38 GEOLOGY. 



to represent a great generative method whereby stellar systems are 

 evolved. Originated, by hypothesis, in dispersion, it is ever diver- 

 gent in its own tendencies, passing into the gaseous state on the one 

 hand, or into the planetesimal on the other, or, escaping these, it is 

 depleted by contributing its constituents, one by one, to existing suns 

 and planets. 



III. The Planetesimal Hypothesis. 1 



Under the typical form of the planetesimal hypothesis, it is assumed 

 that the parent nebula of the solar system was formed of innumerable 

 small bodies, planetesimals, revolving about a central gaseous mass, 

 much as do the planets today. The evolution of the system con- 

 sisted in the aggregation of these innumerable small bodies into much 

 fewer large ones; in other words, in a transition from a plantesimal to 

 a planetary system. The hypothesis, therefore, postulates no funda- 

 mental change in the system of dynamics, after the nebula was once 

 formed, but only an assemblage of the scattered material. The state 

 of dispersion of the material at the outset and throughout, as now, 

 was maintained by orbital revolution, or, more closely speaking, by 

 the centrifugal acceleration arising from revolution. The planetesimal 

 hypothesis by no means excludes gases from playing a part in the 

 parent nebula, or in its evolution, any more than it denies their presence 

 in the sun or the atmosphere to-day, but it assigns to gaseous action 

 a subordinate place in the evolution of the planetary system after the 

 planetesimal condition had become established. 



Subvarieties of the hypothesis. 



Under the broad general conception that planets grew from planet- 

 esimals, it .is possible to entertain different views as to the particular 

 origin of the planetesimal condition. That condition might be an out- 



1 This hypothesis has been gradually developed by the senior author of this work 

 during the past decade, with the very important advice and aid of Dr. F. R. Moulton 

 and other scientific friends. Partial expositions have been made before scientific 

 societies at different times, and a somewhat extended statement appears in Year 

 Book No. 3, Carnegie Institution, 1905, pp. 208-253, but this is its first full statement 

 with illustrations. A statement from the standpoint of a mathematical astronomer 

 and a master in celestial dynamics will be made by Dr. Moulton in his forthcoming 

 work on astronomy. 



