548 The Genesis of Double Stars 
Further than this, I would ask whether the same train of ideas 
does not also apply to the evolution of animals? A species is well 
adapted to its environment when the individual can withstand the 
shocks of famine or the attacks and competition of other animals ; 
it then possesses a high degree of stability. Most of the casual 
variations of individuals are indifferent, for they do not tell much 
either for or against success in life ; they are small oscillations which 
leave the type unchanged. As circumstances change, the stability of 
the species may gradually dwindle through the insufficiency of some 
definite quality, on which in earlier times no such insistent demands 
were made. The individual animals will then tend to fail in the 
struggle for life, the numbers will dwindle and extinction may ensue. 
But it may be that some new variation, at first of insignificant 
importance, may just serve to turn the scale. A new type may be 
formed in which the variation in question is preserved and augmented ; 
its stability may increase and in time a new species may be 
produced. 
At the risk of condemnation as a wanderer beyond my province 
into the region of biological evolution, I would say that this view 
accords with what I understand to be the views of some naturalists, 
who recognise the existence of critical periods in biological history at 
which extinction occurs or which form the starting-point for the 
formation of new species. Ought we not then to expect that long 
periods will elapse during which a type of animal will remain almost 
constant, followed by other periods, enormously long no doubt as 
measured in the life of man, of acute struggle for existence when the 
type will change more rapidly? This at least is the view suggested. 
by the theory of stability in the physical universe’. 
And now I propose to apply these ideas of stability to the theory 
of stellar evolution, and finally to illustrate them by certain recent 
observations of a very remarkable character. 
Stars and planets are formed of materials which yield to the 
enormous forces called into play by gravity and rotation. This is 
obviously true if they are gaseous or fluid, and even solid matter 
becomes plastic under sufficiently great stresses. Nothing approach- 
ing a complete study of the equilibrium of a heterogeneous star has 
yet been found possible, and we are driven to consider only bodies 
of simpler construction. I shall begin therefore by explaining what 
is known about the shapes which may be assumed by a mass of 
incompressible liquid of uniform density under the influences of 
gravity and of rotation. Such a liquid mass may be regarded as 
1 [ make no claim to extensive reading on this subject, but refer the reader for example 
to a paper by Professor A. A. W. Hubrecht on ‘‘ De Vries’s Theory of Mutations,” Popular 
Science Monthly, July 1904, especially to p. 213. 
