AND ITS INHABITANTS 19 



of the sun. This residue is only a very small fraction of the 

 sun's mass. It would appear probable, however, that the solar 

 disruption was very great in order to give an axial revolution 

 to the reaggregated matter forming the present sun, so that 

 its equator should be, as observed, only seven degrees from 

 the mean plane of all of the planetary orbits. The present 

 revolution of the sun is probably due then to the whirl pro- 

 duced during tidal disruption and not to an axial rotation 

 belonging to the sun before the event took place. 



This brings us to the final stage in the evolution of the 

 planets according to the planetesimal hypothesis. In the arms 

 of the spiral nebula were knots or nuclei of matter constituting 

 the cores of the planets. Four small knots, the earth-moon 

 knot being a double one, represented the beginnings of the 

 four small inner planets (see table, page 5). In the zone of 

 the planetoids there was, however, no dominating nucleus, and 

 they have therefore remained to this day largely in the plan- 

 etesimal state. Four greater nuclei beyond were the begin- 

 nings of the major planets. Smaller nuclei associated with 

 the larger marked the presence of satellites. 



The orbits of the planetary nuclei and of the scattered 

 planetesimal swarm were highly eccentric, having the form 

 of a tangle of ellipses of all forms and sizes but lying in nearly 

 a common plane and with a common direction of revolution 

 about the central body. Collisions would inevitably occur at 

 the crossing of the paths in the course of numberless revolu- 

 tions and the nuclei would have sufficient mass and conse- 

 quent gravitative power to retain the matter colliding with 

 them. In this way, each planet would in the course of time 

 clear up an orbital zone, and these zones, because of the eccen- 

 tricities of the orbits of the component particles, overlapped 

 each other with the exception of a region between Mars 

 and Jupiter. But Moulton has shown that in such planetary 

 growth by accretion, an axial revolution would arise in the 



