[54] Reichert, E. T., and Brown, A. P., “The Differentiation and Specificity of Corresponding Proteins and other Vital Substances in Relation to Biological Classification and Organic Evolution.” Carnegie Institution Publication No. 116, Washington, 1909. As far as the genus is concerned it was found that the hemoglobin crystals of any genus are isomorphous. In some cases this isomorphism may be extended to include several genera, but this is not usually the case, unless as in the case of dogs and foxes, for example, the genera are very closely related. The most important question for us is the following: Are the differences between the corresponding hemoglobin crystals of different species of the same genus such as to warrant the statement that they indicate chemical differences? If this were the case we might say that blood reactions as well as hemoglobin crystals indicate that differences in the constitution of proteins determine the species specificity and, perhaps, also species heredity. The following sentences by Reichert and Brown seem to indicate that this may be true for the crystals of hemoglobin. The hemoglobins of any species are definite substances for that species. But upon comparing the corresponding substances (hemoglobins) in different species of a genus it is generally found that they differ the one from the other to a greater or less degree; the differences being such that when complete crystallographic data are available the different species can be distinguished by these differences in their hemoglobins. As the hemoglobins crystallize in isomorphous series the differences between the angles of the crystals of the species of a genus are not, as a rule, great; but they are as great as is usually found to be the case with minerals or chemical salts that belong to an isomorphous group (p. 326). As Professor Brown writes me, the difficulty in answering the question definitely, whether or not the hemoglobins of different species are chemically different, lies in the fact that there is as yet no criterion which allows us to discriminate between a species and a Mendelian mutation except the morphological differences. It is not impossible that while species differ by the constitution of some or most of their proteins, Mendelian heredity has a different chemical basis. It is regrettable that work like that of Reichert and Brown cannot be extended to other proteins, but it seems from anaphylaxis reactions that we might expect results similar to those in the case of the hemoglobins. The proteins of the lens are an exception inasmuch as, according to Uhlenhuth, the proteins of the lens of mammals, birds, and amphibians cannot be discriminated from each other by the precipitin reaction.[55] [55] Uhlenhuth, _Das biologische Verfahren zur Erkennung und Unterscheidung von Menschen und Tierblut_, Jena, 1905, p. 102.