THE ORIGIN AND DEVELOPMENT OF THE EARTH 1 ia . 
ones occasionally pass through the atmosphere without being 
totally destroyed, and reach the earth as_ meteorites. 
Obviously the process is now going on much more slowly than 
it was during the early history of the system, for the larger 
bodies were undoubtedly acquired early in the growth of the 
planet. 
This conception of the origin of the earth differs, as can be 
readily seen, from the one proposed by Laplace. According 
to it the earth, instead of having shrunk from a ball much 
larger than it now is, has been built up from a mass the 
smallness of which cannot be determined. The moon, instead 
of being descended from a ring left behind by the contracting 
earth, had its origin in much the same manner as did our 
planet. Since it was never so large as the earth, and is quite 
near to it, it is controlled by the earth just as the earth is 
controlled by the sun. ; 
We now come to the question which proved the undoing of 
the Laplacian hypothesis—that of movement and rotation of 
the planets and satellites. But according to Chamberlin’s 
hypothesis there would be no fixed relation between the 
rotation of a planet and the revolution of its satellites. The 
rotation of either a planet or satellite may be forward, or it 
may be retrograde. The former would be the rule and the 
latter the exception, and this is precisely the case with the 
solar system. There are many other features of the solar 
system to be fittingly explained by the planetesimal hypo- 
thesis. Certain of them possess added weight because they 
were not discovered until after the hypothesis had been 
formulated and published. Any hypothesis, if it is to be con- 
sidered at all, must explain the facts which are known and 
considered when it is being formulated. But the hypothesis 
which merits serious study or even acceptance is the one 
which explains conditions that were unknown to its authors. 
This the planetesimal hypothesis of Chamberlin and Moulton 
to do. 
