376 THE LAW OF EVOLUTION CONTINUED. 



in outline, and had its surface more sharply marked off from 

 the surrounding void. Simultaneously, the constituent por- 

 tions of nebulous matter, instead of moving independently 

 towards their common centre of gravity from all points, 

 and revolving round it in various planes, as they would at 

 first do, must have had these planes more and more merged 

 into a single plane, that became less variable as the concen- 

 tration progressed — became gradually defined. 



According to the hypothesis, change from indistinct 

 characters to distinct ones, was repeated in the evolution of 

 planets and satellites; and may in them be traced much 

 further. A gaseous spheroid is less definitely limited than 

 a fluid spheroid, since it is subject to larger and more rapid 

 undulations of surface, and to much greater .distortions of 

 general form ; and, similarly, a liquid spheroid, covered as it 

 must be with waves of various magnitudes, is less definite 

 than a solid spheroid. The decrease of oblateness that goes 

 along with increase of integration, brings relative definite- 

 ness of other elements. A planet having an axis inclined 

 to the plane of its orbit, must, while its form is very 

 oblate, have its plane of rotation much disturbed by the 

 attraction of external bodies; whereas its approach to a 

 spherical form, involving a smaller precessional motion, 

 involves less marked variations in the direction of its 

 axis. 



"With progressing settlement of the space-relations, the 

 force-relations simultaneously become more settled. The 

 exact calculations of physical astronomy, show us how defi- 

 nite these force-relations now are; while their original 

 indefiniteness is implied in the extreme difficulty, if not 

 impossibility, of subjecting the nebular hypothesis to mathe- 

 matical treatment. 



§ 131. From that primitive molten state of the Earth 

 inferable from geological data — a state accounted for by the 

 nebular hypothesis but inexplicable on any other — the 



