The increase of fertility caused by nutrition that is greatly in excess of expenditure is to be detected by comparing populations of the same race or of allied races one of which obtains good and abundant sustenance much more easily than the other. On carrying out such comparisons it is seen that in the human race, as in all other races, such absolute or relative abundance of nutriment as leaves a large excess after defraying the cost of carrying on parental life, is accompanied by a high rate of genesis.
It is also apparent that relative increase of expenditure, leaving a diminished surplus, reduces fertility. That infertility is generally produced in women by mental labour carried to excess is shown in the fact that most of the flat-chested girls who survive their high-pressure education are incompetent to bear a well-developed infant and to supply it with the natural food for the natural period. It is a matter of common remark how frequently men of unusual mental activity leave no offspring.
It is likely to be urged that since the civilised races are on the average larger than many of the uncivilised races, and since they are also somewhat more complex as well as more active, they ought, in accordance with the alleged general law, and other things being equal, to be less prolific. But other things are not equal; and it is to the inequality of the other things that this apparent anomaly is attributable.
One more objection has to be met. Cases may be named of men conspicuous for activity, bodily and mental, who were also noted, not for less generative power than usual, but for more. The cases are analogous to some before-named in which more abundant food simultaneously aggrandises the individual and adds to the production of new individuals—the differences between cases being that instead of a better external supply of material there is a better internal utilisation of materials. Some peculiarity of organic balance, some potency of the digestive juices, gives to the system a perpetual high tide of rich blood that serves at once to enhance the vital activities and to raise the power of propagation. The proportion between individuation and genesis remains the same: both are increased by the increase of the common stock of materials.
Human Population in the Future
Any further evolution in the most highly-evolved of terrestrial beings—man—must be of the same nature as evolution in general. It must be an advance towards completion of that continuous adjustment of internal to external relations which was shown to constitute life.
Looking at the several possibilities, and asking what direction this further evolution, this more complete moving equilibrium, this better adjustment of inner to outer relations, this more perfect co-ordination of action is likely to take:—the conclusion is that it must take mainly the direction of a higher intellectual and emotional development. There is abundant scope for development in ascertaining the conditions of existence to which we must conform; and in acquiring a greater power of self-regulation.
What are those changes in the environment to which, by direct or indirect equilibration the human organism has been adjusting itself, is adjusting itself now, and will continue to adjust itself? And how do they necessitate a higher evolution of the organism? In all cases pressure of population is the original cause. Were it not for the competition this entails, so much thought and energy would not be spent on the business of life; and growth of mental power would not take place. Difficulty in getting a living is alike the incentive to a higher education of children, and to a more intense and long-continued application in adults. Nothing but necessity could make men submit to this discipline; and nothing but this discipline could produce a continued progression.
Excess of fertility is then the cause of man's further evolution. And the obvious corollary is that man's further evolution itself necessitates a decline in his fertility. The further progress of civilisation will be accompanied by an enhanced cost of individuation: whether it be in greater growth of the organs which subserve self-maintenance, in their added complexity of structure, or in their higher activity, the abstraction of the required material, implies a diminished reserve of materials for race maintenance. This greater emotional and intellectual development does not necessarily mean a mentally laborious life—for, as the goal becomes organic, it will become spontaneous and pleasurable.
The necessary antagonism of individuation and genesis not only fulfils the a priori law of maintenance of the race from the monad up to man, but insures final attainment of the highest form of this maintenance—a form in which the amount of life shall be the greatest possible and the births and deaths as few as possible. From the beginning pressure of population has been the proximate cause of progress. After having duly stocked the globe with inhabitants; raised all its habitable parts into the highest state of culture; brought all processes for the satisfaction of human wants to perfection; developed the intellect into complete competency for its work, and the feelings into complete fitness for social life; the pressure of population as it gradually finished its work, must gradually bring itself to an end.