ar to me that I was wrong； and that the larger part of the facts cannot be due to any such cause。。。To have the theory of organic evolution justified was of course to get further support for that theory of evolution at large with which。。。all my conceptions were bound up。〃 （Spencer； 〃Autobiography〃； Vol。 II。 page 50； London； 1904。） Instead of the metaphorical expression 〃natural selection；〃 Spencer introduced the term 〃survival of the fittest；〃 which found favour with Darwin as well as with Wallace。
In working out his ideas of evolution； Spencer found that differentiation was not the only form of evolution。 In its simplest form evolution is mainly a concentration； previously scattered elements being integrated and losing independent movement。 Differentiation is only forthcoming when minor wholes arise within a greater whole。 And the highest form of evolution is reached when there is a harmony between concentration and differentiation； a harmony which Spencer calls equilibration and which he defines as a moving equilibrium。 At the same time this definition enables him to illustrate the expression 〃survival of the fittest。〃 〃Every living organism exhibits such a moving equilibriuma balanced set of functions constituting its life； and the overthrow of this balanced set of functions or moving equilibrium is what we call death。 Some individuals in a species are so constituted that their moving equilibria are less easily overthrown than those of other individuals； and these are the fittest which survive； or； in Mr Darwin's language； they are the select which nature preserves。〃 （Ibid。 page 100。） Not only in the domain of organic life； but in all domains； the summit of evolution is； according to Spencer； characterised by such a harmonyby a moving equilibrium。
Spencer's analysis of the concept of evolution； based on a great variety of examples； has made this concept clearer and more definite than before。 It contains the three elements； integration； differentiation and equilibration。 It is true that a concept which is to be valid for all domains of experience must have an abstract character； and between the several domains there is； strictly speaking； only a relation of analogy。 So there is only analogy between psychical and physical evolution。 But this is no serious objection； because general concepts do not express more than analogies between the phenomena which they represent。 Spencer takes his leading terms from the material world in defining evolution （in the simplest form） as integration of matter and dissipation of movement； but as henot always quite consistently （Cf。 my letter to him； 1876； now printed in Duncan's 〃Life and Letters of Herbert Spencer〃； page 178； London； 1908。）assumed a correspondence of mind and matter； he could very well give these terms an indirect importance for psychical evolution。 Spencer has always； in my opinion with full right； repudiated the ascription of materialism。 He is no more a materialist than Spinoza。 In his 〃Principles of Psychology〃 （paragraph 63） he expressed himself very clearly： 〃Though it seems easier to translate so…called matter into so…called spirit； than to translate so…called spirit into so…called matterwhich latter is indeed wholly impossibleyet no translation can carry us beyond our symbols。〃 These words lead us naturally to a group of thinkers whose starting…point was psychical evolution。 But we have still one aspect of Spencer's philosophy to mention。
Spencer founded his 〃laws of evolution〃 on an inductive basis； but he was convinced that they could be deduced from the law of the conservation of energy。 Such a deduction is； perhaps； possible for the more elementary forms of evolution； integration and differentiation； but it is not possible for the highest form； the equilibration； which is a harmony of integration and differentiation。 Spencer can no more deduce the necessity for the eventual appearance of 〃moving equilibria〃 of harmonious totalities than Hegel could guarantee the 〃higher unities〃 in which all contradictions should be reconciled。 In Spencer's hands the theory of evolution acquired a more decidedly optimistic character than in Darwin's； but I shall deal later with the relation of Darwin's hypothesis to the opposition of optimism and pessimism。
II。 While the starting…point of Spencer was biological or cosmological； psychical evolution being conceived as in analogy with physical； a group of eminent thinkersin Germany Wundt； in France Fouillee； in Italy Ardigo took； each in his own manner； their starting…point in psychical evolution as an original fact and as a type of all evolution； the hypothesis of Darwin coming in as a corroboration and as a special example。 They maintain the continuity of evolution； they find this character most prominent in psychical evolution； and this is for them a motive to demand a corresponding continuity in the material； especially in the organic domain。
