y be impossible after
the point of bifurcation, in which case they coalesce and disappear.
This last case shows that types arise and disappear in pairs, and that
on appearance or before disappearance one must be stable and the other
unstable.)

In nature it is of course only the stable types of motion which can
persist for more than a short time. Thus the task of the physical
evolutionist is to determine the forms of bifurcation, at which he must,
as it were, change carriages in the evolutionary journey so as always
to follow the stable route. He must besides be able to indicate
some natural process which shall correspond in effect to the ideal
arrangement of the several types of motion in families with gradually
changing specific differences. Although, as we shall see hereafter, it
may frequently or even generally be impossible to specify with exactness
the forms of bifurcation in the process of evolution, yet the conception
is one of fundamental importance.

The ideas involved in this sketch are no doubt somewhat recondite, but I
hope to render them clearer to the non-mathematical reader by homologous
considerations in other fields of thought (I considered this subject in
my Presidential address to the British Association in 1905, "Report of
the 75th Meeting of the British Assoc." (S. Africa, 1905), London, 1906,
page 3. Some reviewers treated my speculations as fanciful, but as I
believe that this was due generally to misapprehension, and as I hold
that homologous considerations as to stability and instability are
really applicable to evolution of all sorts, I have thought it well to
return to the subject in the present paper.), and I shall pass on thence
to illustrations which will teach us something of the evolution of
stellar systems.

States or governments are organised schemes of action amongst groups of
men, and they belong to various types to which generic names, such as
autocracy, aristocracy or democracy, are somewhat loosely applied. A
definite type of government corresponds to one of our types of
motion, and while retaining its type it undergoes a slow change as the
civilisation and character of the people change, and as the relationship
of the nation to other nations changes. In the language used before, the
government belongs to a family, and as time advances we proceed through
the successive members of the family. A government possesses a certain
degree of stability--hardly measurable in numbers howe