THE ORIGIN OF THE EARTH. 81 



SUMMARY. 



The planetesimal hypothesis thus assumes that the solar system 

 was derived from a nebula of the most common type, the spiral, and 

 that the matter of this parent nebula was in a finely divided solid or 

 liquid state before aggregation, in harmony with the continuous spectra 

 of spiral nebulae. It regards the knots of the nebula as the nuclei of 

 the future planets, and the nebulous haze as matter to be added to these 

 nuclei to form the planets. It assumes that both the knots and the 

 particles of the nebulous haze moved about the central mass in elliptical 

 orbits of considerable, but not excessive, eccentricity. It postulates 

 a simple mode of origin of the nebula connected with the not improbable 

 event of a close approach of the ancestral sun to another large body, 

 but the main hypothesis is not dependent on this postulate. 



It assigns the gathering-in of the planetesimals to the crossing of 

 the elliptical orbits in the course of their inevitable shif tings. Out 

 of this process and its antecedents, it develops consistent views of the 

 requisite distribution of mass and momentum, of the spacing out of 

 the planets, of their directions of rotation, of their variations of mass, 

 of their varying densities, and of other peculiarities. 



It deduces a relatively slow growth of the earth, with a rising internal 

 temperature developed in the central parts and creeping outward. 

 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. These stages now claim our attention. 



