Fig.103, A, B. C, D. Instances of redundant mammary glands and nipples (hypermastism). Fig. 103 _A, B, C, D._—Instances of redundant mammary glands and nipples (_hypermastism_). _A_ a pair of small redundant breasts (with two nipples on the left) above the large normal ones; from a 45-year-old Berlin woman, who had had children 17 times (twins twice). (From _Hansemann._) _B_ the highest number: ten nipples (all giving milk), three pairs above, one pair below, the large normal breasts; from a 22-year-old servant at Warschau. (From _Neugebaur._) _C_ three pairs of nipples: two pairs on the normal glands and one pair above; from a 19-year-old Japanese girl. _D_ four pairs of nipples: one pair above the normal and two pairs of small accessory nipples underneath; from a 22-year-old Bavarian soldier. (From _Wiedersheim._)
The characteristic composition of the vertebrate body develops from the embryonic structure in the same way in man as in all the other vertebrates. As all competent experts now admit the monophyletic origin of the vertebrates on the strength of this significant agreement, and this “common descent of all the vertebrates from one original stem-form” is admitted as an historical fact, we have found the answer to “the question of questions.” We may, moreover, point out that this answer is just as certain and precise in the case of the origin of man from the mammals. This advanced vertebrate class is also monophyletic, or has evolved from one common stem-group of lower vertebrates (reptiles, and, earlier still, amphibia). This follows from the fact that the mammals are clearly distinguished from the other classes of the stem, not merely in one striking particular, but in a whole group of distinctive characters.
It is only in the mammals that we find the skin covered with hair, the breast-cavity separated from the abdominal cavity by a complete diaphragm, and the larynx provided with an epiglottis. The mammals alone have three small auscultory bones in the tympanic cavity—a feature that is connected with the characteristic modification of their maxillary joint. Their red blood-cells have no nucleus, whereas this is retained in all other vertebrates. Finally, it is only in the mammals that we find the remarkable function of the breast structure which has given its name to the whole class—the feeding of the young by the mother’s milk. The mammary glands which serve this purpose are interesting in so many ways that we may devote a few lines to them here.