There is reason to suppose that many human qualities are more complicated in their transmission, and it is probable that certain phenomena now being studied in plants and animals will throw definite light upon man. Though characters are frequently transmitted on the Mendelian scheme quite independently of one another, there are cases known in which they are linked up more or less completely in the germ cells with the determinant of a particular sex. Sex-limited inheritance of this nature has been carefully worked out in particular cases in Lepidoptera and poultry. As yet there is much to be learnt in this direction, and further progress may be expected to lead eventually to a precise knowledge of the mode of transmission of many human defects, such as colour-blindness and hæmophilia. It is not unlikely that a similar mode of transmission will be found to hold good for many human characters usually classed as normal.
Another set of phenomena which will probably be found of importance in the heredity of man are those included under the terms "coupling" and "repulsion." Characters, each exhibiting simple Mendelian segregation, may become linked together more or less completely in the process of heredity, or the reverse may occur. Our knowledge of these phenomena is at present almost completely confined to cases in plants, but evidence is beginning to be obtained for their occurrence in animals. It is not unlikely that they will be found to play a considerable part in human heredity. For one of the most noticeable things about man is the frequency with which children resemble one or other parent to the seemingly almost complete exclusion of the other. In view of the mongrelisation of the human race, the frequency of these cases is very remarkable, and can hardly fail to suggest that some sort of coupling between characters plays a large part in human heredity.
Except in very few cases, our knowledge of heredity in man is at present far too slight and too uncertain to base legislation upon. On the other hand, experience derived from plants and animals has shewn that problems of considerable complexity can be unravelled by the experimental method, and the characters concerned brought under control. Though the direct method is hardly feasible in man, much may yet be learnt by collecting accurate pedigrees and comparing them with standard cases worked out in other animals. But it must be clearly recognised that the collection of such pedigrees is an arduous undertaking demanding high critical ability, and only to be carried out satisfactorily by those who have been trained in and are alive to the trend of genetic research.