106 THE AMERICAN MIDLAND NATURALIST 
servation of these meteors and meteorites arose the idea that 
the earth had been built un from them, the rate of infall being 
more rapid in the early history of the process. The great 
irregularity in the motions and velocities of the observed 
meteorites soon shows that this explanation fails to account 
for the development of any such orderly and harmonious 
motions as are to be seen in the solar system. 
George Darwin, 2 son of the great Charles Darwin, still 
thought he saw in the infall of meteors and meteorites a 
possible solution of the origin of the solar system. He be- 
lieved that meteorites might be brought together into swarms, 
thus constituting nebulae. These nebulae would, according 
to Darwin, behave essentially as though they were composed 
of gases, and the laws of gases might be used in determining 
their mechanics. If this were the case, the same objections 
which have been raised against the Laplacian hypothesis 
apply to the one sponsored by Darwin and Lockyer, so it need 
not be given further attention. 
3.—The Planetesimal Hypothesis. 
When the failings of the Laplacian hypothesis became so 
evident, and the hypothesis of Lockyer and Darwin showed 
itself to be unreliable—in fact, less satisfactory than that of 
Laplace—an alternative more suited to the facts was looked 
for. Earler astronomers and astrophysicists had maintained 
that the matter of a nebula, if composed of particles revolving 
around their common center of gravity, could not come to- 
‘vether into planets without giving them a backward motion. 
The six inner planets of the solar system have forward rota- 
‘tions, and for the time being all hypothesis of the strictly 
nebula type seemed to be ruled out. A more careful survey, 
by Doctors Moulton and Chamberlin, showed that this con- 
clusion was wrong, and that there was no initial barrier in 
the way of a hypothesis in which the solar system was sup- 
posed to be descended from a nebula. It was also shown by 
astronomic photography that there were many times the 
number of nebulae that there formerly were thought to be, 
and it was to these that Dr. Chamberlin turned his attention. 
The nebulae known at the present time seem to fall into 
two classes, when studied with the spectroscope. The first 
