AND ITS INHABITANTS 23 



matter in the universe pulling upon it in the present. If two 

 bodies without previous motion were to be attracted toward 

 each other, and were able to ignore the gravitative pull of 

 all other bodies, they would move in a straight line toward 

 each other's centers with ever increasing velocity until colli- 

 sion would result. But the least inherited motion in any 

 other direction, or the least deflecting pull upon one of them 

 more than upon the other by other bodies would prevent to 

 that degree a central collision, or in almost all cases any 

 collision whatever. Now the velocities of the stars through 

 space at great distances from each other are so great that 

 individual stars can have almost no attractive influence upon 

 each other. They must move in nearly straight lines past 

 each other unless they happen to pass within a thousandth 

 part of their average distance. It is seen then that the 

 chances of close approach depend primarily upon the acci- 

 dental crossing of their paths and only secondarily upon their 

 mutually attracting each other. For this reason the chance 

 of actual collisions may be regarded as negligible, even con- 

 sidering the vast number of stars. Approach sufficiently 

 near to generate strong tidal forces would, however, have 

 occurred during their long lifetimes as radiant bodies to a 

 considerable number out of the hundred million or more of 

 stars which are known to exist in the stellar system, but for 

 any one individual star, where the spacing is of the magni- 

 tude existing in our part of the stellar system, the chance of 

 such approach even in a billion years would be very small. 

 In fact, it has been estimated as only one chance in 1,800 in 

 that time. It is possible, however, that this happened once 

 to our star, that is, to our sun of that time, in the distant 

 past and from that disruptive tidal force was born our 

 system of planets. If such an event was in fact a necessary 

 antecedent condition, fortunate indeed has been our planetary 

 fate, for not only did this happen so early in the sun's career 



