laboriously brought about by the female through selection of those males a little more ornamented than their fellows. To-day we have found out that in many cases the genetic composition of a male with such ornamentation and of a female without it may be almost identical, except that the genes in one chromosome are duplex in one sex and simplex in the other. Owing to this initial difference, the female in birds produces an internal secretion that suppresses in her the ornamentation shown by the male, and in the mammal an internal secretion produced by the testes causes the full development in the male of the secondary sexual characters. If, as seems probable, these secretions are some particular kind of substance, the condition that led to their appearance historically need not have been very complex; and if not, the problem appears simplified. It still remains to give some reasonable explanation as to why such substances should continue to be produced if their products--the secondary sexual characters--possess no “beauty” for the female. Here more work is necessary, but the modern genetic point of view may possibly give an important clue. We are coming to realize more fully that the hereditary genes generally have more than a single effect on the characters of the animal. The secondary sexual characters may, then, be only by-products of genes whose important function lies in some other direction. If, for example, the secretion produced by the cells of the male have an important influence on his output of energy, or strength, or activity, their secondary influence over certain parts of the body would not call for any further explanation on the modern view of natural selection. If the secretions of the ovary of the female bird have some direct relation to her physiological processes that are important in the development of the oviduct, for instance, it would be a matter of no importance from an evolutionary point of view if that same secretion suppresses in her the development of the high color shown by the male. BIBLIOGRAPHY. ALCOCK, A., 1892. On the habits of _Galasimus annulipes_. An. Mag. Nat. Hist., VI. ANCEL, P., ET BOUIN, 1906. Sur l’effet des injections d’extrait de glande interstitielle du testicule sur la croissance. Compt. Rend. Acad. Sc. Paris, CXLII. ARKELL, T. R., 1912. Some data on the inheritance of horns in sheep. N. H. Agr. Exp. Sta. Bull., 160. --------, 1912. The nature of the inheritance in sheep. Science, n. s., XXXV. --------, and C. B. DAVENPORT, 1912. Horns in sheep as a typical sex-limited character. Science, n. s., XXXV. BATESON, W., and E. R. SAUNDERS, 1902. Experiments with poultry. Reports Evol. Committee, I, Part II. --------, E. R. SAUNDERS, and R. C. PUNNETT, 1905. Experimental studies in the physiology of heredity. Reports Evol. Committee, II. --------, 1906. Poultry. Reports Evol. Committee, III.