All through the middle decades of the century Darwin continued to labour at his vast accumulation of illustrative facts; and side by side with his continuous toil, outside opinion kept paving the way for the final acceptance of his lucid ideas. The public was buying and reading all the time its ten editions of the 'Vestiges of Creation.' It was slowly digesting Lyell's 'Principles of Geology,' in which the old cataclysmic theories were featly demolished, and the uniformitarian conception of a past gradually and insensibly merging into the present was conclusively established. It was getting accustomed to statements like those of the younger St. Hilaire, in 1850, that specific characters may be modified by changes in the environing conditions, and that the modifications thus produced may often be of generic value--may make a difference so great that we must regard the product not merely as belonging to a distinct species, but even to a distinct genus or higher kind. In 1852 Herbert Spencer published in the 'Leader' his remarkable essay, contrasting the theories of creation and evolution, as applied to organic beings, with all the biting force of his profound intelligence; and in 1855, the same encyclopædic philosopher put forth the first rough sketch of his 'Principles of Psychology,' in which he took the lead in treating the phenomena of mind from the point of view of gradual development. In that extraordinary work, the philosopher of evolution traced the origin of all mental powers and faculties by slow gradations from the very simplest subjective elements. The 'Principles of Psychology' preceded the 'Origin of Species' by nearly five years; the first collected volume of Mr. Spencer's essays preceded Darwin's work by some twelve months. Baden-Powell's essay on the 'Philosophy of Creation' (much debated and condemned in ecclesiastical circles), and Professor Owen's somewhat contradictory utterances on the nature of types and archetypal ideas, also helped to keep alive interest in the problem of origins up to the very moment of the final appearance of Darwin's great and splendid solution.