20 EVOLUTION OF THE EARTH 



same direction as the orbital revolution, and that the incor- 

 poration of all the planetesimals would cause the eccentrici- 

 ties to cancel out, giving to the whole mass a nearly circular 

 instead of a highly elliptical orbit. This would lead us to 

 believe that the original nucleus was but a small part of the 

 completed planet, so that its original ellipticity of orbit was 

 submerged beneath the average influence of the added masses. 



Outstanding difficulties of the planetesimal hypothesis. The 

 disruption or spiral nebula hypothesis explains the features 

 of the solar system more successfully than the older nebular 

 hypothesis has thus far been able to do, but there are difficul- 

 ties still remaining, though these may perhaps be the result of 

 special conditions. 



The most striking departure of the real system from that 

 expectation deduced from the hypothesis is found in the rota- 

 tion of the sun. The passing of a star able to drag matter 

 from the sun to the distance of the planet Neptune would be 

 expected to lead to an enormous tidal distortion of the sun's 

 mass. This great tidal wave would involve a lifting and 

 revolution about the sun, tending to give it a certain energy 

 of rotation. A very little stronger action and the sun would 

 in fact have been literally pulled to pieces and its matter 

 scattered beyond its gravitative control. It is possible that 

 it may in this way have lost a part of its mass. Considerable 

 quantities of the expelled matter should have fallen back 

 obliquely in the sun and tended further to increase its velocity 

 of rotation. The path of the approaching star could have 

 had no relation to the previous equatorial plane of the sun. 

 The probabilities would consequently be that the final rotation 

 would be a resultant between the older and newer forces and 

 lie in an intermediate plane at a considerable angle to the 

 plane of the planets' orbits. Now, as a matter of fact, the 

 sun, as has been previously noted, revolves but once in twenty- 

 five days and its equator is inclined but seven degrees to the 



