ON OUR PRESENT KNOWLEDGE OF THE ORIGIN OF MAN. 475 



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 origi- 

 nal 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 nec- 

 essary 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 irom all 

 other vertebrates. From their peculiar combination and mutual rela- 

 tions they can only have been acquired once in the course of descent, 

 and only from one stem-form can they have been transmitted by inher- 

 itance 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 

 titegocephali. Such Carboniferous armored amphibia, like small croco- 

 diles, 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 Quadruped a). 



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- 



