THE ORIGIN OF THE EARTH. 19 



adjacent movements at nearly equal speeds — are very exceptional, 

 and hence the accessions in any given period must be presumed to be 

 few compared to the whole number of meteorites that pass the place 

 of the initiating swarm; for all those that have opposite and transverse 

 courses of any appreciable angle, and all those that, though moving 

 in parallel directions, have appreciably different velocities, will pass 

 through the point of assemblage with dangerous differential velocities. 

 They are therefore liable to break up the initiating swarm by colliding 

 with its members and driving them beyond its gravitative control. 

 This contingency is especially great while the swarm is small, and its 

 gravitative command of its members feeble. Hence arises the question 

 whether the swarm's peril of destruction is not greater than its chance 

 of growing to a self-protecting size, so incomparably greater, indeed, 

 as to render the method an extremely improbable one. 



If such a swarm succeeds in reaching a large mass, the proba- 

 bilities of its holding its own members and of capturing the collid- 

 ing meteorites, become very favorable. The growth of a meteoritic 

 swarm or of a nebula after it has once gained a sufficient massiveness , 

 is therefore simple enough and probable enough; but the starting of a 

 swarm, and the early stages of its growth, are attended by extremely 

 unfavorable contingencies. 



The sparseness of distribution and its bearing. — The extreme tenuity 

 of the celestial matter hypothetically thus dispersed is another vital 

 consideration. The light of a star in a flight of 50 years, at the speed 

 of 186,000 miles per second, does not seem, on the average, to encounter 

 enough dark matter to seriously dim its brightness. All the matter 

 that lies between us and the outermost visible stars does not cut off 

 as much light as the thinnest cloud. On a most liberal estimate of the 

 amount of meteoritic matter encountered by the earth at present, it is 

 computed that it would take a billion years to add an inch to its sur- 

 face. 1 



If all the matter now aggregated in the stellar system, on any 

 reasonable estimate of its mass (and the known distribution and move- 

 ments of the celestial bodies limit such an estimate), were distributed 

 through the space now occupied by the stars, it would not help the 

 case much, so far as meteoritic assemblage is concerned. To illustrate, 

 if the matter of the solar system were scattered through the space about 



1 Young's Astronomy, p. 475; Woodward, Astr. Jour. 1902. 



