DIASTROPHISM AND THE FORMATIVE PROCESSES 403 
41. To follow accurately the logic of the planetesimal hypothesis, 
it is necessary to keep clearly in mind that its point of view is 
pre-eminently dynamic. Its type ideas are not based on material 
forms, but on dynamic organizations. ‘This was set forth in explicit 
terms in the earliest full statement of the hypothesis.t In spite 
of this, the hypothesis has come to be more or less unconsciously 
regarded as dependent on a special interpretation of spiral nebulae, 
because these nebulae have been much used as illustrations of the 
type of deployment supposed to have been involved in the genesis 
of our planetary system, but the theory is not thus dependent, as 
was urged from the outset. It will stand or fall solely on its ability 
to explain the remarkable characteristics and relationships of our 
planetary system. The requirements imposed by these are so 
many and so exacting that no theory but the true one has any 
chance of fully meeting them. We may, of course, think they 
are met when they really are not. We hold it is quite sure, how- 
ever, tha® in time the “vestiges of creation”’ will give convincing 
tests, and the essentials of the whole history of the earth will be 
read from beginning to end. 
42. Two distinct types of planetesimal organization are recog- 
nized: In one, planetary nuclei, serving as collecting centers, 
revolve among the planetesimals and gather them in, forming 
bodies of notable size. In the other, there are no such collecting 
nuclei, and the formation of bodies of planetary size is a practical 
impossibility; the planetesimals remain small and constitute a 
multitude of minute secondaries. 
43. Two distinct classes of planetesimal secondaries are recog- 
nized: (a) those which the parent body may develop by its 
own genetic resources, 1.€., monoecious secondaries, and (6) those 
which can be developed only by the co-operation of another body 
serving as a second dynamic parent, i.e., dioecious secondaries.’ 
‘In outlining the planetesimal hypothesis for the use of students in Chamberlin 
and Salisbury’s Geology, Vol. II (1905), pp. 38-40, it was specifically pointed out that 
the planetesimal condition may arise in different ways, as from a gaseous nebula of the 
Laplacian type, or a meteoritic swarm of the Lockyer type. An origin from a spiral 
nebula was made the leading type for reasons specified, but to this was added: ‘‘ While 
this will be followed as the type view, let it be distinctly noted that the planetesima | 
doctrine of accretion does not stand or fall with this particular conception” (p. 40). 
2 The Origin of the Earth (1916), “Celestial Kinships,” pp. 1o1-2. 
