rise to a height of organization that makes clear as day the immediate transition to the human form. For that reason one of the most profound students of the anatomy of primates, Robert Hartmann, went so far as to separate the entire order of primates into three families: (1) Primarii (man and the anthropoid apes); (2) Simiæ, or apes proper (catarrhines and platyrrhines); (3) Prosimiæ (lemurs). This arrangement seems justified by the interesting statement made by Selenka (1890) that the quite peculiar formation of the placenta of man is found in the anthropoids, but not in the other apes.
Decisive for the question as to which of these various classifications we should prefer was the proposition advanced by Huxley, in 1863, after a careful and critical examination of all the anatomical relations within the order of primates, and which I have called in his honor "Huxley's law," or Huxley's pithecometric proposition: "Whatever system of organs be studied, the comparison of their modifications in the ape series leads us to one and the same result—that the structural differences which separate man from the gorilla and the chimpanzee are not so great as those which separate the gorilla from the lower apes." Thereupon it becomes necessary for every unprejudiced taxonomist to give man a systematic place within the order of the apes. By the most conscientious testing of each difference, and by the most severe logical inference, we can, however, go a step further and instead of using the wider term apes (Simiæ), use the more restricted one of Old-World apes (Catarrhinæ). The standard pithecometric proposition would then be worded in this more exact way: "The comparative auatomy of all organs within the catarrhine group leads us to one and the same result—the morphological differences between man and the anthropomorphous Old-World apes are not so great as those which separate these anthropoids from the papiomorphous baboons, the lowest of the catarrhines."
We can now immediately utilize this incontestable pithecometric proposition, both for firmly establishing the basis of the systematic classification of the primates and for the genealogy of man. For the natural system is, within the order of the primates, an expression of genealogical relationship, just as it is in every other group of the animal and vegetable kingdoms. Hence result the following important inferences as to the genealogical tree of man: (1) The primates form a natural monophyletic group; all "dominant animals," lemurs, apes, and man himself sprang from a common original stem form, a hypothetical Archiprimas. (2) Of the two orders of the legion of the primates the lemurs are the lowest and oldest; from them, later, the true apes (Simiæ) first developed. (3) Among these latter the Old-World apes form a natural monophyletic group; their common hypothetical stem form (Archipithecus) is, directly or indirectly, derived from a branch of the lemurs, no matter what relation they may be assumed to have to the New-World apes. (4) Man is descended from a series
