ADDRESSES 67 
constitute the nuclei of comets and from these gaseous emana- 
tions, developed by the heat of the sun, form comas and tails. 
Later, by dispersion, the fragments are scattered into meteor- 
ites. 
This, in hasty terms, is the planetesimal conception of the 
origin of a family of planets, or rather planetoids, generated 
by centrifugal action from a spheroidal gaseous nebula while it 
is condensing into a sun, Such an origin belongs to the plan- 
etesimal genus of hypotheses in being strictly orbital, but the 
mode is distinctly different from that initiated by a passing 
star, and evolved through the form of a spiral nebula. 
Early in this paper it was intimated that the special prob- 
lem of the planetesimal hypothesis was the origin and growth 
of the present planets only, but that the concept must, none 
the less, be in accord with the greater events of cosmic evolu- 
tion. A few words on this point seem required to fill out 
our theme. 
The origin of our sun, and of suns in general, was defin- 
itely set aside as not falling within the special problem for 
whose solution the planetesimal hypothesis was offered. The 
origin of suns must probably, none the less, be related, in some 
suggestive way, to the processes and principles on which the 
planetesimal hypothesis is based. It will have been noted that 
the planetesimal hypothesis postulates an orbital state of the 
nebular matter. It is thus founded declaredly on orbital dy- 
namics. In the phraseology of the naturalist, it belongs to 
the orbital genus. It is perhaps entitled to be regarded as the 
type of that genus, since it found its field by throwing down 
the doctrinal bars that shut off the orbital state from com- 
petency to give rise to forward rotations. 
But suns are the great examples of gaseous bodies, and as 
such, are controlled by gaseous dynamics in contradistinction to 
orbital dynamics. There do not therefore seem to be obvious 
grounds for supposing that the stars came into being by an 
orbital process. Certain of the stars may indeed be concentra- 
tions of the large knots of the great spiral nebulz, but these 
great knots are assigned to explosive separation from still 
greater gaseous bodies, and the real genesis of the gaseous 
state of the knot goes back to the origin of the great suns 
from which the nebulz are supposed to have sprung. The 
