| OCR Text |
Show 1880.] ARRANGEMENT OF THE MAMMALIA. 65" 6. The vertebrae have epiphyses. 7. The malleus is usually small, the incus relatively large, the stapes stirrup-shaped. 8. The coracoid is almost always much reduced, and it is ankylosed with the scapula. 9. The iliac axis makes a small angle with the sacral axis; and there is no epipubis, or only a fibrous vestige of it. 10. The corpus callosum and the anterior commissure vary widely. In such forms as Erinaeeus and Dasypus they are almost Mono-treme- like. 11. The fcetus is connected with the uterus of the mother by an allantoic placenta. The umbilical sac varies in size; and in some lower forms (e. g. Lepus) it is, at first, highly vascular, and perhaps plays a quasi-placental part during the early stages of development. It is obvious that, in all these respects, we have the mammalian type in a higher stage of evolution than that presented by the Prototheria and the Metatheria. Hence we may term forms which have reached this stage the Eutheria. It is a fact, curiously in accordance with what might be expected on evolutionary principles, that while the existing members of the Prototheria and the Metatheria are all extremely modified, there are certain forms of living Eutheria which depart" but little from the general type. For example, if Gymnura possessed a diffuse placentation, it would be an excellent representative of an undifferentiated Eutherian. Many years ago, in m y lectures at the Royal College of Surgeons, I particularly insisted on the central position of the Insectivora among the higher Mammalia; and further study of this order and of the Rodentia has only strengthened m y conviction, that any one who is acquainted with the range of variation of structure in these groups possesses the key to every peculiarity which is met with in the Primates, the Carnivora, and the Ungulata. Given the common plan of the Insectivora and of the Rodentia, and granting that the modifications of the structure of the limbs, of the brain, and of the alimentary and reproductive viscera which occur among them may exist and accumulate elsewhere, and the derivation of all the Eutheria from animals which, except for their simpler placentation, would be Insectivores, is a simple deduction from the law of evolution. There is no known Monotreme which is not vastly more different from the Prototherian type, and no Marsupial which has not far more widely departed from the Metatherian type, than Gymnura or, indeed, Erinaeeus, have from the Eutherian type. The broadest physiological distinction between the Prototheria, the Metatheria, and the Eutheria, respectively, lies in the differences which the arrangements for prolonging the period of intra-uterine and extra-uterine nutrition by the parent present in each. The possibility of a higher differentiation of the species is apparently closely connected with the length of this period. Similarly, the broadest morphological distinction which can be drawn among the Eutheria lies in their placentation. All forms of deciduate placentation commence by being non-deciduate; and the intimate connexion |
