ORIGIN AND DEVELOPMENT OF THE EARTH 311 



form of rings ; and many other difficulties of an even more 

 fundamental nature have been pointed out. A new theory, 

 which seems to explain the facts we now have at command 

 better than did the older hypotheses, has recently been worked 

 out on a quite different basis. It is known as the Planetesi- 

 mal theory. 1 



The planetesimal theory. Many more nebulae are known 

 now than were known when Laplace advanced his theory. 

 But among them all none have been found which have a cen- 

 tral mass surrounded by a ring. On the other hand, re- 

 searches show that the great majority of nebulas are spiral 

 in form (Figs. 315 and 316). Such a nebula consists of a 

 luminous center with spiral arms or streamers issuing from 

 opposite sides. On these arms there are knotlike' condensa- 

 tions at various points. The spiral form suggests that the 

 whole mass has a whirling motion about the center. It is 

 thought that the thinner portions of these nebulae are com- 

 posed of scattered particles of various sizes, while in the knots 

 the particles are less scattered. All of the particles, including 

 those of the knots, are believed to be revolving, each in its 

 own independent orbit, about the central mass, much as the 

 earth revolves about the sun. Since these bodies behave like 

 tiny planets, they are called planetesimals. The suggestion 

 is made that our solar system may have grown from such a 

 nebula. It is conceived that the knotlike bodies in the arms 

 became the centers of growth for the planets, gradually gather- 

 ing in the planetesimals about them. Some of the " shooting 

 stars " or meteorites that enter our atmosphere are probably 

 but planetesimals still being gathered in, a suggestion that 

 the growth of the earth is still continuing, although with 

 exceeding slowness. 



On the new hypothesis, then, the earth grew from a nebular 

 knot to its present size by the slow ingathering of the smaller 

 particles or planetesimals. In the earlier stages of its existence 



1 The work of Professor T. C. Chamberlin of the University of Chicago, 

 assisted on the mathematical side by Dr. F. R. Moulton. 