To Wundt and Fouillee the concept of will is prominent。 They see the type of all evolution in the transformation of the life of will from blind impulse to conscious choice； the theories of Lamarck and Darwin are used to support the view that there is in nature a tendency to evolution in steady reciprocity with external conditions。 The struggle for life is here only a secondary fact。 Its apparent prominence is explained by the circumstance that the influence of external conditions is easily made out； while inner conditions can be verified only through their effects。 For Ardigo the evolution of thought was the starting…point and the type： in the evolution of a scientific hypothesis we see a progress from the indefinite （indistinto） to the definite （distinto）； and this is a characteristic of all evolution； as Ardigo has pointed out in a series of works。 The opposition between indistinto and distinto corresponds to Spencer's opposition between homogeneity and heterogeneity。 The hypothesis of the origin of differences of species from more simple forms is a special example of the general law of evolution。
In the views of Wundt and Fouillee we find the fundamental idea of idealism： psychical phenomena as expressions of the innermost nature of existence。 They differ from the older Idealism in the great stress which they lay on evolution as a real； historical process which is going on through steady conflict with external conditions。 The Romantic dread of reality is broken。 It is beyond doubt that Darwin's emphasis on the struggle for life as a necessary condition of evolution has been a very important factor in carrying philosophy back to reality from the heaven of pure ideas。 The philosophy of Ardigo； on the other side； appears more as a continuation and deepening of positivism； though the Italian thinker arrived at his point of view independently of French…English positivism。 The idea of continuous evolution is here maintained in opposition to Comte's and Mill's philosophy of discontinuity。 From Wundt and Fouillee Ardigo differs in conceiving psychical evolution not as an immediate revelation of the innermost nature of existence； but only as a single； though the most accessible example； of evolution。
III。 To the French philosophers Boutroux and Bergson； evolution proper is continuous and qualitative； while outer experience and physical science give us fragments only； sporadic processes and mechanical combinations。 To Bergson； in his recent work 〃L'Evolution Creatrice〃； evolution consists in an elan de vie which to our fragmentary observation and analytic reflexion appears as broken into a manifold of elements and processes。 The concept of matter in its scientific form is the result of this breaking asunder； essential for all scientific reflexion。 In these conceptions the strongest opposition between inner and outer conditions of evolution is expressed： in the domain of internal conditions spontaneous development of qualitative formsin the domain of external conditions discontinuity and mechanical combination。
We see； then； that the theory of evolution has influenced philosophy in a variety of forms。 It has made idealistic thinkers revise their relation to the real world； it has led positivistic thinkers to find a closer connection between the facts on which they based their views； it has made us all open our eyes for new possibilities to arise through the prima facie inexplicable 〃spontaneous〃 variations which are the condition of all evolution。 This last point is one of peculiar interest。 Deeper than speculative philosophy and mechanical science saw in the days of their triumph； we catch sight of new streams； whose sources and laws we have still to discover。 Most sharply does this appear in the theory of mutation； which is only a stronger accentuation of a main point in Darwinism。 It is interesting to see that an analogous problem comes into the foreground in physics through the discovery of radioactive phenomena； and in psychology through the assumption of psychical new formations （as held by Boutroux； William James and Bergson）。 From this side； Darwin's ideas； as well as the analogous ideas in other domains； incite us to renewed examination of our first principles； their rationality and their value。 On the other hand； his theory of the struggle for existence challenges us to examine the conditions and discuss the outlook as to the persistence of human life and society and of the values that belong to them。 It is not enough to hope （or fear？） the rising of new forms； we have also to investigate the possibility of upholding the forms and ideals which have hitherto been the bases of human life。 Darwin has here given his age the most earnest and most impressive lesson。 This side of Darwin's theory is of peculiar interest to some special philosophical problems to which I now pass。
IV。
Among philosophical problems the problem of knowledge has in the last century occupied a foremost place。 It is natural； then； to ask how Darwin and the hypothesis whose most eminent representative he is； stand to this problem。
Darwin started an hypothesis。 But every hypothesis is won by inference from certain presuppositions； and every inference is based on the general principles of human thought。 The evolution hypothesis presupposes； then； human