If we suppose that the plus race and the minus race differ from each other by certain “modifiers,” we can not suppose that the plus and the mutant races differ by these same modifiers. They differ in some other _single_ respect; perhaps that in which they differ is the _main_ hooded factor. Are we, then, to suppose that the plus and the minus races _do not_ differ as regards this same main factor? This can not be stated, but we see no reason for considering them identical as regards that factor. It appears that the mutant race arose from the plus race by a single large plus variation, which seems to have its determiner in some single component of the germ-cell. But the fact that this change came as a large quantitative variation does not show that small variations are impossible in that same cell component. It seems to us quite improbable that the plus mutation could have arisen in the minus selection series. We believe that the repeated selection which was practised had something to do with inducing this change in the plus direction. If one can increase at will the “modifiers” which make the pigmentation more extensive, it does not seem strange that after a time a readjustment should occur within the cell which should incorporate modifiers in that part of the cell which is responsible for the unit-character behavior of the hooded pattern. This would amount to a quantitative change in the unit-character for hooded pigmentation. BIBLIOGRAPHY. CASTLE, W. E. 1905. Heredity of coat characters in guinea-pigs and rabbits. Carn. Inst. Wash. Pub. 23. 1906. The origin of a polydactylous race of guinea-pigs. Carn. Inst. Wash. Pub. 49. 1912. The inconstancy of unit-characters. American Naturalist, vol. 46, pp. 352-362. CASTLE, W. E., and ALEXANDER FORBES. 1906. Heredity of hair-length in guinea-pigs and its bearing on the theory of pure gametes. Carn. Inst. Wash. Pub. 49. DE VRIES, H. 1901-1903. Die Mutationstheorie. Veit & Co., Leipzig. EAST, E. M. 1912. The mendelian notation as a description of physiological facts. American Naturalist, vol. 46, pp. 633-655. JENNINGS, H. S. 1909. Heredity and variation in the simplest organisms. American Naturalist, vol. 43, pp. 321-337. 1910. Experimental evidence on the effectiveness of selection. American Naturalist, vol. 44, pp. 136-145. JOHANNSEN, W. 1909. Elemente der exakten Erblichkeitslehre. G. Fischer, Jena. MACCURDY, H., and W. E. CASTLE. 1907. Selection and cross-breeding in relation to the inheritance of coat-pigments and coat-patterns in rats and guinea-pigs. Carn. Inst. Wash. Pub. 70. PEARL, R. 1913. Genetics and breeding. Science, n. s., vol. 37, pp. 539-546. TABLES.