PLANETARY EVOLUTION 



but only streams of meteors or shooting-stars, 

 such as are now poured down upon the earth 

 and its neighbour planets at the rate of a hun- 

 dred thousand million each year. The researches 

 of the past ten years have gone far to show that 

 such meteoric streams differ from nebulous com- 

 ets in no respect save in their greater aggrega- 

 tion ; the difference being similar to the differ- 

 ence between a cloud and a shower of raindrops. 

 We are constantly encountering portions of 

 these condensed comets and uniting them with 

 our own planetary substance. And in this way 

 the integration of the outlying portions of our 

 primitive nebula is, at this late day, still going 

 on. 



As we pause to survey, in a single compre- 

 hensive glance, this gigantic process of Plan- 

 etary Evolution, in which the integration of 

 matter and concomitant dissipation of molec- 

 ular motion, kept up during untold millions of 

 ages, has brought about the gradual transfor- 

 mation of a relatively homogeneous, indefinite, 

 and incoherent mass of nebular vapour into 

 a decidedly heterogeneous, definite, and coher- 

 ent system of worlds ; we are at first struck by 

 the peculiarity that the process has apparently 

 long since come to a close in the establishment 

 of a complete moving equilibrium. Habituated 

 as we are to the contemplation of fleeting phe- 

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