Apart from those creatures whose fragmentary remains have been considered in the last chapter, and which belong to the earliest of mammaliferous strata, the remains of Mammalia are all referable to existing orders. In the pages which follow we shall therefore deal with the actual representatives of living families side by side with their extinct relatives. The existing orders of Mammalia, together with those of their fossil allies, can be plainly divided into two great subdivisions, or, as we shall term them, sub-classes; the Mammalia as a whole being termed a class of the Vertebrata comparable with the class Reptilia, etc. It has been usual, owing to the initiative of Professor Huxley, to divide the Mammalia into three divisions of primary importance. We shall adduce reasons later for not accepting this mode of division, but that which allows of only two primary divisions. These two divisions are (1) Prototheria and (2) Eutheria. Whether the Multituberculata, Trituberculata, and Triconodonta, considered in the last chapter, are really to be distributed among these two sub-classes is a matter upon which it is possible to form an opinion, but not to dogmatise. The Prototheria stand at the base of the mammalian series, and present many likenesses to the Sauropsida; the Eutheria are the animals which are most fully differentiated as mammals. We shall commence with
To this group belongs the order Monotremata, and possibly also the so-called Allotheria or Multituberculata. As, however,the latter are only known from very fragmentary remains, which are not sufficient to determine the systematic position of the animals of which they are fragments, I have not thought it worth while to attempt a serious definition of the order Multituberculata. I have introduced a short account of the principal facts which are known concerning the creatures grouped together under this name into the historical sketch of the progress of mammalian life in Chapter IV. As to the Monotremata, there is no question that they are entitled to rank in a group equivalent to that including all other mammals of which we have sufficient knowledge to construct a classificatory scheme. There have been, indeed, naturalists, such as Meckel, who would altogether deny the mammalian rank of these creatures.
The Monotremata or Ornithodelphia may be thus defined:—
Mammalia with no teats, but with a temporary pouch in which the young are hatched, or to which they are transferred after hatching, and into which open the ducts of the mammary glands. An anterior abdominal vein, or at least the membrane supporting it, persists throughout the abdominal cavity. Heart with an incomplete and largely fleshy right auriculo-ventricular valve. Brain without a corpus callosum. Shoulder girdle with a large coracoid reaching the sternum; clavicles and an interclavicle present. There are "marsupial" or epipubic bones attached to the pelvis. Vertebrae with no epiphyses for the most part. Ribs with only capitulum and no tuberculum. Mammary glands of the sudoriparous and not the sebaceous type of epidermic gland.[50]Oviparous, with a large-yolked and meroblastic ovum, enclosed within a follicle of two rows of cells.
To call these animals Mammalia is of course an abuse of the meaning of that word in one sense, but it is not in another; since the pouch of these Monotremes is, as has been explained elsewhere (p.16), the real equivalent of a teat, and not of the pouch of the Marsupials.
The most salient characteristic of this group of mammals in the estimation of their position in the vertebrate series is not so much the fact that they are oviparous as that the eggs are large-yolked, and develop therefore, so far as regards their early stages, after the fashion of the egg of a reptile. The layingof eggs, or at least ovoviviparity, would follow from the structure of the egg, since the abundance of yolk would do away with the necessity for a placenta. That the eggs had this Saurian characteristic was first definitely made known by Professor Poulton[51]forOrnithorhynchus, and his results were confirmed later forEchidna.[52]The structure of the eggs has, however, already been dealt with on p.72. The fact that these animals lay eggs appears to have been known for a very long time, though rediscovered so lately as 1884 by Mr. Caldwell.[53]In connexion with the structure of the ova, the ovaries themselves and the oviducts are built upon the Sauropsidan plan. In the male the testes retain the primitive abdominal position. The fact that the urinary and genital products escape by means of their ducts into a chamber which also receives the end of the alimentary tract is not a distinctive feature of this group, inasmuch as it is seen in the Marsupials, and also in certain low Eutheria, such as the Beaver and other Rodents, and a few Insectivores. As to external features, the Monotremata show certain archaic characters. The unspecialised arrangement of the mammary glands has already been described. These animals are plantigrade, if the term may be used also to describe the aquaticOrnithorhynchus. The ears are absolutely destitute of a conch. The remarkable spur upon the hind-legs furnished with a gland, which is more marked in the male, and indeed disappears in the female ofOrnithorhynchus, is a structure which argues the specialised condition of these two modern representatives of what must have been a large order in the past.
Fig. 51.
Fig.51.—Ventral view of skull ofEchidna aculeata, and right half of mandible,ang, Angle of mandible;aud.oss, auditory ossicles;cond, condyle of mandible;cor, coronoid process;max, maxilla;oc.cond, occipital condyle;pal, palatine;p.max, premaxilla;pt, pterygoid;sq, squamosal;ty, tympanic ring. (After Parker and Haswell.)
The skeleton shows numerous ancient characteristics. In the skull there is no demarcation of the orbit from the temporal fossa, a feature widely found in archaic mammals. The tympanic remains as a slender ring, there being no auditory bulla formed either from this or from any other bone. The malleus and incus are large, and thus reminiscent of the quadrate and articular bone of reptiles. In the lower jaw the absence of a marked coronoid process, and the absence of a firm ossification at the meeting of the two rami, may be a primitive state of affairs. It must be remembered, however, that the Cetacea show the same characters, though it is possible that they too are developed from a low mammalian stock. In the vertebral column we find the typical mammalian seven cervicals; but those characteristically mammalian structures the epiphyses are totally absent inEchidna, and only to be seen in the tail-region inOrnithorhynchus. In having only the capitular head to the ribs, these mammals are evidently far removed from all other mammals, and are even more reptilian than the Theromorphous reptiles. The large clavicles and the interclavicle (Fig. 52,p. 109) are characteristic of the group, and the latter bone is peculiar to the Monotremata among mammals. So, too, is the large coracoid. In the scapula there is a spine which coincides with the anterior border of that bone. The arrangement of the muscles in this region proves conclusively that this projection is the homologue of the spine and the acromion of other mammals. Here, again, we have a point of likeness to the Cetacea.[54]In the pelvis the acetabulum is perforate (inEchidna), as in Sauropsida.
