Gruenbaum, of Liverpool,[61] has been fortunate enough to procure a considerable quantity of the blood of three large anthropoid apes—the gorilla, chimpanzee, and ourang-outang. He has been able to prove that the serum of animals injected with man’s blood gives a precipitate not only with this blood but also with that of the above-mentioned apes. It was impossible for him “to distinguish this precipitate as regards quality and quantity from that which is obtained with human blood.” To verify this result, Gruenbaum prepared the serum of animals injected with the blood of the gorilla, chimpanzee, and ourang-outang. These three kinds of serum gave precipitates with the blood of these three apes, and to the same extent with the blood of man. It is therefore evident that there exists between the human species and the anthropoid apes not only a superficial analogy of body and of the principal organs, but a close blood relationship. Facts of this kind could not be foreseen when the theory of the simian origin of man was put forward. In spite of this they have arisen to confirm it in a truly astonishing way. It is therefore impossible to doubt that man is a member of the group of primates having a close connection with the higher monkeys of the present time. This result is of great importance in all questions relating to human nature. It would certainly be of considerable interest to know more exactly what steps were followed in this simian descent of man. On this question our knowledge is still very imperfect. In his researches on anthropoid apes, Selenka insists on a more intimate relationship between the chimpanzee and man. “The great resemblance of the pre-molars and of the molars in the permanent dentition of the chimpanzee with human teeth appears to indicate that the chimpanzee and man have a common origin, and descend from extinct forms like _Dryopithecus_. This conclusion, however, is contradicted by the fact that the milk teeth of the chimpanzee are much nearer those of the ourang-outang than those of man.”[62] It is evident that to clear up this question it would be necessary to have a greater knowledge of fossil anthropoids such as _Dryopithecus_ and its allies. In the present state of knowledge only a very general hypothesis can be formulated as to the exact mode of human descent.