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. An inexorable law would determine that there could be no mental evolution, for the best of the race would cease to propagate their kind. All who would arrive at this standard of mental growth would become barren. And against this there could be no remedy.