THE ORIGIN OF THE EARTH. 53 



tion that the sidereal system has had a very prolonged history, and 

 that the ancestral sun played its part in it as the solar system does 

 now. 



The contingencies of stellar collision. — There are probably at least 

 100,000,000 suns in the present sidereal galaxy, and these are moving, 

 so far as now determined, in various directions and at various velocities, 

 much like the molecules of an exceedingly tenuous gas. Besides these 

 suns, there is possibly even a larger number of dark bodies asso- 

 ciated with them as extinct suns, planets, planetoids, and satellites. 

 It would be strange, therefore, if there were not occasional collisions, 

 especially in the more crowded portions, as in the Milky Way, how- 

 ever rare these might be relatively. It is in or near the Milky Way 

 that new stars appear, and also there that the free-molecular nebulae 

 chiefly abound. This coincidence of distribution obviously does not, 

 of itself, carry so broad a conclusion as that the new stars and the 

 free-molecular nebulge necessarily arise from collision; but the con- 

 tingency of collision, when such a multitude of bodies are moving 

 in such diverse ways, can scarcely be questioned, notwithstanding 

 the vast distances involved and the consequent smallness of the con- 

 tingency for a given body within a limited time. But to the phenomena 

 of collision no appeal is here made, except as a part of a consistent 

 conception of the whole group of associated phenomena. 



The contingencies of close approach. — If there are contingencies of 

 actual collision between stars, there must be much larger possibil- 

 ities of their close approach to one another and to dark bodies. 

 Celestial bodies possess velocities inherited from past states, and veloci- 

 ties due to the existing attraction of other bodies, so that close approach 

 is assignable in part to the crossing of predetermined paths, and in 

 part to the mutual attraction of the great bodies themselves. The 

 contingencies of collision and of close approach arising from these two 

 factors follow different laws. Considering inherited motions alone, 

 the contingencies of collision are greatest when the velocities are 

 highest and the bodies thickest. The contingencies of close approach 

 by mutual gravity are greatest when previously acquired velocities 

 are least and conflicting attractions are fewest and feeblest. It is 

 obvious that as the inherited velocities approach infinity, collision 

 approaches infinite frequency, while gravitational deflection approaches 

 the infinitesimal. On the other hand, if inherited velocities were 



