sh. v.] PLANETARY EVOLUTION. 383 



Martial summer. Coincidences like these bear sufficient 

 testimony to a general resemblance between Mars and the 

 earth. For where there are oceans and clouds and an 

 atmosphere and polar snows, there must also be currents, 

 aerial and oceanic, as well as rains, rivers, and sedimentary 

 rocks; so that the surface of Mars must probably present 

 geologic phenomena not essentially unlike those witnessed 

 upon the earth. Whether such geologic similarity has 

 entailed a further resemblance in the case of organic and 

 super-organic phenomena, must be left for the more profound 

 deductive science of some future day to determine. 



Thus from whatever point of view we study our planetary 

 system, we find such a congeries of phenomena as would 

 have been produced by the gradual development of the 

 system from a homogeneous nebula. On summing up the 

 conspicuous facts already cited, we see that the nebular hypo- 

 thesis fully explains the shapes of the planetary orbits, and 

 their slight inclinations to the plane of the solar equator ; the 

 shapes of the satellite-orbits, and their proximate coincidence 

 with the equatorial planes of their primaries ; the inclina- 

 tions of the planetary axes to their orbit-planes ; the oblate 

 figures of the planets ; their velocities of rotation ; the direc- 

 tions in which they revolve ; and the directions in which 

 they rotate. To this last clause the apparent obstacle pre- 

 sented by the retrograde rotation of Uranus (and possibly of 

 Neptune also) is seen on closer examination to be no real 

 obstacle ; and the fact that the exception occurs among the 

 outermost planets, just where we might expect it to occur, if 

 at all, is a powerful argument in favour of the general theory. 

 A like powerful argument is furnished by the existence of 

 apparently-continuous rings about Saturn, the planet upon 

 which the centrifugal force bears the highest ratio to gravity. 

 Still more convincing is the testimony rendered by the dis- 

 tribution of satellites, — a testimony well-nigh meeting all 

 tiie requirements of crucial proof. Irregular as are the sizes 



