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 interest 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.
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 evident.
Let the genealogist, then, bring together data on every trait he can think of. As a guide and stimulus, he should read the opening chapters of Herbert's Spencer's Autobiography, or of Karl Pearson's, Life, Letters and Labors of Sir Francis Galton, or C. B. Davenport's study[172] of C. O. Whitman, one of the foremost American biologists. He will also find help in Bulletin No. 13 of the Eugenics Record Office, Cold Spring Harbor, Long Island, New York. It is entitled, How to Make a Eugenical Family Study, and gives a list of questions which should be answered, and points which should be noted. With some such list as this, or even with his own common-sense, the genealogist may seek to ascertain as much as possible about the significant facts in the life of his ancestors, bearing in mind that the geneticist will ask two questions about every trait mentioned:
1. Is this characteristic inherited?
2. If so, how?