XXVIIT 
THE GENESIS OF DOUBLE STARS 
By Sir Greorce Darwin, K.C.B., F.RS. 
Plumian Professor of Astronomy and Experimental Philosophy in the 
University of Cambridge. 
IN ordinary speech a system of any sort is said to be stable when 
it cannot be upset easily, but the meaning attached to the word is 
usually somewhat vague. It is hardly surprising that this should be 
the case, when it is only within the last thirty years, and principally 
through the investigations of M. Poincaré, that the conception of 
stability has, even for physicists, assumed a definiteness and clearness 
in which it was previously lacking. The laws which govern stability 
hold good in regions of the greatest diversity ; they apply to the 
motion of planets round the sun, to the internal arrangement of those 
minute corpuscles of which each chemical atom is constructed, and to 
the forms of celestial bodies. In the present essay I shall attempt to 
consider the laws of stability as relating to the last case, and shall 
discuss the succession of shapes which may be assumed by celestial 
bodies in the course of their evolution. I believe further that homo- 
logous conceptions are applicable in the consideration of the trans- 
mutations of the various forms of animal and of vegetable life and in 
other regions of thought. Even if some of my readers should think that 
what I shall say on this head is fanciful, yet at least the exposition will 
serve to illustrate the meaning to be attached to the laws of stability 
in the physical universe. 
I propose, therefore, to begin this essay by a sketch of the 
principles of stability as they are now formulated by physicists. 
I. 
If a slight impulse be imparted to a system in equilibrium one of 
two consequences must ensue ; either small oscillations of the system 
will be started, or the disturbance will increase without limit and the 
arrangement of the system will be completely changed. Thus a stick 
may be in equilibrium either when it hangs from a peg or when it is 
balanced on its point. If in the first case the stick is touched it will 
swing to and fro, but in the second case it will topple over. The first 
