None of the writers of the class here quoted seem to have made themselves acquainted with the researches of Herbert Spencer, Sir F. Galton, and others, as to the natural laws which determine the rate of increase of population when those laws are allowed to operate freely under rational and moral social conditions. A short statement of these laws will therefore be given. In a remarkable essay, first published in 1852, H. Spencer, with his usual philosophical insight, examined the facts of reproduction and population throughout the whole of the animal kingdom, and showed that the duration of the individual life and the increase of the race varied inversely, those groups which have the simplest organisation and the shortest lives producing the greatest number of offspring; in other terms, individuation and reproduction are antagonistic. But individuation depends almost entirely on the development and specialisation of the nervous system, through which alone all advance in instinct, emotion, and intellect is rendered possible. The actual rate of increase in man has been determined by the necessities of the savage state, in which, as in most species of mammals, it is usually what is just required to maintain a limited average population. But with a true advance in civilisation the average duration of life increases, and the possible increase of population under favourable conditions becomes very great, because fertility is greater than is needed under the new conditions. At present, however, no general advance in intellectuality has taken place; but that the facts do accord with the theory is indicated by the common observation that highly intellectual parents do not have large families, while the most rapid increase occurs in those classes which are engaged in healthy manual labour. But a law founded on such a broad physiological basis of observation is sure to continue in action, and we may therefore feel certain that as the intellectual level of the whole race is raised by general culture and physical health, the law of diminishing fertility will act, and will tend in the remote future to bring about an exact balance between the rate of increase and that of mortality.