24 EVOLUTION OF THE EARTH 



that Its radiant energy has been able to endure through all 

 the ages needed for organic evolution, but the spacing of 

 the stars Is so wide and the chance of approach so rare that 

 no other of them has since advanced sufficiently near to 

 throw this system into disorder, or to disrupt and sweep 

 away the earth and Its sister planets as a wasted effort, and 

 start the re-creation of a new heaven and a new earth. 



Hypothesis of Earth-Growth by Slow Accretion 

 OF Planetesimals 



Under the terms of either nebular or planeteslmal hypoth- 

 esis a scattered state of the planetary material is implied 

 as a stage antecedent to the origin of the planets. Was this 

 growth of the planets geologically slow or rapid? Did It 

 take tens or hundreds of millions of years, or was it on the 

 contrary largely accomplished in tens or hundreds of thou- 

 sands of years? Was the material largely in dust-like or 

 molecular form, or was it to a large extent In nuclei of con- 

 siderable size? From these different postulates very diver- 

 gent consequences may be traced In the formative stages of 

 the earth; and finally the present nature of the earth Itself 

 may speak In favor of one or the other of these views. 



Chamberlln, who has been the chief writer on this sub- 

 ject, adopts the hypothesis that the stages of earth-growth 

 were very prolonged, even geologically speaking, and that 

 the accretion was domlnantly of dust-like or molecular par- 

 ticles. According to him the building up of the planets fol- 

 lowed three stages: first, the direct condensation of the 

 nuclear knots of the spirals into liquid or solid cores; second, 

 the less direct collection of the outer, or orbital and satelli- 

 teslmal matter; third, the still slower gathering up of the 

 planeteslmal material scattered over the zone between 

 adjacent planets. This third factor, in Chamberlln's view, is 



