her 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?

To have any effect at all on the birth-rate, the instinct would have
either to be killed or to be so reduced in intensity as to stop
marriage, or to delay it till very late in life.

When once marriage was contracted sexual union once in every two years,
would, under strictly normal conditions, result in a very large family.

For according to Mr. Spencer's theory, it is the instinct that is
weakened not the power of the spermatozoa to fertilize.

Evidence is wanting, however, to show that there is a decrease in the
sexual power of any nation.

France might be flattered to be told that her low birth-rate is due