sart, Bordet, and Gabrichevsky, that the leucocytes are attracted by the chemical poisons secreted by the micro-organisms; or the protein of the bacterial cells themselves may bring them on the spot, as is maintained by Buchner, who also has conclusive experiments in favor of his theory. Only further research will be able to decide which of these views is correct, and to what extent. But under the present state of knowledge the question can not be answered with certainty—the more so as Behring, Kitasato, Buchner, Emmerich, Vaillard, Tizzani, Cattani, Ch. Richet, and many others have weighty arguments in favor of the opinion that the immunity of animals depends upon some ferment-like albuminous substance contained in the serum of their blood. Strenuous efforts have been made of late by Koch, Buchner, E. H. Hankin,^[1] and many others to come to some more definite knowledge of these "defensive proteins," which are known in science under the names of "alexines," "sozins," "phylaxins," and so on. But it will probably take some time before our notions about these substances take a definite form. One thing seems, however, to become more and more certain—namely, that the serum of the blood of immune or vaccinated animals, although in many cases it does not destroy the microbes themselves, is nevertheless possessed of a vaccinating power. This fact is settled beyond doubt; it is continually confirmed by fresh experiments; and it is recognized by the followers of the biological theory as well. As to its explanation, it may be sought for in the direction indicated by Metchnikoff—namely, that the serum, though not destroying the microbes themselves, destroys the poisonous substances which they are developing in the organism. In such case, organisms would be endowed with two means of defense instead of one; the two theories would naturally complete each other; and, may be, in some not very distant future they would enable man to combat with success some of the worst microscopic enemies of the human race.—Nineteenth Century.