14 evolution: the ages and tomorrow 



the result of an explosion which threw part of its atmosphere 

 far out into space. He thought that the planets condensed 

 from this gas (the so-called nebular hypothesis) and their 

 motions were the result of the sun's original revolution 

 around its own axis. Analysis eventually revealed difficulties 

 in the Laplace hypothesis, at least as expressed by its author, 

 and at the turn of the twentieth century science returned to 

 Buif on's idea of a two-parent collision. 



The modern collision hypothesis, formulated independ- 

 ently by Sir J. H. Jeans, T. C. Chamberlin, and F. R. 

 Moulton, assumes that the planets are the result of a giant 

 tidal wave raised on the surface of the sun by the near- 

 approach of another star. In this theory tidal action is sub- 

 stituted for Buifon's direct collision, largely because the 

 former is more probable. Many years of careful analysis by 

 the authors of this theory have shown it to be capable of ex- 

 plaining most of the characteristics of the solar system, but 

 it is not all-inclusive. Some of its failures may be fatal to the 

 theory, and in recent years modifications have been pro- 

 posed that begin to pile theory on theory, always a sign of 

 weakness. H. N. Russell has proposed that before the en- 

 counter with the passing star, the sun was a twin star. Such 

 twins are common enough in the heavens, but the develop- 

 ment of this modification has not appeared to be of much 

 help to the original theory. 



All critics have pointed out that collisions or near-colli- 

 sions between stars would be exceedingly rare— in fact, so 

 rare in the present form of the universe as to make worlds 

 such as ours almost unique in a whole galaxy of stars. If the 

 universe is expanding, a concept which will be discussed 

 later, near-collisions would have been much more common 

 some 3,000,000,000 years ago when the stars were closer 

 together; but this assumption would tend to set up planetary 

 systems of nearly all the same age, a condition not too de- 

 sirable in in evolutionary process where one might long for 

 an eternal succession of worlds. 



