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



not been able to initiate an organic form. If the existing living species of this 

 earth were destroyed, we do not know by what process a beginning could again 

 be made. So much, however, is plain : First, that all of the existing organic forms 

 have had the initial stages of their development in aquatic conditions, for there 

 alone can the earlier stages of development be attained. Second, that the 

 aqueous stages of the forms which now inhabit the land must have required 

 a very long period of such life before the creatures were ready to enter on the 

 more difficult conditions of the land. It may safely be presumed that a period 

 of development such as is represented by thousands of species of successive 

 forms was necessary to bring the terrestrial organisms into conditions of structure 

 and function where even as the lowliest plants they were fit for stations in the 

 air. This process of reconciliation with the environment demands, among other 

 things, means whereby the spores may be diffused, and with all plants of rapid 

 growth, such as have to be assumed if they are to give color to the surface of the 

 moon, it requires a soil or air for food supply. 



It is a favorite assumption with selenographers who adopt the hypothesis 

 of plant life on the moon a pure assumption that there may be a thin atmos- 

 phere of carbon dioxide next the surface and that in such an air plants would 

 grow with rapidity. This is a natural view, for it is based on the well-known fact 

 that the carbon of plants is largely obtained by the decomposition of that gas, the 

 carbon being taken into the structure and the oxygen set free. But the experi- 

 ments made by a committee of the British Association for the Advancement of 

 Science clearly showed that terrestrial plants, even the lowlier cryptogams, were 

 not sensibly helped by an increase in the amount of C O 2 in the air and that any 

 considerable augmentation of that gas was hurtful to them. 



Therefore, in view of these facts : that terrestrial plants, notwithstanding all 

 their ample opportunities for so doing, have never been able to reconcile them- 

 selves to the conditions which exist at heights where the density of the air is not 

 more than one-third of what it is at the sea-level ; that all organic life necessarily 

 had its beginning in the seas or other masses of water ; that the conditions of its 

 origin are so peculiar that we have never been able to reproduce them ; and that 

 the development of every organic species known to us requires a considerable 

 supply of water, it appears most unlikely that the moon is now or has ever been 

 the seat of organic life of the sort that exists on this earth. 



It cannot well be denied that there may be on the other celestial spheres 

 than this earth forms of association of matter in which other fluids than water 

 may serve as the menstruum in which vital activities develop, and that the essen- 

 tial results accomplished in the organic forms of our planet may be thus attained. 

 But, so far as we know, organic individuals are limited to very narrow conditions : 

 to those in which water is exposed to temperatures between the freezing point 

 and about sixty degrees Centigrade, and which afford air such as that of-_the earth 

 in density equivalent to not less than what corresponds to a pressure of one-third 

 that normally existing at sea-level. These conditions clearly do not exist, and, so 

 far as we can determine, have never existed on the lunar surface. It is, in fact, 



