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 1 . 



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 I 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. 



