Some serious factor, inoperative during the thirty years prior to 1877 must have suddenly been introduced into the social system, to work such a marvellous revolution during the last twenty years. Some economic writers find it easy here to discover a law, and declare that the birth-rate is in inverse ratio to the abundance of food. (Doubleday quoted by Nitti, Population and the Social System, p. 55). Other economic writers of recent date attribute this great change in ratio of increase to economic causes. Only a few find the explanation in biological laws. Herbert Spencer is the champion of the biological explanation of a decreasing birth-rate. With the intellectual progress of the race there is a decadence of sexual instinct. In proportion as an individual concentrates his energies and attention on his own mental development, does the instinct to, and power of, generation decrease. It may be true, it certainly is true, that if an individual's energies are concentrated in the direction of development of one system of the body, the other systems to some extent suffer. A great and constant devotion to the development of the muscular system will produce very powerful muscles, and great muscular energy, with a strong tendency to, and pleasure in exercise. It is true also, that time and energy are monopolized in this creation of muscle, and that less time and energy are available for mental pursuits and mental exercise. Up to a certain point muscular exercise aids mental development, but beyond that point concentration of effort in the direction of muscular development starves mental growth. On the other hand, if the education and exercise of the mind receive all attention, the muscular system will suffer, and to some extent remain undeveloped. Or generally, one system of the body can be highly developed only at the expense of some other system, not immediately concerned. It is true that the more an individual concentrates his efforts on his own intellectual development, the more his sexual system suffers, and the less vigorous his sexual instincts. And the converse of this is also true, for examples of those with great sexual powers are numerous. In plant life, this same law is also in operation. If one system in a plant, the woody fibre for instance, takes on abundant growth, the fruit is starved and is less in quality and quantity, and _vice versa_. But to what extent does this affect fertility? Sexual power and fertility are not synonymous terms. The vast profusion of seed in plant and animal life, would allow of an enormous reduction in the amount produced, without the least affecting fertility. Even admitting the application of Spencer's law to sexual vitality, and allowing him to claim that, with the progress of "individuation," there is a decline in sexual instinct, would the fertility of the race be affected thereby?