THE ORIGIN OF THE EARTH 425 



than the inner parts, and since the planets were formed from these 

 arms, -the inner ones should have higher specific gravities than the 

 outer ones, as is the fact. 



Other peculiarities of the solar system seem to find a fitting ex- 

 planation in the planetesimal hypothesis, but most of these must 

 be passed without mention here. 



The meetings and unions of planetesimals and nuclei at the 

 crossings of their orbits imply a relatively slow evolution of the 

 nebula into a solar system, however inevitable the evolution must 

 be with the requisite time. The planetesimal hypothesis therefore 

 implies a relatively slow growth of the earth. With such a mode 

 of growth, the stages of the earth's early history necessarily depart 

 widely from those postulated by the Laplacian and the meteoritic 

 hypotheses. 



