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.
One of the main contentions of this work is that the best have to a large extent ceased to propagate their kind, but it is not maintained that this is the result of a biological law, over which there is no control.

It can be safely claimed that to Malthus's three checks to population--vice, misery, and moral restraint, the demographic phenomena of a century have added no other.

The third check, however, moral restraint, must be held to include all restraint voluntarily placed by men and women on the free and natural exercise of their powers of procreation.
Malthus used the term "moral" in this connection, not so much in relation to the _motive_ for the restraint, but in relation to the result, viz., the limitation of the family.