Dr. Castle's paper treats of an ancestral or atavistic coat-character of the guinea-pig present in most wild rodents but frequently absent in their tame varieties and which may be obtained by crossing certain domesticated varieties of guinea-pig. A full account is given of how the reversion in question may be secured, how its reappearance can be accounted for, and in what way the recovered character may be "fixed" or reestablished as a racial character together with the complete data which form the basis of this account. Dr. Little analyzes the observed phenomena of color inheritance in mice on the basis of Mendelian heredity and shows the relations of the various factors involved in the production of colors, to each other and to the coat and eye colors produced. The paper includes the analysis and description of the color factors and gives the experimental data on which the conclusions are based. -Classified First of Publications of the Carnegie Institution of Washington [1918]