It seemed, however, as if there was one fatal exception to the scheme of naturalistic optimism. The Rev. Thomas Malthus had originally published his _Essay on Population_ (1798) as a telling answer to the "infidel" Godwin's _Political Justice_ (1793), the bolder precursor of _Social Statics_. The argument was that the tendency of population to outrun the means of subsistence put human perfectibility out of the question. It had been suggested by the idealists, Mill among the number, that the difficulty might be obviated by habitual self-restraint on the part of married people. But Spencer, with great ingenuity, made the difficulty its own solution. The pressure of population on the means of subsistence is the source of all progress; and of progress not only in discoveries and inventions, but also, through its increased exercise, in the instrument which effects them--that is, the human brain. Now, it is a principle of Aristotle's, revived by modern biology, that individuation is antagonistic to reproduction; and increasing individuation is the very law of developing life, shown above all in the growing power of life's chief instrument, which is thought's organ, the brain. For, as Spencer proceeded {140} to show in his next work, the _Principles of Psychology_, life means a continuous series of adjustments of internal to external relations. Therefore the rate of multiplication must go on falling with the growth of intellectual and moral power until it only just suffices to balance the loss by death. The next step was to revive Laplace's nebular hypothesis, and to connect it through Lyell's uniformitarian geology with Lamarck's developmental biology, thereby extending the same evolutionary process through the whole history of the universe. Nor was this all. Milne-Edwards, by another return to Aristotle, had pointed to the "physiological division of labour" as a mark of ascending organic perfection, to which Spencer adds integration of structure as its obverse side, at the same time extending the world-law, already made familiar in part through its industrial applications by Adam Smith, to all orders of social activity. Finally, differentiation and integration were stretched back from living to lifeless matter, thus bringing astronomy and geology, which had already entered into the causal series of cosmic transformations, under one common law of evolution; while at the same time, seeing it to be generally admitted that inorganic changes originated from the operation of purely mechanical forces, they suggested that mechanism, without teleology, could adequately explain organic evolution also. Finally came the great discovery of Darwin and Wallace, with its extension of Malthus's law to the whole world of living things. Spencer had just touched, without grasping, the same idea years before. He now gladly accepted Natural Selection as supplementing without superseding Lamarck's theory of spontaneous adaptation. {141}