THE BIRTH AND DEATH OF WORLDS 421 



which this rotatory motion might have been acquired, and 

 perhaps shed some light upon the modes of planetary formation. 

 Some of the more distinct nebulae appear to show knots or 

 clumps of denser formation, as though they in no wise possess 

 a uniform constitution, but that within the mass several centres 

 of condensation have begun to appear. 



We have just a hint as to how these centres or nuclei might 

 be formed. We know on the one hand that the number of 

 nebulae is enormous ; on the other hand, that space is simply 

 teeming with meteoritic or cometary swarms. These range in 

 size all the way from grains of sand and brickbats up to bodies 

 of considerable magnitude. When after uncounted aeons a 

 nebula had become somewhat thickened, if it were then to 

 encounter a meteoritic swarm, it might readily arrest the motion 

 of the latter and imprison it within its meshes. The motion 

 of the meteorites would be partly dissipated in heat, partly 

 in imparting motion to the nebula itself. It is conceivable 

 that if such a swarm were very great, and the nebula already 

 of some density, this might give rise in time to just the ap- 

 pearances which we perceive. 



It may be that this imprisonment of meteorites within the 

 spider-web of the nebulae is no isolated event, but that it is a 

 continuous process, like unto the sweeping up of the meteorites 

 by the earth, the sun, and other large bodies. We know that 

 some of the nebulae at least are in very rapid motion, at speeds 

 quite comparable to that of the stars themselves. It is very 

 easy to suppose that they would of necessity encounter vast 

 quantities of meteors. 



Occasionally they would encounter a sun. It may be that 

 this explains the blazing up of " new " stars, now a well- 

 recognised fact of stellar observation. In whatever way these 

 centres of condensation may form, it is easy to see that by 

 further condensation, they might readily result in the ring- 

 like or Saturn-like appearances which some of the nebulae 

 present. If this were true the difficulty of understanding how 

 these zones or rings were formed by the condensation of a nebular 

 mass would be dissipated. 



Moreover, it may be that the mode of evolution is not 

 single, but that the process varies considerably under varying 

 physical conditions. Still there is unmistakably in nature a 

 certain unity. The modes of reproduction of life, whether of 



