tricities of the orbits of the planetesimals it gathers in on either hand. 



Competitive nuclei and selective dominance. — It is obvious that 

 there may have been two or more nuclei originally within the same 

 zone. If one of these was notably smaller than the other, it might 

 be picked up by the latter, the same as if it were a planetesimal. Two 

 of equal size might perhaps unite, though this would not necessarily 

 take place. Two nuclei in nearly the same zone must feed upon the 

 same belt of planetesimals, and must mutually interfere with one 

 another's growth. If there were little difference in their masses at the 

 outset, that one which was best spaced out from the nuclei in neighbor- 

 ing zones would be likely to become dominant by superior growth, 

 for it would have a better feeding-ground, so to speak. Even a nucleus 

 that was smaller at the outset, if well separated from large competitors, 

 might become the dominant one by a better growth. 



If there were originally many nuclei of minor mass, and if these 

 Were much scattered, especially if the planes of their orbits were di- 

 verse, the dominance of any one might be avoided, and a scanty growth 

 of all result, as in the case of the asteroids. 



The shifting of orbits. — It seems to be a sure inference that, in the 

 process of growth, the nucleus must have worked towards the center of 

 the zone from which it gathered, as a consequence of the superior feed- 

 ing on the richer side. For example, if more planetesimals were picked 

 up on orbits smaller than its own, its orbit must have grown smaller 

 as a mechanical result of the accretion, for a new orbit, arising from 

 the union of two bodies, is intermediate between the two previous 

 orbits, and hence smaller than the larger one. If more planetesimals 

 were picked up on the outer side, the orbit of the nucleus must have 

 grown larger. The nucleus, therefore, must have worked toward the 

 center of the richer feeding-ground, or in average cases of equable 

 original distribution, toward the ground not preyed upon by other 

 nuclei. 



The spacing-out of the planets. — The foregoing processes tended 

 toward a selection of nuclei for dominance, and to an automatic spacing- 

 out of the successful nuclei. This process, if our hypothesis be true, 

 should find verification in the actual distribution of the planets, and 

 be an explanation of it. This distribution should correspond to the 

 eccentricities of the nuclei, modified by the proportions of planetesi- 



