ch. v.] PLANETARY EVOLUTION. 389 



as the comets are chiefly distributed about the po^es of 

 our solar system, their orbits cutting its equatorial plane at 

 great angles, so the nebulae are found in greatest numbers 

 about the poles of the galaxy. It seems unlikely that this 

 parallelism, which Mr. Spencer was the first to point out, 

 should be accidental. It indicates a common mode of evo- 

 lution of the whole starry system. It vaguely points to a 

 gigantic process of concentration going on throughout the 

 galaxy, analogous to the local process of concentration which 

 has gone on in our own little planetary group. Still more 

 obvious will this become when we consider the explanation 

 of these phenomena which Mr. Spencer has offered. 



Observation shows that while the more consolidated 

 nebulas are oval or spheroidal in shape, the less consolidated 

 nebulae are often extremely irregular, throwing out long arms 

 of vaporous matter into the adjacent spaces. This agrees 

 with what we have learned to expect in any rotating mass 

 which gravitation is slowly drawing closer and closer together. 

 The oval form is due, as we have seen, to the combined 

 effects of gravitation and rotatory movement. But this im- 

 plies an earlier state in which the figure was irregular. Now 

 while the heavier portions of the mass were being drawn 

 together so as to acquire a spheroidal contour, the lighter 

 portions, floating farther from the centre of gravity, would 

 remain like detached shreds of cloud, or like long luminous 

 streaks. And while all these would ultimately be compelled 

 by gravitation to revolve about the centre of the mass, never- 

 theless the lightest and outermost shreds would be a long 

 time in acquiring a definite direction of revolution. While 

 the greater number would be doubtless drawn in and ab- 

 sorbed by the main mass at an early stage, the chances are 

 that some would not arrive until the main mass had become 

 considerably contracted. Now it is easy to see that such 

 iate arriving flocculi, coming toward the centre of gravity 

 from a great distance, and therefore having small angular 



