PLANETARY EVOLUTION 



yields but a simple hydrogen-line, to silch a 

 highly evolved body as our own sun with the 

 many lined vapour of iron abundant in its heated 

 atmosphere. But into this fascinating region of 

 speculation it would be somewhat premature for 

 us now to enter. Merely indicating what a rich 

 harvest of discovery is here likely to reward 

 the labourers of the immediate future, I would 

 call attention to an interesting speculation of 

 Mr. Spencer's, the possible inadequacy of which 

 need not weaken the effect of the evidence above 

 cited from planetary phenomena, and which is 

 in every way worthy of serious consideration. 



According to Mr. Spencer, the distribution 

 of nebulae affords a significant illustration of the 

 nebular hypothesis. Speaking generally, nebulae 

 occur in regions where developed stars are scarce. 

 The vast groups of spherical nebulae, here and 

 there partly developed into starry clusters, which 

 constitute the so-called Magellanic Clouds, are 

 situated in a district of the sky that is other- 

 wise starless. Now by far the most striking of 

 this class of facts is one which serves to bring 

 the entire sidereal system into direct comparison 

 with that little portion of it to which we belong. 

 Just as the planets lie almost entirely in a singie 

 plane, so the stars are distributed in almost in- 

 finite numbers in the plane of the Milky Way, 

 while elsewhere they occur rarely. And just as 

 the comets are chiefly distributed about the 

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