PLANETARY EVOLUTION 



possesses climates analogous to our own might 

 be inferred from the inclination of his axis to 

 his orbit-plane, and is inductively proved by the 

 fact that we can actually see his polar snows 

 accumulate during the Martial winter and melt 

 away at the approach of the 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 geo- 

 logic phenomena not essentially unlike those 

 witnessed upon the earth. Whether such geo- 

 logic similarity has entailed a further resem- 

 blance in the case of organic and super-organic 

 phenomena, must be left for the more profound 

 deductive science of some future day to deter- 

 mine. 



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 con- 

 spicuous facts already cited, we see that the neb- 

 ular hypothesis 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 



vol. n. 289 



