4. Fifty or sixty years ago surgeons did not hesitate to transfuse the blood of animals into human beings. The practice was a failure, and Landois[39] showed by experiment that if blood of a foreign species was introduced into an animal the blood corpuscles of the transfused blood were rapidly dissolved and the animal into which the transfusion was made was rendered ill and often died. The result was different when the animals whose blood was used for the purpose of transfusion belonged to the same species or a species closely related to the animal into which the blood was transfused. Thus when blood was exchanged between horse and donkey or between wolf and dog or between hare and rabbit no hemoglobin appeared in the urine and the animal into which the blood was transfused remained well.[40] This was the beginning of the investigations in the field of serum specificity which were destined to play such a prominent rôle in the development of medicine. Friedenthal was able to show later that if to 10 c.c. of serum of a mammal three drops of defibrinated blood of a foreign species are added and the whole is exposed in a test tube to a temperature of 38°C. for fifteen minutes the blood cells contained in the added blood are all cytolyzed; that this, however, does not occur so rapidly when the blood of a related species is used. He could thus show that human blood serum dissolves the erythrocytes of the eel, the frog, pigeon, hen, horse, cat, and even that of the lower monkeys but not that of the anthropoid apes. The blood of the chimpanzee and of the human are no longer incompatible, and this discovery was justly considered by Friedenthal as a confirmation of the idea of the evolutionists that the anthropoid apes and the human are blood relations.[41] [39] Landois, L., _Zur Lehre von der Bluttransfusion_. Leipzig, 1875. [40] This is probably true only within the limits of exactness used in these experiments. [41] Friedenthal, H., “Experimenteller Nachweis der Blutverwandtschaft.” _Arch. f. Physiol._, 1900, 494. This line of investigation had in the meanwhile entered upon a new stage when Kraus, Tchistowitch, and Bordet discovered and developed the precipitin reaction, which consists in the fact that if a foreign serum (or a foreign protein) is introduced into an animal the blood serum of the latter acquired after some time the power of causing a precipitate when mixed with the antigen, _i. e._, with the foreign substance originally introduced into the animal for the purpose of causing the production of antibodies in the latter; while, of course, no such precipitation occurs if the serum of a non-treated rabbit is mixed with the serum of the blood of the foreign species.