400 COSMIC FIJILOSOPIIY. [ft. U, 



tion of this state of things, we found an adequate explanation 

 in the rapid loss of heat which the small size of the moon 

 has entailed. It is not likely, therefore, that the moon can 

 ever have "been the theatre of a geologic and organic develop- 

 ment so rich and varied as that which the earth has witnessed. 1 

 In the second place, the following chapter will show that 

 the chief circumstance which has favoured terrestrial hetero- 

 geneity has been the continuous supply of molecular motion 

 fn >m the sun. To this source may be traced all the aqueous 

 phenomena, save the tides, which concur in maintaining the 

 diversity of the earth's surface. And having thus seen how 

 a complex geologic evolution is rendered possible, we shall 

 further discern that organic evolution also — that highly 

 specialized series of terrestrial events — is rendered possible 

 by the same favouring circumstance. 



1 An example of the too hasty kind of inference which, is often drawn in 

 discussing the question of life upon other planets, may be found in a recent 

 lucid and suggestive pamphlet by Prof. Winchell, entitled " The Geology of 

 the Stars." " The zoic age of the moon," says the author, " was reached 

 while yet our world remained, perhaps, in a glowing condition. Its human 

 period was passing while the eozobn was solitary occupant of our primeval 

 ocean. " More careful reflection will probably convince us that, with such a 

 rapid succession of geologic epochs, the moon can hardly have had any human 

 period. For the purposes of comparative geology, the earth and the moon 

 may be regarded as of practically the same antiquity. Now, supposing the 

 earliest ape-like men to have made their appearance on the earth, say during 

 the Miocene epoch, we must remembev that at that period the moon must 

 have advanced in refrigeration much farther than the earth. Supposing 

 organic evolution to have gone on with equal pace in the two planets, it might 

 be argued that the moon would be fast becoming unfit for the support of 

 organic life at about the time when man appeared on the earth. Still more, 

 it is a fair inference from the theory of natural selection, that upon a small 

 planet there is likely to be a slower and less rich and varied evolution of life 

 than upon a large planet. On the whole, therefore, it does not seem likely 

 that the moon can ever have given rise to organisms nearly so high in tho 

 scale of life as human beings. Long before it could have attained to any such 

 point, its surface is likely to have become uninhabitable by air-breathing 

 organisms. Long before this, no doubt, its surface air and water must have 

 sunk into its interior, and left it the mere lifeless ember that it is. The moon 

 would thus appear to bo not merely an extinct world, but a partially aborted 

 world ; and the still smaller asteroids are perhaps totally aborted worlds. 

 Nevertheless, from the earth down to the moon, and from the moon down to 

 an asteroid, the differences are at bottom only differences of degree ; though 

 the differences in result may range all the way from a world habitable bv 

 civilized men down to a mere dead ball of planetary matter. An interesting 

 example, if it be sound, of the continuity of cosmical phenomena. 



