CLASSIFICATION. Linnaeus (170(0 divided the mammals into seven orders: Cete, Bellua?, Pecora, Glires, Fenc. Bruta. Primates. Of these orders the Cete, Glires, and perhaps Bellme and Pecora were natural groups quite similar to some of our modern orders. But under Fern were grouped opossums, moles. hedgehogs, weasels, bears, dogs. cats, and seals; evidently the use of animal food was the principal character common to such an assembly. Under Bruta, moreover, such widely different animals as elephants, anteaters, and sloths were united. Finally, in the Primates, not only were man and monkeys associated, but even the bats were included. Cuvier reformed this arrangement as a result of extensive anatomic-al investigations. He recognized nine orders of mammals, as follows: Bimana, Quadrumana. earn ivora , Ma rsupialia, Rodent ia Pachydermata, Ruminantia. Cetacea. There is little that is really unnatural in this arrange ment, and it remained in popular use down to the very close of the nineteenth century. Cuvier regarded man's intellectual qualities as such as to rank him in an order apart from the monkeys, a backward step from Linnasus's arrangement. His Bimana and Quadrumana are now fused as Linneus's 'Primates.' and Cuvier's Pachydermata and Ruminantia are likewise united: but his other orders stand substantially as made. After the doctrine of organic evolution was generally accepted, the tendency was toward a far more elaborate and complex classification among both animals and plants. As a result, some of the proposed classifications of mammals suggested no less than twenty-five orders; but the pendulum soon reached its limils on that side, and we are now far nearer a natural classification than we have ever been previously. It is generally agreed now that the class Mammalia is very naturally divisible into two subclasses. according to the ar rangement of the reproductive organs and the relations between mother and offspring,. The pri mary divisions ought to be based nn such char acters, for it is in just those diameters that the mammals stand out in sharpest eontrast to the other vertebrates. These two subclasses are: ( I) Prototheria, or Ornithodelphia—mammals without teats.
(2) Theria, or Eutheria—mammals with teats, arranged according to two plans: (a) Metatheria, nr Hide1phia.
(b) Eutheria (proper), or Monodelphia.
In the Prototheria the female reproductive organs resemble those of birds. There is no vagina, the oviducts opening separately into the cloaca. and the uteri are merely enlargements of the oviducts. The egg: contain much yolk,. are supplied with albumen and covered with a shell as in reptiles and birds. They are laid in a nest at the end of a burrow and are hatched very
.non. possibly in twenty-four hours. The mam mary glands have no teats, any part of the longi tudinal depression along the centre of the abdo men (the primitive pouch) serving that. purpose whom the sucking mouth of the you'lg one is applied to it. group contains but one order, Monotremat The Theria are mammals in which the mam mary glands are provided with teats, and there is a niore or less perfect allantoic development. These two facts, especially, have led to join into a single group the Metatheria and Entheria, hitherto regarded as separate groups of en;;rdinate rank with Prototheria. These group: are thus distinguished: In the Metatheria (Dide1phia of early authors) the oviducts of the female are united in a longer or shorter part of their length, and there is no cloaca. though the urogenital and anal openings are surrounded by a common sphincter muscle; each oviduct possesses a well-fo•med uterus. There are thus two uteri, whence the name `Didelphia."fhe eggs of the Metatheria have little yolk and no shell. They lie in the uteri without becoming very firmly united to the walls, in sonic species not at all: in others. however, there is formed quite a distinct placenta. (The foregoing groups have been called 'Aplaeentalia.') IMt the piling remain in the uteri only a short time: even in the giant kangaroo, the largest of the subclass, they are horn on the 39th day. When born, they are transferred by the mother to her pouch, where each one is placed on a teat. to which it clings automatically. This interest ing subclass, like the preceding, contains but one order. the Marsupialia (q.v.).
The Entheria comprise all the remaining (higher) mammals. In these the marsupium is absent, the vagina is fully formed and single, and there is only one uterus. The egg of the Mono delphian mammal is very minute, almost with out yolk, and becomes closely connected with the wall of the uterus as soon as it enters that organ. (Hence this group has been called 'Pla centalia.') The young develop there in closest union with the mother until their various organs are all distinctly formed and they are in condition to take milk from the mammary glands by their own efforts; but.even after birth they are tended and protected by the mother. The vast majority of the 10.000 known species of mammals belong in this subclass. It has proved the dominant, the successful type. Australasia is the only land the placentals do not possess, and there geo graphical barriers have served to protect the marsupials from being crushed by their success ful rivals. The classification of Entheria is not easy to agree upon, but the following outline is that of Parker and Haswell (see CLASSIFICATION