and, lastly, all those varied conditions which make for greater or
lesser mental and physical activity.

Fertility, broadly speaking, varies in inverse proportion to the
degree of nervous energy or what we may call vitality.

Conditions, therefore, which lower the general vitality below the
normal produce abnormal fertility. This excessive child-bearing under
present conditions still further lowers the standard of life and the
health of the mother, hence a vicious circle is set up, the only
escape from which will come by such consideration of the laws of
health relating to work, housing, food and recreation as shall ensure
the maximum of vitality to the workers. This is the true method of
conception control.

There comes a point in the development of nervous energy which is
productive of sterility. It is true that principles based on so many
varying factors will necessarily appear to fail in individual cases.
Environment with its influence on the nervous energy of the individual
will be modified by the inherited tendency of that individual towards
fertility or the reverse. We find, therefore, isolated cases of large
families among the well-to-do and small families among those whose
vitality is below the normal, but if the general principle is true we
should expect to find a larger number of _sterile_ marriages among the
well-to-do than among those whose lives are more full of hardship, and
this undoubtedly is the case.

This aspect of the problem is deserving of careful study. The desire
for children in so many homes where every advantage could be given,
may be gratified when more knowledge of how wisely to modify the