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 evident.