Some facts in the brain development of women point to the same conclusion.  The growth of the brain is relatively more rapid in women than in men before the twentieth year.  Between 15 and 20 it has reached its maximum, and from that time there is a gradual decline in weight until about the fiftieth year, when there is an acceleration of growth, followed by a renewed diminution after the sixtieth year.  The maximum of brain weight is almost reached by men at 20 years, but there is a slow increase until 30 or 35 years.  There is then a diminution until the fiftieth year, followed by an acceleration, and at 60 years again a rapid diminution in weight; but the acceleration is more marked and the final diminution less marked in woman than in man.[93] A table prepared by Topinard shows that woman from 20 to 60 years of age has from 126 to 164 grams less brain weight than man, while her deficit from 60 to 90 years is from 123 to 158 grams.[94]

The only explanation at hand of this relative superiority of brain weight in old women is that with the close of the period of reproduction (the anabolic surplus being no longer consumed in the processes associated with reproduction) the constructive tendency still asserts itself, and a slight access of growth and vitality results to the organism.

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It must be confessed that the testimony of anthropologists on the difference in variability of men and women is to be accepted with great caution.  As a class they have gone on the assumption that woman is an inferior creation, and have almost totally neglected to distinguish between the congenital characters of woman and those acquired as the result of a totally different relation to society on the part of women and men.  They have also failed to appreciate the fact that differences from man are not necessarily points of inferiority, but adaptations to different and specialized modes of functioning.  But, whatever may be the final interpretation of details, I think the evidence is sufficient to establish the following main propositions:  Man consumes energy more rapidly; woman is more conservative of it.  The structural variability of man is mainly toward motion; woman’s variational tendency is not toward motion, but toward reproduction.  Man is fitted for feats of strength and bursts of energy; woman has more stability and endurance.  While woman remains nearer to the infantile type, man approaches more to the senile.  The extreme variational tendency of man expresses itself in a larger percentage of genius, insanity, and idiocy; woman remains more nearly normal.

The fact that society is composed of two sexes, numerically almost equal, but differing in organic and social habits, is too significant to remain without influence on the structural and occupational sides of human life, and in the following chapters we shall note some of the influences of sex, and of the differences in bodily habit of men and women, on social forms and activities.