Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/393

Rh CLASSIFICA riO.X.] M A M M A L I A 371 Monotremes, though lost in most other members of the class. These &quot; Prototheria,&quot; as Professor Huxley terms them, have in their turn probably been derived from the same source as that in which the existing Amphibia on the one hand and the Sauropsida on the other have had their origin. The great divisions of the Vertebrata may be looked upon therefore as parallel, or rather diverging groups, each tending towards its own specialization, not in any way in the light of ancestor and descendant. No further advance of the Sauropsidian type, which has reached its highest perfection in the modern volant birds, would bring it nearer to the mammalian organization. Restricting ourselves now to the class of mammals, as differentiated from other vertebrates, it will be unnecessary to repeat the oft-told history of the various attempts to express the prevailing knowledge of their structure and affinities in a systematic manner called a classification. The systems of Ray, Linnaeus, Cuvier, Owen, Milne- Edwards, Huxley, and others mark successive epochs of that knowledge. A perfect arrangement of any group of animals can only be obtained simultaneously with a perfect knowledge of their structure and life history, and from this, it need scarcely be said, we are still very far removed. If, as was formerly the case, classifications could be con fined to existing species, the work would be far less difficult. By the extinction of intermediate forms the surviving groups have mostly come to be much isolated, and their limits can be readily determined and defined. The discovery of extinct species, which appears to be taking place at a constantly increasing rate, is by degrees breaking down these boundaries, and making definitions impossible, though at the same time it is throwing much light upon the affinities and probable origin of many groups now widely separated. A source of difficulty, and perhaps error, which this advancing knowledge has introduced, arises from the necessity of determining the position and relation of so many forms by the bones and teeth alone, without any hope of deriving aid from all those other structures of which we avail ourselves in the case of recent animals. These considerations will show that any classi fication advanced at present must be regarded as pro visional. There are, however, some positions which seem to be so firmly established that it is very unlikely that we shall be dislodged from them by any further increase of knowledge, and which we should carefully distinguish from others which are acknowledged to be doubtful, and adopted rather for convenience, owing to the necessity of having some arrangement, than as representingunimpeachable truth. Iree One of the most certain and fundamental points in the [-at classification of the Mammalia is, that all the animals now rul : al composing the class can be grouped primarily in three IS&amp;gt; natural divisions, which, presenting very marked differen tiating characters, and having no existing, or yet certainly demonstrated extinct, intermediate or transitional forms, may be considered as subclasses of equal value, tax- onomically speaking, though very different in the numbers and importance of the animals at present composing them. These three groups are often called by the names originally proposed for them by De Blainville (1) Ornithodelphia, ( 2) Didelphia, (3) Monodelphia, the first being equivalent to the order Monotremata, the second to the Marsupialia, and the third including all the remaining members of the class. Although actual palseontological proof is wanting, there is much reason to believe that each of these, as now existing, are survivors of distinct branches to which the earliest forms of mammals have successively given rise, and for which hypothetical branches Huxley has proposed the names of Protolheria, Metatheria, and Eutheria, 1 names 1 Proceedings of the Zoological Society, 1880, p. 649. which, being far less open to objection than those of Blainville, we shall here use as equivalents for the latter. The characters of the Prototheria can only be deduced Proto- from the two existing families, as hitherto no extinct animals tlieria. which can be referred to other divisions of this remarkable and well-characterized group have been discovered. These two isolated forms, in many respects widely dissimilar, yet having numerous common characters which unite them together and distinguish them from the rest of the Mammalia, are the Ornitliorhynchidx and the Echidnidse, both restricted in their geographical range to the Australian region of the globe. Taken altogether they represent the lowest type of evolution of the mammalian class, and most of the characters in which they differ from the other two subclasses tend to connect them with the inferior verte brates, the Saiiropsida and Amphibia ; for, though the name Ornitkodelphia owes its origin to the resemblance of the structure of the female reproductive organs to those of birds, there is nothing especially bird-like about them, Their principal distinctive characters are these. The brain has a very large anterior commissure, and a very small corpus callosum, agreeing exactly in this respect with the next group. The cerebral hemispheres, in Echidna at least, are well developed and convoluted on the surface. The auditory ossicles present a low grade of development, the malleus being very large, the incus small, and the stapes columelliform. They have no true teeth, though the jaws of Ornithorhynchus are provided with horny productions, which functionally supply their place. The coracoid bone is complete, and articulates with the sternum, and there is a large &quot; interclavicle &quot; or episternum in front of the sternum, and connecting it with the clavicles. There arc also &quot; epipubic &quot; bones. The oviducts (not differentiated into uterine and Fallopian portions) are completely dis tinct, and open as in oviparous vertebrates separately into a cloacal chamber, and there is no distinct vagina. The testes of the male are abdominal in position throughout life, and the vasa deferentia open into the cloaca, not into a distinct urethral passage. The penis, attached to the ventral wall of the cloaca, is perforated by a canal in the greater part of its length, but not at the base, which is open as in reptiles and those birds which have such an organ, and brought only temporarily in contact with the termination of the vasa deferentia, so as to form a seminal urethra when required, but never transmits the urinary secretion. This condition is a distinct advance on that of the Sauropsida in the direction of the more complete development of these parts in most of the other Mammalia. The ureters do not open into the bladder, but behind it into the dorsal wall of the genito-urinary passage. The mammary glands have no distinct nipple, but pour out their secretion through numerous apertures in the skin. The early stages of the development of the young are not yet fully known, but they are produced in a very rudimentary condition, and appear never to be nourished by means of an allantoic placenta. The Metatheria or Didelphia are represented at present Meta- by numerous species, presenting great diversities of general thcria. appearance, structure, and habits, although all united by many essential anatomical and physiological characters, which, taken altogether, give them an intermediate posi tion between the Prototheria and the Eutheria. In the structure of the brain and the presence of epipubic bones they agree with the former, while in the structure of the ear bones and the shoulder girdle and the presence of teats on the mammary glands they resemble the latter, the repro ductive organs belonging to neither one nor the other type, but presenting a special character representing an inter mediate grade of development. The ureters open into the base of the bladder. The oviducts are differentiated into