One of the present objects of the student of heredity, perhaps his chief object, is to be able to state the facts of human heredity in Mendelian terms, reducing many of the complex human traits to their simpler elements. Some of the chief objections to the use of the statistical formula of heredity are that apparently it is applicable only to the fluctuating variabilities of organisms; that it rarely takes into account the presence of (and therefore the heredity of) true variations or mutations—and we have seen that it is just these characters that are of the greatest value in evolution; and that heredity is after all fundamentally an individual relation which loses much of its definiteness and significance when we merge the individual in with a crowd. To some these seem fatal objections to any use of the statistical formula and it is certainly true that they greatly limit its value. But for the present at least the statistical statement of certain facts of heredity is still useful in this bio-social field. We may therefore use the statistical formulas of heredity as a kind of temporary expedient, enabling us to make statements regarding inheritance of certain characters in the group or class, pending the time when we shall be able to give the facts a more precise and more "final" expression in Mendelian formulas. Many human traits are indeed already known to Mendelize. Most of these are, however, "abnormal" traits or pathological conditions; we are still in the dark regarding the actually Mendelian or non-Mendelian inheritance of most of man's normal characteristics. We might enumerate the following Mendelizing human characters—eye color, color blindness, hair color and curliness, albinism (absence of pigment), brachydactylism (two joints instead of three in fingers and toes), syndactylism (union of certain fingers and toes), polydactylism (one or more additional fingers or toes in each hand or foot), keratosis (unusually thick and horny skin), hæmophilia (lack of clotting property in the blood), nightblindness (ability to see only in strong light—a retinal defect usually), certain forms of deaf mutism and cataract, imbecility, Huntington's chorea (a form of dementia).
In observing Mendelian heredity we should bear in mind that a given character may be due either to the presence or to the absence of a "determiner" in the germ. Long hair such as is characteristic of many "Angora" varieties of the guinea pig and cat, for example, is believed to be due to the absence of a determiner which stops its growth. Blue eyes are due to the absence of a brown pigment determiner, et cetera. The presence or absence in the offspring of such characters as we know do Mendelize can be predicted when we know the parental history for two generations.
Turning now to the inheritance of mental traits and including, of course, moral traits here as well, we find that we are almost entirely limited to the statistical statement of results. Pearson found upon examining data from a large number of school children, brothers and sisters, that the coefficients of heredity between them were the same as for their physical traits. His results are summarized in Figure 12. The physical traits measured were, in the order plotted in the figure—health, eye color, hair color, hair curliness, cephalic index (ratio between breadth and length of cranium), head length, head breadth, head height. These gave an average of .54 in brothers, .53 in sisters, and .51 in brothers and sisters. The psychical traits in order were—vivacity, assertiveness, introspection, popularity, conscientiousness, temper, ability, handwriting. The corresponding averages were .52, .51, .52.
Fig. 12.—Coefficients of heredity of physical and psychical characters in school children. Characters enumerated in text. (From Pearson.)
Galton's pioneer works on "Hereditary Genius," "English Men of Science," and "Natural Inheritance" showed with great clearness the fact of mental and moral heredity. Wood's recent extensive study of "Mental and Moral Heredity in Royalty" shows the same thing, although not all the results of these investigations are given in mathematical form. Little can be said regarding Mendelian heredity of mental traits because the psychologist has not yet told us how to analyze even the common and simpler psychic characters into their fundamental units; since we do not know what the mental hereditary units are, obviously we cannot work with them. Much of our knowledge in this field does not permit of very accurate summary, though pointing indisputably to the fact of mental inheritance in spite of the very great influences of training and education, environment and tradition, in moulding the mental and moral characteristics—influences with much greater effect here than in connection with physical characters.
Galton studied the parentage of 207 Fellows of the Royal Society, a Fellowship which is a real mark of distinction. He assumed that one per cent of the individuals represented by the class from which his observations were drawn, that is the higher intellectual classes, might be expected to be "noteworthy": among the general population the average is really about one in 4,000 or one fortieth of one per cent. On the one per cent basis Galton found that Fellows of the Royal Society had noteworthy fathers with 24 times the frequency to be expected in the absence of heredity; noteworthy brothers with 31 times the expected frequency; noteworthy grandfathers 12 times; and so on through various grades of relationship.
Schuster examined the class lists of Oxford covering a period of 92 years and found that first honor men had 36 per cent first or second honor fathers; second honor men had 32 per cent first or second honor fathers; ordinary degree men 14 per cent first or second honor fathers. These percentages are far in excess of that to be expected—perhaps 0.5 per cent—on the assumption that ability is not inherited. Schuster also determined the coefficients of heredity between fathers and sons as regards intellectual ability, the evidence being class marks in Oxford and Harrow; these he found to be about .3 for the parental relation and .4 for the fraternal. The intensity of heredity in many forms of insanity has been determined and this runs up much higher—.57 parental and .50 fraternal.
It is clear I take it, that the fact of human heredity does not concern only physical traits but extends to psychical traits as well, and with about the same intensity. This fact has been found true also for still less analyzable characters such as length of life, fertility or infertility and the like, and again about the same intensity of resemblance is found.
Human heredity is a fact then just as human variability is a fact. We have truly the raw materials and the means for racial improvement. The ability to direct the evolution of the human race makes this our supremest duty.