THE LAW OF EVOLUTION CONCLUDED. 397 



ible. • Meanwhile, there goes on that further in- 



tegration by which the motions of all the parts of the Solar 

 System are rendered mutually dependent. Locally in each 

 planet and its satellites, and generally in the Sun and the 

 planets, we have a system of simple and compound rhythms, 

 with periodic and secular variations, forming together an 

 integrated set of movements. 



The matter which, in its original diffused state, had 

 motions that were confused, indeterminate, or without 

 sharply-marked distinctions, has, during the evolution of 

 the Solar System, acquired definitely heterogeneous mo- 

 tions. The periods of revolution of all the planets and satel- 

 lites are unlike; as are also their times of rotation. Out of 

 these definitely heterogeneous motions of a simple kind, 

 arise others that are complex, but still definite; — as those 

 produced by the revolutions of satellites compounded with 

 the revolutions of their primaries; as those of which pre- 

 cession is the result; and as those which are known as per- 

 turbations. Each additional complexity of structure has 

 caused additional complexity of movements ; but still, a defi- 

 nite complexity, as is shown by having calculable results. 



§ 141. While the Earth's surface was molten, the cur- 

 rents in the voluminous atmosphere surrounding it, mainly 

 of ascending heated gases and of descending precipitated 

 liquids, must have been local, numerous, indefinite, and but 

 little distinguished from one another. But as fast as the 

 surface cooled, and solar radiation began to cause appre- 

 ciable differences of temperature between the equatorial 

 and polar regions, a decided atmospheric circulation from 

 poles to equator and from equator to poles, must have slowly 

 established itself: the vast moving masses of air becoming, 

 at last, trade-winds and other such permanent definite cur- 

 rents. These integrated motions, once com- 

 paratively homogeneous, were rendered heterogeneous as 

 great islands and continents arose, to complicate them by 



