value lie in the help they may give
to the historian, in investigating a certain period of civilisation,
to enable him to discover the interrelations among the diverse features
which it presents. They are, as some one has said, an instrument of
heuretic method.

20. The men engaged in special historical researches--which have been
pursued unremittingly for a century past, according to scientific
methods of investigating evidence (initiated by Wolf, Niebuhr,
Ranke)--have for the most part worked on the assumptions of genetic
history or at least followed in the footsteps of those who fully grasped
the genetic point of view. But their aim has been to collect and sift
evidence, and determine particular facts; comparatively few have given
serious thought to the lines of research and the speculations which
have been considered in this paper. They have been reasonably shy
of compromising their work by applying theories which are still much
debated and immature. But historiography cannot permanently evade the
questions raised by these theories. One may venture to say that no
historical change or transformation will be fully understood until it is
explained how social environment acted on the individual components of
the society (both immediately and by heredity), and how the individuals
reacted upon their environment. The problem is psychical, but it is
analogous to the main problem of the biologist.

XXVIII. THE GENESIS OF DOUBLE STARS. By Sir George Darwin, K.C.B.,
F.R.S.

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. Poincare, 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 m