A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 75 



very doubtful if any other body in our solar system with the exception of the 

 planet Mars is now in a viable state, for it is not likely that any but the earth, and 

 possibly the Martial sphere, has the necessary combination of water and solar 

 heat which has long existed on this planet. 



The foregoing considerations concerning the possibilities of organic life 

 on the moon show clearly that we must exhaust every valid hypothesis to explain 

 the occurrence of changes on that sphere before we assume that they are due to 

 the development of living forms. I would suggest that the patent facts of color- 

 change shown by the blotches and rays, which gain intensity as the sun goes 

 higher, lead naturally to the supposition that these other conditions of darkening 

 are due to a like though somewhat diverse action. We may fairly suppose that 

 the regions which thus darken are covered with crystals which reflect or refract 

 the sun's light in such a manner that they send us less of it when the sun is about 

 vertical than when it is relatively low. We have command of three certainly 

 warranted agents for explaining changes of color in the moon : that of reflec- 

 tions from crystalline surfaces ; that of refraction taking place in the interior of 

 translucent crystals ; and that of fluorescence. We have a right to combine these 

 actions as needs be to account for such phenomena of varying color as may be 

 observed, for all of them are well within the limits of what we note on the earth, 

 but we have not a like right to bring in hypotheses of organic life when all we know 

 of its conditions on this planet shows that it cannot exist on the lunar surface. 



It is naturally painful to conclude that the moon is and always has been 

 deprived of those features of existence which we deem the nobler ; that it has 

 never known the stir of air or water or the higher life of beings who inherit the 

 profit of experience and thereby climb the way that has led upward to man. That 

 these large gifts have been denied to the nearest companion of the earth has its 

 lessons for the naturalist, since it clearly shows how vast are the effects arising from 

 the interrelation of actions. The fate of our satellite was probably in large part 

 determined by the ratio between its gravitative force and the energy of the kinetic 

 movement of the gases such as constitute the atmosphere. If that energy had 

 been sufficient to retain them on the satellite, there is no reason,'at least so long 

 as the original rotation on its axis continued, why it should not have had the 

 history of a miniature earth. As it is, from the beginning it appears to have 

 been determined that it should have no share in the solar energy which has given 

 the most of the dynamic and all of the organic activities of the earth, and there 

 is no imaginable accident that can alter its state except some catastrophe which 

 may return the solar system to a nebulous mass. Just as it is, our moon is likely 

 to see the sun's light go out. 



SUGGESTIONS CONCERNING THE STUDY OF THE MOON. 



From the point of view of the geologist and geographer I venture to make 

 certain suggestions concerning the future work of selenographers. In the first 

 place, it may be said that, while the delineation of lunar features has, within a 



