$ 320] The Evolution of the Solar System 409 



in which the earth rotated on its axis, which was then a 

 little over two hours.. The two bodies, in fact, were moving 

 as if they were connected ; it is difficult to avoid the 

 probable inference that at an earlier stage the two really 

 were one, and that the moon is in reality a fragment of the 

 earth driven off from it by the too-rapid spinning of the 

 earth, or otherwise. 



Professor Darwin has also examined the possibility of 

 explaining in a similar way the formation of the satellites 

 of the other planets and of the planets themselves from 

 the sun, but the circumstances of the moon-earth system 

 turn out to be exceptional, and tidal influence has been 

 less effective in other cases, though it gives a satisfactory 

 explanation of certain peculiarities of the planets and their 

 satellites. More recently (1892) Dr. See has applied a 

 somewhat similar line of reasoning to explain by means 

 of tidal action the development of double stars from an 

 earlier nebulous condition. 



Speaking generally, we may say that the outcome of the 

 1 9th century study of the problem of the early history 

 of the solar system has been to discredit the details of 

 Laplace's hypothesis in a variety of ways, but to establish 

 on a firmer basis the general view that the solar system 

 has been formed by some process of condensation out of 

 an earlier very diffused mass bearing a general resemblance 

 to one of the nebulae which the telescope shews us, and 

 that stars other than the sun are not unlikely to have been 

 formed in a somewhat similar way ; and, further, the theory 

 of tidal friction supplements this general but vague theory, 

 by giving a rational account of a process which seems to 

 have been the predominant factor in the development of 

 the system formed by our own earth and moon, and to have 

 had at any rate an important influence in a number cf 

 other cases. 



