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 to the high intellectual attainments of her people, and that the rapidly decreasing birth-rate is due to a rapid increase of her intellectual power during recent years.
Ireland and New Zealand would be equally pleased could they believe that their low, and still decreasing birth-rate is due to the lessening of the sexual instinct, attendant upon, and resulting from a high and increasing intellectual power and activity.
The fact is, that the sexual instinct is so immeasurably in excess of the maximum power of procreation in the female, that an enormous reduction in sexual power would require to take place before it would have any effect on the number of children born.
The number of children born is controlled by the capacity of the human female to bear children, and one birth in every two years during the child-bearing period of life is about the maximum capacity.
A moderate diminution in the force of the sexual instinct might lead to a decrease in the marriage rate, but it would require a very serious diminution bordering on total extinction of the instinct to exert any serious effect on the fecundity of marriage.
All that can be claimed for this theory of population is, that, reasoning from known physiological analogies, we might expect a weakening of the desire for marriage, coincident with the general development of intellect in the race.
There are as yet no facts to prove that such weakening has taken or is taking place, nor are there facts to prove that population has in any way suffered from this cause.
If such a law obtained, and resulted in a diminished birth-rate, the future of the race would be the gloomiest possible.