During the forty years which have elapsed since Darwin's first publication of his theories an enormous literature, discussing the _general problems_ of transformism as well as its special application to man, has been published. In spite of the wide divergence of the different views, all agree in one main point: the natural development of man cannot be separated from general transformism. There are only two possibilities. Either all the various species of animals and plants have been created independently by supernatural forces (and in this case the creation of man also is a miracle); or the species have been produced in a natural way by transmutation, by adaptation and progressive heredity (and in this case man also is descended from other vertebrates, and immediately from a series of primates). We are absolutely convinced that only the latter theory is fully scientific. To prove its truth, we have to examine critically the strength of the different arguments claimed for it. I. First, we have to consider the relative place which comparative anatomy concedes to man in the 'natural system' of animals, for the true value of our 'natural classification' is based upon its meaning as a pedigree. All the minor and major groups of the system--the classes, legions, orders, families, genera, and species--are only different branches of the same pedigree. For man himself, his place in the pedigree has been fixed since Lamarck,[4] in 1801, defined the group of vertebrates. The most perfect[5] of these are the Mammalia; and at the head of this class stands the order of Primates, in which Linnæus, in 1735, united four 'genera'--Homo, Simia, Lemur, and Vespertilio. If we exclude the last-named, the Chiroptera of modern zoology, there remain three natural groups of Primates--the Lemures, the Simiæ, and the Anthropi or Hominidæ. This is the classification of the majority of zoologists; but if we compare man with the two chief groups of monkeys--the Eastern monkeys (or Catarrhinæ) and the Western or American monkeys (Platyrrhinæ)--there can be no doubt that the former group is much more closely related to man than is the latter. In the natural order of the Catarrhinæ we find united a long series of lower and higher forms. The lowest, the Cynopitheci, appear still closely related to the Platyrrhinæ and to the Lemures; while, on the other hand, the tailless apes (Anthropomorphæ) approach man through their higher organization. Hence one of our best authorities on the Primates, Robert Hartmann,[6] proposed to subdivide the whole order of the Simiæ into three groups: (1) Primarii, man together with the other Anthropomorphæ, or tailless apes; (2) Simiæ, all the other monkeys; (3) Prosimiæ, or Lemurs. This arrangement has received strong support from the interesting discovery by Selenka that the peculiar placentation of the human embryo is the same as in the great apes, and different from that of all the other monkeys. Our choice between these different