re easily be brought home to us by
the examination of some actual pedigrees and family histories. We may
look at a few representative cases which will serve to bring out some
additional aspects of the significance to society of the demonstrated
fact of heredity. In the examination of single family histories we
should remember that a single pedigree may not accurately illustrate a
general law of heredity--again, an individual case may belong to a
group of cases without representing them fairly. Even in observing
illustrations of Mendel's laws allowance has to be made for the
variability due to "chance" meetings of germ cells. It is only when
large numbers of individuals are observed that the typical Mendelian
fractions and ratios can be strictly observed. It must be borne in
mind then that the histories given below illustrate the nature of the
facts of heredity rather than the laws of heredity. Some special
cautions in the interpretation of certain pedigrees will be suggested
in particular cases. Many of the figures are taken from the extremely
valuable "Treasury of Human Inheritance," now being published by the
Eugenics Laboratory of the University of London. In these figures and
some others a uniform series of symbols is used. Successive horizontal
lines designated by Roman numerals indicate generations; within a
single generation the individuals are numbered consecutively simply
for purposes of reference. The meaning of the more common symbols is
as shown in Table IV. We may first consider a few pedigrees showing
the heredity of physical abnormalities or defects.

TABLE IV.

_Symbols used in Pedigrees. As adopted by the Galton Eugenics
Laboratory._

[Symbol] Male and female respectively, not possessing the trait
under consideration.
[Symbol] Male and female possessing the trait under consideration.
[Symbol] Unknown sex--normal or affected.
[Symbol] Trait incompletely developed.
[Symbol] Neither presence nor absence of trait can be affirmed.
[Symbol] With a deformity or disease of special character which
may possibly be associated with that under consideration.
[Symbol] Twins.
[Symbol] Indicates number of children.
[Symbol] Marriage.
[Symbol] Number of children unknown.
[Symbol] Number and character of children unknown.
_S. P._ _Sine prole._ (No offspring.)

Fig. 13 illustrates a family history where brac