68o T. C. CHAMBERLIN 



if they were not thrown out by collision, which would be rare in 

 such vast space, or perturbed by other bodies, among which the 

 earth-core would be the most influential in most cases. But such 

 perturbations work very slowly, and their effects on the orbits 

 involved are not easily visualized by any except experts in orbital 

 dynamics. It is easy, however, to see that the case is far different 

 from the direct collapse of gaseous particles and that it must occupy 

 much greater time. In the lack of any rigorous determination of 

 just how much longer the ingathering process would take, we may 

 merely note that if it be taken as no more than two or three times 

 longer, the total period would at least equal the biological require- 

 ments given above on the older geological scale. 



But such bodies in heterogeneous orbits belong to the meteor- 

 itic type, and would not arise normally from the dynamic influences 

 postulated by the planetesimal hypothesis, nor would their aggre- 

 gation give rise to planets in concurrent revolution, for lack of 

 the requisite moment of momentum, unless it were assigned them 

 by some supplementary hypothesis such as revolution of the whole 

 assemblage. As in the preceding case this assumption only serves 

 as a step toward the real case. 



4. The distinctive feature of the postulated planetesimals was 

 that they were moving in the same general direction as the collecting 

 body and at the same general rate of speed. The process of collection 

 was therefore confined to overtakes and to convergencies of orbits. 

 The differences between this and the preceding case may be com- 

 pared to the different degrees of danger of collision between auto- 

 mobiles when, in one case, they are running in a common direction, 

 on the right side of the road, under fairly well regulated speeds, 

 and, in the other case, running wildly at random in both direc- 

 tion and speed. So planetesimals, circling about the sun in more 

 or less concurrent orbits, only collide and coalesce in so far as they 

 deviate from concurrence with the rest of the system or are per- 

 turbed in their independent orbits and drawn into coalescence by 

 overtakes or convergencies. In so far as their orbits were con- 

 current, the moment of momentum of the combined mass was nearly 

 as high as the sum of the individual moments of momenta, and so, 

 if at any stage the orbits became adjusted to one another, they 



