114 UNIVERSITY OF MISSOURI BULLETIN 



faster it would probably not remain in a disc-like shape, but 

 rather draw out like an egg rotating about its shorter axis. 

 As time went on, the egg-shape would change into the shape 

 of a dumb-bell, and finally the two spheres of the dumb-bell 

 would separate and revolve about their common center of 

 gravity. After that, each sphere would cause tides in the 

 other, and the effect of these would be to make them sepa- 

 rate to a considerable distance. Unfortunately, the mathe- 

 matical difficulties of such a theory are very great, and in 

 order to simplify it Darwin was obliged to make certain 

 assumptions that do not quite correspond with the facts. 

 Moreover, while his theory accounts very well for the forma- 

 tion of the moon, which probably really was thrown off from 

 the earth in just this way, it does not explain satisfactorily 

 the very small satellites of the other planets, or the formation 

 of the planets themselves out of the sun. 



Quite recently, Chamberlain and Moulton of the Univer- 

 sity of Chicago elaborated what they call the "planetesimal 

 hypothesis." According to this, the whole solar system was 

 once a single body traveling through space. Finally, it hap- 

 pened to come nearly in contact with another similar body. 

 This is not impossible, for there are many millions of stars 

 in space, and it is reasonable enough to suppose that collisions, 

 or at any rate very close approaches, do occasionally occur. 

 When two such bodies approach each other, the chances are 

 very great that they will not actually collide, but will swing 

 around one another, each one tracing out a curved path, and 

 then separate again. This is a result of their gravitational 

 attraction for one another. Now, if body A attracts body 

 B, the force of gravitation will be greatest on those parts of 

 B which are nearest to A, and least on those parts which are 

 farthest from A. We have an example of this in the tides, 

 for the attraction of the moon is greatest on that side of the 

 earth which is nearest to the moon and least on that side 

 which is farthest, and this causes a piling-up of the water of 



