minence is
greater than would be expected on the basis of pure chance. More data
are needed to clear up this problem.[168]

2. The advantage to a child of being a member of a large or small family
is a question of importance. In these days of birth control, the
argument is frequently heard that large families are an evil of
themselves, the children in them being handicapped by the excessive
child-bearing of the mother. The statistics cited in support of this
claim are drawn from the slums, where the families are marked by poverty
and by physical and mental inferiority. It can easily be shown, by a
study of more favored families, that the best children come from the
large fraternities. In fact Alexander Graham Bell found evidence,[169]
in his investigation of the Hyde Family in America, that the families of
10 or more children were those which showed the greatest longevity (see
Figs. 42 and 43). In this connection, longevity is of course a mark of
vitality and physical fitness.

3. The question of the effect of child-bearing on the mother is equally
important, since exponents of birth control are urging that mothers
should not bear more children than they desire. A. O. Powys' careful
study[170] of the admirable vital statistics of New South Wales showed
that the mothers who lived longest were those who bore from five to
seven children.

4. The age at which men and women should marry has not yet been
sufficiently determined, on biological grounds. Statistics so far
compiled do not indicate that the age of the father has any direct
influence on the character of the children, but the age of the mother
undoubtedly exercises a strong influence on them. Thus it is now well
established[171] that infant mortality is lowest among the children of
young mothers,--say from 20 to 25 years of age,--and that delay in
child-bearing after that age penalizes the children (see Fig. 44). There
is also some evidence that, altogether apart from the infant mortality,
the children of young mothers attain a greater longevity than do those
of older women. More facts are needed, to show how much of this effect
is due to the age of the mother, how much to her experience, and how
much to the influence of the number of children she has previously
borne.

5. Assortative mating, consanguineous marriage, the inheritance of a
tendency to disease, longevity, sex-linked heredity, sex-determination,
the production of twins, and many other problems of i