odern times that generalisation is most fruitful； but even in these it may be contended that it furnishes only partial explanations。
17。 The truth is that Darwinism itself offers the best illustration of the insufficiency of general laws to account for historical development。 The part played by coincidence； and the part played by individualslimited by； and related to； general social conditionsrender it impossible to deduce the course of the past history of man or to predict the future。 But it is just the same with organic development。 Darwin （or any other zoologist） could not deduce the actual course of evolution from general principles。 Given an organism and its environment； he could not show that it must evolve into a more complex organism of a definite pre…determined type； knowing what it has evolved into； he could attempt to discover and assign the determining causes。 General principles do not account for a particular sequence； they embody necessary conditions； but there is a chapter of accidents too。 It is the same in the case of history。
18。 Among the evolutional attempts to subsume the course of history under general syntheses； perhaps the most important is that of Lamprecht； whose 〃kulturhistorische Methode；〃 which he has deduced from and applied to German history； exhibits the （indirect） influence of the Comtist school。 It is based upon psychology； which； in his view； holds among the sciences of mind （Geisteswissenschaften） the same place （that of a Grundwissenschaft） which mechanics holds among the sciences of nature。 History； by the same comparison； corresponds to biology； and； according to him； it can only become scientific if it is reduced to general concepts （Begriffe）。 Historical movements and events are of a psychical character； and Lamprecht conceives a given phase of civilisation as 〃a collective psychical condition （seelischer Gesamtzustand）〃 controlling the period； 〃a diapason which penetrates all psychical phenomena and thereby all historical events of the time。〃 （〃Die kulturhistorische Methode〃； Berlin； 1900； page 26。） He has worked out a series of such phases； 〃ages of changing psychical diapason；〃 in his 〃Deutsche Geschichte〃 with the aim of showing that all the feelings and actions of each age can be explained by the diapason； and has attempted to prove that these diapasons are exhibited in other social developments； and are consequently not singular but typical。 He maintains further that these ages succeed each other in a definite order； the principle being that the collective psychical development begins with the homogeneity of all the individual members of a society and； through heightened psychical activity； advances in the form of a continually increasing differentiation of the individuals （this is akin to the Spencerian formula）。 This process； evolving psychical freedom from psychical constraint； exhibits a series of psychical phenomena which define successive periods of civilisation。 The process depends on two simple principles； that no idea can disappear without leaving behind it an effect or influence； and that all psychical life； whether in a person or a society； means change； the acquisition of new mental contents。 It follows that the new have to come to terms with the old； and this leads to a synthesis which determines the character of a new age。 Hence the ages of civilisation are defined as the 〃highest concepts for subsuming without exception all psychical phenomena of the development of human societies； that is； of all historical events。〃 （Ibid。 pages 28； 29。） Lamprecht deduces the idea of a special historical science； which might be called 〃historical ethnology；〃 dealing with the ages of civilisation； and bearing the same relation to （descriptive or narrative） history as ethnology to ethnography。 Such a science obviously corresponds to Comte's social dynamics； and the comparative method； on which Comte laid so much emphasis； is the principal instrument of Lamprecht。
19。 I have dwelt on the fundamental ideas of Lamprecht； because they are not yet widely known in England； and because his system is the ablest product of the sociological school of historians。 It carries the more weight as its author himself is a historical specialist； and his historical syntheses deserve the most careful consideration。 But there is much in the process of development which on such assumptions is not explained； especially the initiative of individuals。 Historical development does not proceed in a right line； without the choice of diverging。 Again and again； several roads are open to it； of which it chooses onewhy？ On Lamprecht's method； we may be able to assign the conditions which limit the psychical activity of men at a particular stage of evolution； but within those limits the individual has so many options； such a wide room for moving； that the definition of those conditions； the 〃psychical diapasons；〃 is only part of the explanation of the particular development。 The heel of Achilles in all historical speculations of this class has been the role of the individual。
The increasing prominence of economic history has tended to encourage the view that history can be explained in terms of general concepts or types。 Marx and his school based their theory of human development on the conditions of production； by which； according to them； all social movements and historical changes are entirely controlled。 The leading part which economic factors play in Lamprecht's system is significant； illustrating the fact that economic changes admit most readily this kind of treatment； because they have been less subject to direction or interference by individual pioneers。
Perhaps it may be thought that the conception of SOCIAL ENVIRONMENT （essentially psychical）； on which Lamprecht's 〃psychical diapasons〃 depend； is the most valuable and fertile conception that the historian owes to the suggestion of the science of biologythe conception of all particular historical actions and movements as （1） related to and conditioned by the social environment； and （2） gradually bringing about a transformation of that environment。 But no given transformation can be proved to be necessary （pre…determined）。 And types of development do not represent laws； their meaning and 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 researcheswhich 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 may be assumed by celestial bodies in the course of their evolution。 I believe further that homologous conceptions are applicable in the consideration of the transmutations 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 position is a stable one； the second is unstable。 But this case is too simple to illustrate all that is implied by stability； and we must consider cases of stable and of unstable motion。 Imagine a satellite and its planet； and consider each of them to be of indefinitely small size； in fact particles； then the satellite revolves round its planet in an ellipse。 A small disturbance imparted to the satellite will only change the ellipse to