During the past two years the existence of “biologic forms” in the Erysiphaceæ has been definitely proved. We know now that there exist in this group of parasitic fungi races of individuals which are morphologically identical, but which differ physiologically—or biologically—in possessing distinctive and sharply defined powers of infection. Thus we find a fungus A growing on host-plant a, and a fungus B growing on host-plant b. Now the fungus A belongs to the same species as fungus B, and is identical in every morphological character, yet the fungus A cannot infect the host-plant b, nor can the fungus B infect the host-plant a. The two fungi A and B thus prove to be “biologic forms” of the same morphological species. This specialization of parasitism was first observed, (1) (2) (5), as regards the Erysiphaceæ, in the conidial (Oidum) stage of these fungi. At the beginning of the present year I published a paper “On the Infection-powers of the Ascospores in the Erysiphaceæ”(4), in which I gave the results of experiments which proved that the ascospores of “biologic forms” are similarly specialized in their powers of infection. Soon afterwards Professor E. Marchal published a summary of experiments in which the same results were obtained (3).