146 THE EVOLUTION OF 



same regions to the Moon, as the Moon already presents 

 the same face to it. At the ends of the terrestrial 

 diameter that will then be constantly pointing towards 

 the Moon, permanent high tides will accumulate ; ebb 

 and flow, and with it tidal friction will cease, and a state 

 of stable equilibrium will be reached 1 . It is impossible 

 to determine the epoch of this stage, but under the most 

 favourable conditions it must be measured by hundreds 

 of millions of years from the present time. 



There can be little doubt that we have in these actions 

 an explanation of the fact that the Moon continually pre- 

 sents the same face to the Earth. The mass of the 

 Earth, which determines its attractive power, is eighty 

 times that of the Moon. Its tide-generating power is 

 therefore eighty times as great. There is no water upon 

 the Moon, so that tidal friction seems to have been 

 sufficient to bring its periods of rotation and revolution 

 into coincidence while it was still in a molten condition. 



It is interesting to follow the future of the Earth-Moon 

 system ; but it is of greater interest to trace the past. In 

 the past the Earth must have rotated more rapidly, the 

 Moon must have been nearer, and it must have revolved 

 in a shorter period than at present. From the application 

 of mathematics to the problem, Darwin has shown that 

 there must have been a time when the Moon was quite 

 close to the surface of the Earth, and, when in this con- 

 dition, the further suggestive fact appears that its period 

 of revolution, the month, coincided, as it will again 

 coincide in the last stage, with that of the Earth's 

 rotation, the day. Both must then have been between 



1 The argument assumes that the terrestrial oceans will remain fluid. 

 If owing to decrease in solar radiation the oceans should become 

 frozen, tidal friction will cease before the final condition described 

 above is reached, and, with it, further development of the process. 



