should they be called prosimians, or should they figure among the primates?
The primates might be well defined by saying that they are placentary mammalia, non-aquatic—which excludes the cetaceans, sirens, and pinnipeds; without hoofs—which excludes the ungulates and proboscidians; having three kinds of teeth—which removes the rodents and edentata; and having molars neither with cutting blades nor with sharp, conical points—which excludes the carnivora and insectivora. They have no absolutely peculiar characters in common, naturalists not regarding the type of the cerebral circumvolutions. They have a discoidal placenta, and a uterus with a cavity not two-horned; the cheiroptera or bats have likewise the third characteristic. They have two pectoral mammæ; but so have the bats and the lamantins.
The teeth vary among them as to number, form, and permanence. They appear more specialized, brought nearer to one another, and more fixed in their general form, as we ascend toward man. There are four stages in the last category—the lemurs, the monkeys of the old continent, the monkeys of the new continent, and man.
The replacement of claws by nails forms one of the most important characteristics of the primates. Claws are designed and formed for attack and defense; the hoofs of the ungulates form hard soles for the feet, protecting them from contact with the ground and facilitating the march of the animal; while the nails are so shaped as to be adapted to the purpose of prehension. This adaptation is more or less perfect, and extends to more or fewer fingers among the primates, permitting another division into the perfect primates, like man and all the monkeys but one group, and imperfect primates. Another adaptive characteristic, the corollary of the nails, is the well-developed thumb, removed from the other fingers, and opposable to them. More completely than they, it also indicates an organ made to clasp, to seize. The primates may also be divided by this feature into three groups Man, with whom the thumb is opposable only on the fore-limbs the monkeys, with which it is opposable on all four of the limbs and the imperfect primates, with which the adaptation is less exact or less marked on the hinder than on the fore limbs. Other characteristics, usually graduated in the ascending series of the primates, might be mentioned; but these are enough for our purpose.
To regard the primates in this way is a little to prejudge the solution we are seeking. From the instant we suppose a progressive development of characteristics in the series and divide the primates into superior, medium, and inferior, we are tempted to be indulgent in respect to characteristics which may be little accentuated or wanting in the last. From this, to assuming that
