PLURALITY OF WORLDS 171 



It is completely covered with cloud or mist. For this 

 reason no definite surface markings can be made out, 

 and it is still uncertain how fast it rotates on its axis 

 and in which direction the axis lies. One curious theory 

 may be mentioned though it should perhaps not be taken 

 too seriously. It is thought by some that the great 

 cavity occupied by the Pacific Ocean is a scar left by the 

 moon when it was first disrupted from the earth. Evi- 

 dently this cavity fulfils an important function in drain- 

 ing away superfluous water, and if it were filled up 

 practically all the continental area would be submerged. 

 Thus indirectly the existence of dry land is bound up 

 with the existence of the moon. But Venus has no moon, 

 and since it seems to be similar to the earth in other 

 respects, it may perhaps be inferred that it is a world 

 which is all ocean — where fishes are supreme. The 

 suggestion at any rate serves to remind us that the 

 destinies of organic life may be determined by what 

 are at first sight irrelevant accidents. 



The sun is an ordinary star and the earth is an 

 ordinary planet, but the moon is not an ordinary satel- 

 lite. No other known satellite is anything like so large 

 in proportion to the planet which it attends. The moon 

 contains about 1/80 part of the mass of the earth which 

 seems a small ratio; but it is abnormally great compared 

 with other satellites. The next highest ratio is found 

 in the system of Saturn whose largest satellite Titan has 

 1/4000 of the planet's mass. Very special circum- 

 stances must have occurred in the history of the earth 

 to have led to the breaking away of so unusual a frac- 

 tion of the mass. The explanation proposed by Sir 

 George Darwin, which is still regarded as most prob- 

 able, is that a resonance in period occurred between 

 the solar tides and the natural free period of vibration 



