THE INSTABILITY OF THE HOMOGENEOUS. 417 



regions in which both red and blue stars are rare; there are 

 regions in which the blue occur in considerable numbers, 

 and there are other regions in which the red are comparative- 

 ly abundant. Yet one more irregularity of like significance 

 is presented by the nebulae, aggregations of matter which, 

 whatever be their nature, most certainly belong to our 

 sidereal system. For the nebulae are not dispersed with any- 

 thing like uniformity ; but are abundant around the poles of 

 the galactic circle and rare in the neighbourhood of its 

 plane. ISTo one will expect that anything like a 



definite interpretation of this structure can be given on the 

 hypothesis of Evolution, or any other hypothesis. The most 

 that can be looked for is some reason for thinking that 

 irregularities, not improbably of these kinds, would occur in 

 the course of Evolution, supposing it to have taken place. 

 Any one called on to assign such reason might argue, that if 

 the matter of which stars and all other celestial bodies con- 

 sist, be assumed to have originally existed in a diffused form 

 throughout a space far more vast even than that which our 

 sidereal system now occupies, the instability of the homo- 

 geneous would negative its continuance in that state. In de- 

 fault of an absolute balance among the forces with which 

 the dispersed particles acted on each other (which could not 

 exist in any aggregation having limits) he might show that 

 motion and consequent changes of distribution would neces- 

 sarily result. The next step in the argument would be that 

 in matter of such extreme tenuity and feeble cohesion there 

 would be motion towards local centres of gravity, as well as 

 towards the general centre of gravity; just as, to use a 

 humble illustration, the particles of a precipitate aggregate 

 into flocculi at the same time that they sink towards the 

 earth. He might urge that in the one case as in the other, 

 these smallest and earliest local aggregations must gradually 

 divide into groups, each concentrating to its own centre of 

 gravity, a process which must repeat itself on a larger and 

 larger scale. In conformity with the law that motion once 



