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’, 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. 
36—2 
