CELESTIAL EVOLUTION. 235 



contact. In other words, the Moon originally 

 formed part of the Earth, which, in consequence 

 of its short - rotation period, and probably also 

 owing to the interference of the solar tide, split 

 into two portions, and the smaller of these now 

 forms the Moon. It is likely that the matter 

 now forming the Moon was detached from the 

 Earth in separate particles. Just as the tides 

 raised by the Moon tend to retard the motion 

 of the Earth, so the Earth tides raised in the 

 Moon have already done their work. The Moon 

 now rotates on its axis in the same time as it 

 revolves round the Earth. Part of the evolu- 

 tion of the Earth -Moon system is completed. 

 Schiaparelli's discovery that the rotation periods 

 of both Venus and Mercury coincide with their 

 times of revolution is distinctly confirmatory of 

 Darwin's theory. 



In his chapter on the " Evolution of Celestial 

 Systems " in his book on ' The Tides/ Darwin dis- 

 cusses the distribution of the satellites of the Solar 

 System. He says of the evolution of a planet : 

 "We have seen that rings should be shed from 

 the central nucleus when the contraction of the 

 nebula has induced a certain degree of augmen- 

 tation of rotation. Now, if the rotation were 

 retarded by some external cause, the genesis of 

 a ring might be retarded or entirely prevented." 



