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
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,