ADDRESSES 63 
faster at the critical point, it is clear that if a collision occurs 
when the body in the smaller orbit is approaching the junction, 
the rotational effect will be forward, but if the body in the 
inner orbit has passed the junction, the effect will be the op- 
posite. In a large number of cases both phases are quite sure 
to occur, The total effect when a multitude of planetesimals 
are added to a knot will be determined by the mean effect of 
all. The areas within which collisions tend to forward ro- 
tation are greater than the areas of the opposite class, and 
thus the probabilities distinctly favor forward rotation, but ir- 
regular or special distributions make possible retrograde or 
oblique rotations. 
Quite as important as the direction of rotation is the fact 
that velocity of rotation is not measured by the simple sum 
of collisional effects of like sign, but rather the algebraic sum 
of effects of opposite signs, and so the rotation may be low or 
high according to the propositions of the opposing phases of 
collision. 
Now these features fit the case in hand, for the velocities, 
axes, and directions of rotation of the planets are quite various 
and do not conform to the systematic requirements of concen- 
tration from a common source. Rotations, slow or fast, for- 
ward, backward or oblique, with axes differently inclined, are 
all consistent with concentration from orbits such as are postu- 
lated by the planetesimal hypothesis. There was indeed a 
lion in the way, but he was chained to a very special and ex- 
ceptional case. 
Perhaps the most important test that can be brought to 
bear upon theories of the origin of the planets is found in the 
planets themselves, the ultimate product; in our earth 1n par- 
ticular, because it is accessible to close inspection. The La- 
placian and the planetesimal hypotheses have been carried 
down to their specific planetary applications, and have offered 
us definite stories of the early stages of the earth. These stages 
form the first chapter of earth history. The harmony of the 
stories with the later events of the self-recorded history, bear 
critically on the verity of these stories of earth genesis. 
The series of Laplacian pictures is very familiar: (1) a globe 
of gas; (2) a globe of white hot lava enshrouded in a vast at- 
mosphere; (3) a globe crusted over and covered by a nearly 
