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