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.
A more immediate and effective check to rapid increase of population will, however, be brought about by the social reforms already suggested. When poverty is abolished and neither economic nor social advantages will be gained by early marriage, there can be no doubt it will be generally deferred to a later age. Still more effective will be the extension of the period of education or training for the whole population for several years longer than at present, together with the growth of public opinion against all marriages between persons who have not yet begun the serious work of life. It would also be an essential part of education to inculcate the delay of
marriage till every opportunity has been afforded both of the parties concerned of becoming thoroughly acquainted with each other before undertaking so serious a responsibility as marriage usually involves.
The effect of even a few years' delay of marriage on population is very considerable. Sir F. Galton has shown from the best statistics available that if we compare women married at twenty with those at twenty-nine, the comparative fertility is as 8 to 5. But this does not represent the whole effect on increase of population. When marriage is delayed, the time between successive generations is correspondingly increased; and yet another effect in the same direction is produced by the fact that the greater the average age of marriage the fewer generations are alive at the same time, and it is the combined effect of these three factors that determines the actual increase of the population due to this cause.
Sir F. Galton gives a remarkable table showing this combined result of these causes. He finds that if one hundred mothers and their daughters in each successive generation marry at twenty, there will be an increase of such mothers in each successive generation of 1·15. If,
however, they marry at twenty-nine, each successive generation of mothers diminishes in the proportion of 0·85. If this goes on for 108 years, the hundred mothers who marry at twenty have increased to 175, and in 216 years to 299; while those who marry at twenty-nine will have decreased to 61 and 38 respectively. It is therefore shown that under present social conditions the age of marriage necessary to preserve a stationary population will be somewhere between twenty and twenty-nine. The above figures are, however, founded on special cases, and the actual facts are so complicated by the number of childless marriages, the rate of infantile mortality and other causes, that they must be taken only as establishing a law of rather rapid decrease of fertility with each year's addition to the average age of marriage of the mother.