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 classifications of Primates is best determined by the important thesis of Huxley, in which, in 1863, he carried out a most careful and critical comparison of all the anatomical gradations within this order. In my opinion, this ingenious thesis—which I have called the Huxleyan Law, or the 'Pithecometra-thesis of Huxley'—is of the utmost value. It runs as follows: 'Thus, whatever system of organs be studied, the comparison of their modifications in the ape series leads 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.' If we accept the Huxleyan law without prejudice, and apply it to the natural classification of the Primates, we
