What, it may be asked, is the relation of these facts to the general principle that inheritance is through the gametes? Why, when a gamete with the median element unites with a gamete without that element, does the zygote develop a soma that in some cases shows a nine-tenths median and sometimes a one-tenth median element? We have seen that the Y comb is a heterozygous form due to imperfection of dominance of the median element; but why this variation in the perfection of the median element? This is probably a piece of the question, why any dominance at all. We find, in general, that the determiner of a well-developed organ dominates in the zygote over the determiner of a slightly developed condition of that organ or its obsolete condition. It is as though there were in the zygote an interaction between the strong and the weak form of the determiner, and the strong won; but sometimes the victory is imperfect. In the specific case of comb the interaction between median and no-median leads to a modification, weakening, or imperfection of the median element, and this weakening varies in degree. Sometimes the weakening is inappreciable--when the comb is essentially single; sometimes it is great, and the result is a comb in which the median element is reduced to one-half; sometimes, finally, the determiner of median comb is so completely weakened by its dilution with "no-median" as not to be able to develop, and we have the cup comb with only a trace of the median element. Nevertheless, such a cup comb is heterozygous and produces both single-combed and Polish-combed germ-cells. Thus the variation in the extent of the median comb seems to point to variations in relative potency of the median comb over its absence. CHAPTER II. POLYDACTYLISM. The possession of extra toes is a character that crops out again and again among the higher, typically 5-toed vertebrates. Many cases have been cited in works on human and mammalian teratology (_cf._ Bateson, 1904, and Schwalbe, 1906), and it is recognized that this abnormality is very strongly inherited in man. Bateson and Saunders, and Punnett (1902 and 1905), Hurst (1905), and Barfurth (1908), as well as myself in my earlier report, have demonstrated the inheritableness of the character in poultry. Bateson and Punnett (1905, p. 114) say: "The normal foot, though commonly recessive, may sometimes _dominate_ over the extra-toe character, and this heterozygote may give equality when bred with recessives, just as if it were an ordinary DR." Altogether, the inheritance of extra-toe diverges so far from typical Mendelian results as to deserve further study. A. TYPES OF POLYDACTYLISM.