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]

7. The serum of certain humans may cause the destruc­tion or agglutina­tion of blood corpuscles of certain other humans. This fact of the existence of “isoagglutinins” seems to have been established for man, but Hektoen states that he has not been able to find any isoagglutinins in the serum of rabbits, guinea-pigs, dogs, horses, and cattle. Landsteiner found the remarkable fact that the sera of certain individuals of humans could hemolyze the corpuscles of certain other individuals, but not those of all individuals. A systematic investiga­tion of this variability led him to the discovery of three distinct groups of individuals, the sera of each group acting in a definite way towards the corpuscles of the representatives of each other group. Later observers, for example Jansky and Moss, established four groups. These groups are, according to Moss,[56] as follows:

Group 1. Sera agglutinate no corpuscles.
Corpuscles agglutinated by sera of Groups 2, 3, 4.

Group 2. Sera agglutinate corpuscles of Groups 1, 3.
Corpuscles agglutinated by sera of Groups 3, 4.