oh. v.] PLANETARY EVOLUTION, 391 



hand the comet is an object of inconsiderable mass, though, 

 often of considerable volume. The slight concentration of 

 which it is capable will not produce planetary systems or 

 even asteroids, but only streams of meteors or shooting-stars, 

 such as are now poured down upon the earth and its neigh- 

 bour planets at the rate of a hundred thousand million each 

 year. The researches of the past ten years have gone far 

 to show that such meteoric streams differ from nebulous 

 comets in no respect save in their greater aggregation ; the 

 difference being similar to the difference between a cloud 

 and a shower of rain-drops. AVe are constantly encounter- 

 ing 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 comprehensive glance, 

 this gigantic process of Planetary Evolution, in which the 

 integration of matter and concomitant dissipation of mole- 

 cular motion, kept up during untold millions of ages, has 

 brought about the gradual transformation of a relatively 

 homogeneous, indefinite, and incoherent mass of nebular 

 vapour into a decidedly heterogeneous, definite, and coherent 

 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 

 phenomena, the stars in their courses have become the 

 types of permanence; and the stability of our planetary 

 system has furnished a fruitful theme for the admiring com- 

 ments of the mathematician and the theologian. In so far 

 as this appearance of eternal stability is well founded, it 

 admirably illustrates the theorem, already cited in our dis- 

 cussion of the rhythm of motion, that wherever the forces 

 in action are few in number and simple in composition, the 



