2IO CAKNEGIE INSTITUTION OF WASHINGTON. 



drawn toward the center, collides, and rebounds after the fashion 

 of gaseous molecules, as conceived by Lockyer and Darwin. The 

 meteoroids that are formed by the dispersion of a comet, such as 

 constitute the belts that give rise to the August and November me- 

 teoritic showers, are probably in the planetesimal rather than the 

 collision- rebound condition, and are becoming more and more 

 .scattered and individually independent as time goes on. 



As the basis for developing the typical form of the planetesimal 

 hypothesis, I have assumed that the parent nebula had a plan- 

 etesimal organization from the outset. The conception is a rather 

 radical departure from the gaseous conception of the familiar neb- 

 ular hypothesis, and from the meteoritic conception of Lockyer and 

 Darwin, so far as fundamental dynamics and mode of evolution are 

 concerned. To develop the hypothesis as definitely and concretely 

 as possible, I have further chosen a special case from among those 

 that might possibly arise, viz, the case in which the nebula is sup- 

 posed to have ari.sen from the dispersion of a sun as a result of close 

 approach to another large body. The case does not involve the 

 origin of a star nor even the primary origin of the solar system, but 

 rather its rejuvenation and the origin of a new family of planets. 

 The general planetesimal doctrine does not stand or fall with the 

 merits or demerits of this special phase of it, but to be of much 

 real service in stimulating and guiding investigation, a hypothesis 

 must be carried out into working detail so that it may be tested bj^ 

 its concrete and specific application to the phenomena involved, and 

 hence the reason for developing a specific sub-hypothesis. This 

 particular sub-hypothesis was selected for first development (i) be- 

 cause it postulates as simple an event as it seems possible to assign 

 as the source of so great results, (2) because that event seems verj- 

 likely to have happened, (3) because the form of the nebula sup- 

 posed to arise in this way is the most common form known, the 

 spiral, and (4) because spectroscopic observations seem at present 

 to support the constitution assigned this class of nebulae, although 

 it must be noted that spectroscopic observations have not reached 

 such a stage of development as to demonstrate the motions of the 

 nebular constituents. In future spectroscopic determinations lies 

 one of the crucial tests which the hypothesis must j'et undergo, for 

 there is little doubt that spectroscopic work will in time reach such 

 a degree of refinement as to demonstrate the motions of the con- 

 stituents of the spiral nebulae. 



