A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 73 



prove, as already noted, that at a mile above the surface there is no trace of an 

 atmosphere, surely not more than the thousandth part of our own. The law of 

 diffusion of gases makes it impossible that there should be any great increase in 

 the density of such air at its contact with the sphere. How, then, could vapors 

 slowly float away as clouds from a crater ? If they came forth they should be swiftly 

 and uniformly diffused in the essential vacuum. Change of hue due to the angle 

 of illumination or fluorescence, or both actions combined, affords a far more satis- 

 factory explanation of the observed facts. This explanation has difficulties, but 

 they are much less serious than those we encounter in a hypothesis of volcanic 

 action still existing and producing clouds. 



The observations which indicate that extended fields of the lunar surface 

 darken with its advancing day are extremely interesting for the reason that they 

 show a departure from the general tendency of the surface to become brighter 

 with the higher sun. There is no doubt that these changes are of great import- 

 ance, but I cannot regard them as suggesting the development of any kind of 

 organic life. This question as to the probability of life on the lunar surface has 

 never been adequately discussed, and as the suggestion is recurrent I purpose to 

 set forth below certain considerations which, in my opinion, make it appear to be 

 most improbable that anything like organic structures can possibly develop there. 



It is, in the first place, to be noted that all organic forms, from the lowest to 

 the highest, plant and animal alike, absolutely depend for their existence on the 

 solvent action of water on various substances. The conditions of life are that 

 this water shall be readily obtainable either directly from the fluid in which the 

 creatures dwell, from the rain, or from the moisture of the air. In all cases 

 this water must contain free oxygen and carbonic dioxide, as well as certain 

 minerals in solution. Although it is stated that certain lichens develop in rocks 

 within the antarctic circle, where the temperature has never been observed above 

 the freezing point, it may be safely assumed that these plants have now and then 

 received during their growing period and have retained in their bodies water in 

 the fluid state, otherwise their organic processes could not go on. Wherever on 

 high mountains, say above the level of 20,000 feet, the surface of the rock has 

 been examined, no resident life has been discovered. Thus in an air which is 

 surely many times as dense as any that can exist on the moon, terrestrial life, for 

 all its ample opportunities to become reconciled to such environment, has not 

 succeeded in establishing itself at these great altitudes. The conditions for the 

 formation of organisms suited to the higher peaks of the earth are vastly more 

 favorable than they could have been on the moon, yet the result is that they 

 have failed to develop in such conditions. 



Whatever were the circumstances, as yet unknown, which led to the be- 

 ginning of life on this earth, they were evidently of rare occurrence. The succes- 

 sions of organic forms suggest that they have been derived from few if not from 

 one original form ; and, further, that these initial stages have long since been 

 lost. It is unlikely that fresh starts in the origination of the lowliest organisms 

 are now making, for with all the skill of a host of well-trained inquirers we have 



