HUMAN SELECTION. ic 3
rate of increase of population caused by a delay in the average period of marriage. For when the age of marriage is delayed the time between successive generations is correspondingly length- ened ; while a still further effect 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 rate of increase of the popula- tion.*
But there is yet another factor tending to check the increase of population that would come into play in a society such as we have been considering. In a remarkable essay on the Theory of Population, Herbert Spencer has shown, by an elaborate discus- sion of the phenomena presented by the whole animal kingdom, that the maintenance of the individual and the propagation of the race vary inversely, those species and groups which have the shortest and most uncertain lives producing the greatest number of offspring ; in other words, individuation and reproduction are antagonistic. But individuation depends almost entirely on the development and specialization of the nervous system, through which, not only are the several activities and co-ordinations of the various organs carried on, but 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 animal species, it has usually been only just sufficient to maintain a limited average population. But with civilization the average duration of life increases, and the possible increase of population under favorable conditions becomes very great, because fertility is greater than is needed under the new conditions. The advance in civilization as regards the preserva- tion of life has in recent times become so rapid, and the increased development of the nervous system has been limited to so small a portion of the whole population, that no general diminution in fertility has yet occurred. That the facts do, however, accord with the theory is indicated by the common observation that highly intellectual parents do not as a rule have large families, while the most rapid increase occurs in those classes which are engaged in the simpler kinds of manual labor. But in a state of society in which all have their higher faculties fully cultivated and fully exercised throughout life, a slight general diminution of fertility would at once arise, and this diminution, added to that caused by the later average period of marriage, would at once bring the rate of increase of population within manageable limits. The same general principle enables us to look forward to
��* See Inquiries into Iluman Faculty and its Development, p. 321 ; and Hereditary Genius, p. 353.