nterest to the
general public as well as to the biologist, are awaiting the collection
of fuller data. All such problems will be illuminated, when more
genealogies are kept on a biological basis.

[Illustration: INFLUENCE OF MOTHER'S AGE

FIG. 44.--As measured by the percentage of infant deaths, those
children show the greatest vitality who were born to mothers between the
ages of 20 and 25. Infant mortality increases steadily as the mother
grows older. In this case the youngest mothers (those under 20 years of
age) do not make quite as good a showing as those who are a little
older, but in other studies the youngest mothers have made excellent
records. In general, such studies all show that the babies are penalized
if marriage is delayed beyond the age of 25, or if child-bearing is
unduly delayed after marriage. Alexander Graham Bell's data.]

Here, however, an emphatic warning against superficial investigation
must be uttered. The medical profession has been particularly hasty,
many times, in reporting cases which were assumed to demonstrate
heredity. The child was so and so; it was found on inquiry that the
father was also so and so: _Post hoc, ergo propter hoc_--it was
heredity. Such a method of investigation is calculated to bring genetics
into disrepute, and would hazard the credit of genealogy. As a fact, one
case counts for practically nothing as proof of hereditary influence;
even half a dozen or a dozen may be of no significance. There are two
ways in which genealogical data can be analyzed to deduce biological
laws: one is based on the application of statistical and graphic methods
to the data, and needs some hundreds of cases to be of value; the other
is by pedigree-study, and needs at least three generations of pedigree,
usually covering numerous collaterals, to offer important results. It is
not to be supposed that anyone with a sufficiently complete record of
his own ancestry would necessarily be able by inspection to deduce from
it any important contribution to science. But if enough complete family
records are made available, the professional geneticist can be called
into cooperation, can supplement the human record with his knowledge of
the results achieved by carefully controlled animal and plant breeding,
and between them, the genealogist and the geneticist can in most cases
arrive at the truth. That such truth is of the highest importance to any
family, and equally to society as a whole, must be e