The experiments on the heredity of pigments are at present almost the only ones which can be used for an analysis of the chemical nature of the character and its possible determiner. The important work of G. Bertrand[211] and of Chodat[212] on the production of black pigment in the cells of animals and plants with the aid of enzymes has paved the way for such work. Bertrand has shown that tyrosine (_p_-oxyphenylaminopropionic acid) is transformed into a black pigment by an enzyme tyrosinase which occurs in numerous organisms and is obviously the cause of pigment and colouration in a great number of species. This discovery was utilized in the study of the heredity of pigments by Miss Durham, Gortner,[213] and very recently by Onslow.[214] The latter showed that from the skins of certain coloured rabbits and mice a peroxidase can be extracted which behaves like a tyrosinase toward tyrosine in the presence of hydrogen peroxide. This peroxidase was found in the skins of black agouti, chocolate and blue rabbits, but not in yellow or orange rabbits. The recessive whiteness in rabbits and mice according to this author is due to the lack of the peroxydase. There exists a dominant whiteness in the English rabbit which is due to a tyrosinase inhibitor which destroys the activity of the tyrosinase “and the dominant white bellies of yellow and agouti rabbits are due to the same cause.” “Variations in coat colour are probably due to a quantitative rather than to a qualitative difference in the pigment present.” [211] Bertrand, G., _Ann. d. l’Inst. Pasteur_, 1908, xxii., 381; _Bull. Soc. Chim._, 1896, xv., 791. [212] Chodat, R., _Arch. d. Sc. phys. et nat._, 1915, xxxix., 327. [213] Gortner, R. A., _Trans. Chem. Soc._, 1910, xcvii., 110. [214] Onslow, H., _Proc. Roy. Soc._, 1915, B. lxxxix., 36.