Sketch of the process of evolution 563 



inferences drawn from several sources. We have first to rely on the 

 general principles of stability, according to which we are to look for 

 a series of families of forms, each terminating in an unstable form, 

 which itself becomes the starting-point of the next family of stable 

 forms. Secondly we have as a guide the analogy of the successive 

 changes in the evolution of ideal liquid stars ; and thirdly we 

 already possess some slender knowledge as to the equilibrium of 

 gaseous stars. 



From these data it is possible to build up in outline the probable 

 history of binary stars. Originally the star must have been single, 

 it must have been widely diffused, and must have been endowed with 

 a slow rotation. In this condition the strata of equal density must 

 have been of the planetary form. As it cooled and contracted the 

 symmetry round the axis of rotation must have become unstable, 

 through the effects of gravitation, assisted perhaps by the increasing 

 speed of rotation 1 . The strata of equal density must then become 

 somewhat pear-shaped, and afterwards like an hour-glass, with the 

 constriction more pronounced in the internal than in the external 

 strata. The constrictions of the successive strata then begin to rupture 

 from the inside progressively outwards, and when at length all are 

 ruptured we have the twin stars portrayed by Roberts and by 

 others. 



As we have seen, the study of the forms of equilibrium of rotating 

 liquid is almost complete, and Jeans has made a good beginning in the 

 investigation of the equilibrium of gaseous stars, but much more 

 remains to be discovered. The field for the mathematician is a wide 

 one, and in proportion as the very arduous exploration of that field 

 is attained so will our knowledge of the processes of cosmical 

 evolution increase. 



From the point of view of observation, improved methods in the 

 use of the spectroscope and increase of accuracy in photometry will 

 certainly lead to a great increase in our knowledge within the next 

 few years. Probably the observational advance will be more rapid 

 than that of theory, for we know how extraordinary has been the 

 success attained within the last few years, and the theory is one 

 of extreme difficulty ; but the two ought to proceed together hand 

 in hand. Human life is too short to permit us to watch the leisurely 

 procedure of cosmical evolution, but the celestial museum contains 

 so many exhibits that it may become possible, by the aid of theory, 

 to piece together bit by bit the processes through which stars pass in 

 the course of their evolution. 



1 I learn from Professor Jeans that he now (December 1908) believes that he can 

 prove that some small amount of rotation is necessary to induce instability in the sym- 

 metrical arrangement. 



362 



