From previous transfusion experiments it had been learned that the affinity of mammals is connected to a certain extent with their chemical blood-relationship. If the living blood of two nearly related animals of the same family, such as the dog and the fox, or the rabbit and the hare, is mixed together, the living blood-cells of each species remain uninfluenced. But if we mix the blood of the dog and the rabbit, or the fox and the hare, a struggle for life immediately takes place between the two kinds of blood-cells. The watery fluid or serum destroys the blood-cells of the rodent, and _vice versâ_. It is the same with specimens of the blood of the various primates. The blood of the lower apes and lemurs, which are close to the common root of the primate stem, has a destructive effect on the blood of the anthropoid apes and man, and _vice versâ_. On the other hand, the human blood has no injurious effect when it is mixed with that of the anthropoid apes. In recent years these interesting experiments have been continued by other physiologists and physicians, such as Professor Uhlenhuth at Greifswald and Nuttall at London, and they have proved directly the blood-relationship of various mammals. Nuttall studied them carefully in 900 different kinds of blood, which he tested by 16,000 reactions. He traced the gradation of affinity to the lowest apes of the New World; and Uhlenhuth continued as far as the lemurs. By these results the affinity of man and the anthropoid apes, long established by anatomy, has now been proved physiologically to be in real "blood-relationship."[9] Not less important are the embryological discoveries of the deceased zoologist, Emil Selenka. He made two long journeys to the East Indies, in order to study on the spot the embryology of the Asiatic anthropoid apes, the orang and gibbon. By means of a number of embryos that he collected he showed that certain remarkable peculiarities in the formation of the placenta, that had up to that time been considered as exclusively human, and regarded as a special distinction of our species, were found in just the same way in the closely related anthropoid apes, though not in the rest of the apes. On the ground of these and other facts, I maintain that the descent of man from extinct Tertiary anthropoid apes is proved just as plainly as the descent of birds from reptiles, or the descent of reptiles from amphibians, which no zoologist hesitates to admit to-day. The relationship is as close as was claimed by my former fellow-student, the Berlin anatomist, Robert Hartmann (with whom I sat at the feet of Johannes Müller fifty years ago), in his admirable work on the anthropoid apes (1883). He proposed to divide the order of primates into two families, the _primarii_ (man and the anthropoid apes), and _simianæ_ (the real apes, the catarrhine or eastern, and the platyrrhine or western apes).