of these is the transformation of the articulation of the mandible or lower jaw, which in mammals is quite strikingly different from that of all other vertebrates. This joint, by which the lower jaw moves upon the temporal bone, is in mammals a temporal joint, while the original joint of its reptilian and amphibian ancestors was a quadrate joint. The latter is, in the mammalia, taken up into the tympanic cavity and there represented by the articulation of two of the special bones of the ear, the malleus and the incus; the malleus was formed from the original joint piece of the lower jaw, while the incus is the quadrate bone or jaw pedicel of the reptilian ancestors.
But apart from these and other anatomical peculiarities which all mammals have in common, and which elevate them above all other vertebrates, in order to recognize their difference it will only be necessary to look at a single drop of blood under a microscope. "Blood is a very peculiar juice." The small red-blood corpuscles which, heaped up by millions, occasion the red color of the blood of vertebrates were all originally elliptical disks, thicker in the middle (biconvex), as it was here that the nucleus lay. Only in the mammals have these lost their nucleus, then appearing thinner in the middle (biconcave), as small circular disks. These and other important peculiarities occur, without exception, among all mammals, and separate them from all other vertebrates. From their peculiar combination and mutual relations they can only have been acquired once in the course of descent, and only from one stem-form can they have been transmitted by inheritance to all members of the class.
The older portion of the genealogical history of the human species leads us still farther back into the domain of the lower vertebrates, into that dark, immeasurably long age of the Paleozoic era, which with its uncounted millions of years (according to recent estimations, at least a thousand) was certainly much longer than the succeeding Mesozoic age. Here we first come upon the important fact that in the earliest portion of the Paleozoic period, in the Permian age, no mammals yet existed, but instead lung-breathing reptiles, as the oldest amnion animals. They belong partly to the Tocosauria, the oldest and lowest group of reptiles, partly to the strange Theromera, which by many characters approach the mammals. These reptiles are preceded in the lower Carboniferous period by true amphibia, such as the armored stegocephali. Such Carboniferous armored amphibia, like small crocodiles, are the oldest vertebrates, who by their creeping method of loco- motion adapted themselves to the firm ground, and in whom the fins of swimming fishes and the paddles of swimming amphibians (Dipneusta) had been modified into the typical five fingered extremity of a four-footed animal (Tetrapoda or Quadrupeda).
We only need to compare carefully the skeleton of the four legs of our salamanders and frogs with the bony framework of our own four limbs to convince ourselves that with these amphibians the same char-
