The Spencerian philosophy is the most comprehensive and ambitious application of the principle of evolution ever attempted. Without showing anywhere that mastery of detail and that power of marshalling facts in evidence which give Darwin's great work its unequalled significance, the _Synthetic Philosophy_ yet reaches at both ends beyond the limits Darwin set himself. Mr. Spencer begins by recognising three kinds of evolution, in the spheres of the inorganic, the organic and the super-organic; and all the parts of the _Synthetic Philosophy_ find a place under one or other of these; but the treatment of the first part is omitted as less pressing and as adding too greatly to the magnitude of the scheme. After the _First Principles_, in which are laid down the limits of the knowable and the unknowable, there follows therefore the _Principles of Biology_ (1864-1867), where the evolution of life, the gradual differentiation of functions and kindred topics are treated. Still within the sphere of the evolution of the organic we have next the _Principles of Psychology_ (1855), where organisms exhibiting the phenomena of mind are examined from various points of view to determine so far as possible the nature of mind, its relations with the universe, the composition of its simpler elements, etc. From psychology we step to super-organic evolution in the _Principles of Sociology_ (1876-1896), which is probably regarded by the majority as the most characteristic part of the Spencerian philosophy. It is certainly one of the most interesting; for it combines in an unusual measure the best results of ancient thought with full justice to modern individualism. Mr. Spencer is a consistent individualist, but a far-sighted one. He sees that 'the survival of the fittest,' and with it progress, are impossible unless 'the fittest' both wins and keeps advantage to himself. Unlimited altruism would be as bad as unlimited egoism, and would indeed foster egoism, for it would in the end mean the stripping of generosity to pamper greed. On the other hand, pure egoism is fatal to society; and the animal for whom gregariousness is an advantage must fail in the struggle if he is unfaithful to the social principle. Hence there arises a society which is a balance between the two principles. It demands sacrifices from the individual in return for benefits; but the law of its existence prohibits the extension of this demand beyond the point where the individual 'fittest' survives and prospers. If the demand goes beyond this the course is downwards; for, as society is composed of individuals, a society in which the strongest has no advantage is a society in which progress is impossible, but, on the contrary, deterioration is sooner or later certain. There is no room on Spencerian principles for any socialism which does not recognise difference of reward according to difference of capacity.