An impartial and thorough comparison of the bodily structure of the primates forces us to distinguish two orders in this most advanced legion of the mammalia--half-apes (_prosimiae_ or _hemipitheci_) and apes (_simiae_ or _pitheci_). The former seem in every respect to be the lower and older, the latter to be the higher and younger order. The womb of the half-ape is still double, or two-horned, as it is in all the other mammals. In the true ape, on the contrary, the right and left wombs have completely amalgamated; they blend into a pear-shaped womb, which the human mother possesses besides the ape. In the skull of the apes, just as in that of man, the orbits of the eyes are completely separated from the temporal cavities by an osseous partition; in the _prosimiae_ this is either entirely wanting or very imperfect. Finally, the cerebrum of the _prosimia_ is either quite smooth or very slightly furrowed, and proportionately small; that of the true ape is much larger, and the gray bed especially, the organ of higher psychic activity, is much more developed; the characteristic convolutions and furrows appear on its surface exactly in proportion as the ape approaches to man. In these and other important respects, particularly in the construction of the face and the hands, _man presents all the anatomical marks of a true ape_. The extensive order of apes was divided by Geoffroi, in 1812, into two sub-orders, which are still universally accepted in systematic zoology--New World and Old World monkeys, according to the hemisphere they respectively inhabit. The American "New World" monkeys are called _Platyrrhinae_ (flat-nosed); their nose is flat, and the nostrils divergent, with a broad partition. The "Old World" monkeys, on the contrary, are called collectively _Catarrhinae_ (narrow-nosed); their nostrils point downward, like man's, and the dividing cartilage is narrow. A further difference between the two groups is that the tympanum is superficial in the _platyrrhinae_, but lies deeper, inside the petrous bone, in the _catarrhinae_; in the latter a long and narrow bony passage has been formed, while in the former it is still short and wide, or even altogether wanting. Finally, we have a much more important and decisive difference between the two groups in the circumstance that all the Old World monkeys have the same teeth as man--_i. e._, twenty deciduous and thirty-two permanent teeth (two incisors, one canine, two premolars, and three molars in each half of the jaw). The New World monkeys, on the other hand, have an additional premolar in each half-jaw, or thirty-six teeth altogether. The fact that these anatomical differences of the two simian groups are universal and conspicuous, and that they harmonize with their geographical distribution in the two hemispheres, fully authorizes a sharp systematic division of the two, as well as the phylogenetic conclusion that for a very long period (for more than a million years)