156 THE EVOLUTION OF 



this high proportion that so great a separation has been 

 possible as the result of tidal friction. 



The Earth-Moon scheme of development does not 

 therefore replace that of Laplace. Neither does it, so 

 far as is yet known, exclude it. Laplace imagined a 

 nebula strongly condensed towards the centre ; Poincare" 

 one of uniform density throughout. It by no means 

 follows that the developments of such different bodies 

 will proceed along similar lines, but the relation between 

 them may conceivably be a problem to be successfully 

 attacked in the future. 



There is, however, another problem to which we may 

 reasonably hope to apply the Earth- Moon method of 

 development with greater success. It is to systems of 

 double stars. In all cases in which comparison is possible, 

 the components of a double star do not differ greatly in 

 mass, and therefore a wide separation by tidal friction is 

 conceivable. If, in any double star, the rates of rotation 

 and revolution of the components were known, and also 

 the dimensions of the system, Darwin's method would 

 enable us to trace it back to the first stage at which tidal 

 friction became an operative factor, and, if the components 

 should then appear to be in close proximity, their origin 

 would not be doubtful ; but, unfortunately, it seems im- 

 possible to expect that such complete data will ever be at 

 our disposal. There is, however, another way in which 

 we may hope to test the theory. If it furnishes a true 

 expression of the development of a double star, it is 

 reasonable to expect that its different stages will be 

 represented in existing systems, that there are, in addi- 

 tion to the cases revealed by the telescope, in which the 

 separation is necessarily great, and in which accordingly we 

 are tempted to see systems in an advanced stage of their 

 life-histories, others in which the separation is small, and 