Fig. 52.
Fig.52.—Side view of right half of the shoulder girdle of a young Echidna (Echidna aculeata). × 1.a, Acromion;c, coracoid;cb, coracoid border;cl, clavicle;css, coraco-scapular suture;ec, epicoracoid;gb, glenoid border;gc, glenoid cavity;ic, interclavicle;pf, postscapular fossa;ps, presternum;s, spine;ss, suprascapular epiphysis;ssf, subscapular fossa. (From Flower'sOsteology.)
Considering the numerous very archaic features which the general structure of this group displays, it is surprising to find how typically mammalian they are in certain other peculiarities. The mammalian diaphragm, one of the distinguishing features of the class, is perfectly normal in the Monotremata. The alimentary canal shows no great divergences from the normal structure. The stomach is almost globular, with a projecting pyloric region inOrnithorhynchus; the intestine is divided into a "small" and "large" intestine by a slender caecum. The liver has the subdivisions that this organ usually shows in the Mammalia. However, the presence of the ventral mesentery and of the abdominal vein inEchidnaandOrnithorhynchushas already been mentioned as a distinctive character. The peculiar and apparently partly primitive valve of the right ventricle has been described above (see p.66). The brain is in most respects mammalian in its characters, but naturally shows some important differences. Dr. Elliot Smith, who has most recently studied this question,[55]is of opinion that the size of the cerebral hemispheres is not at all reptilian; indeed, it "greatly exceeds that ofmany other mammals." InEchidna, too, but not inOrnithorhynchus, the hemispheres are well convoluted, though the arrangement of these convolutions cannot be brought into line with what is known concerning the convolutions upon the hemispheres of other mammals. It had been stated that in these animals, at least inEchidna, there were only two optic lobes, as in lower vertebrates, instead of the mammalian four. The late Sir W. H. Flower set this matter at rest,[56]and showed thatEchidnawas in this respect typically mammalian. The absence of the corpus callosum is one of the principal features separating the Monotremes from other mammals.
The Monotremata are represented to-day by two types,OrnithorhynchusandEchidna, which are no doubt worthy of being placed in separate families. Fossil remains of the group (apart from the problematical Multituberculata) are only known from Pleistocene times in Australia, and consist of the bones of a large species ofEchidna, and some fragments ofOrnithorhynchus, indicating a smaller animal than the living Platypus.
Fig. 53.
Fig.53.—Brain ofEchidna aculeata, dorsal view. (Nat. size.) (From Parker and Haswell'sZoology.)
Fam. 1. Echidnidae.—This family contains two genera, of whichEchidnais the older and much the better known. The skin is abundantly covered with spines, with which are mingled hairs. The snout is tapering, the tail rudimentary, and the fingers and toes five in number. The spur and gland upon the calcaneum are smaller than inOrnithorhynchus. The claws are very strong, serving to tear open the ants' nests, upon the inhabitants of which the Echidna feeds, licking them up with a long extensile tongue like that ofMyrmecophaga. In relation to this habit the salivary glands are enormously developed, and indeed the animal has been confounded withMyrmecophaga,[57]as the vernacular name "Australian Anteater" exemplifies.
In the skull the Echidna differs fromOrnithorhynchusin the greater extension backwards of the palatines, and the larger size of the pterygoids. The extent and relations of these bones to each other is not at all unlike that which obtains in many Whales. The premaxillae show traces of the same divergence followed by convergence of their ends that is seen in the Platypus. There are only sixteen pairs of ribs, and either three or four lumbar vertebrae.Echidnahas no trace of teeth, and there are no horny pads which take their place; the mouth is as edentulous as in the true American Anteaters. The brain (Fig. 53) is marked by sulci, contrary to what we find inOrnithorhynchus. The genus has been divided into three species, but it is doubtful whether more than one can be allowed, which ranges from Australia through the Papuan region. While there is but one species of trueEchidna, a New Guinea species must clearly be referred to a distinct genusProechidna.[58]This animal is to be distinguished by the fact that there are usually but three toes on each foot. But there are copious rudiments of the other phalanges, upon which claws are sometimes developed. The beak is curved downwards, and the back is rather arched; the whole animal has the most singular likeness to an Elephant! The ribs are increased by one pair, and there are four lumbar vertebrae. The one species is namedP. bruijnii. The Hon. W. Rothschild[59]distinguishes a formP. nigroaculeata, which is allowed by Mr. Lydekker.
Fig. 54.
Fig.54.—Australian Anteater.Echidna aculeata.×1⁄6.
The Echidna feeds like anteaters, by thrusting its tongue into an ant-hill, and waiting until it is covered with indignant and marauding ants, which are then swallowed. But this animal also devours worms and insects, which are extracted from their hiding-places by the tongue. It is mainly nocturnal, and prefers the seclusion of the densest scrubs of the bush, or rocky spots where it is free from intrusion. Dr. Semon did not find that the spur of this animal was used at all in self-defence; but he thinks that possibly the weapon may be used, in the breeding season only, in the combats of the males for the females, when perhaps, as has been shown to be the case inOrnithorhynchus, the gland attached to it produces a poisonous secretion.
The egg, as it appears, is transferred to the pouch by the mouth of the mother; the shell is broken by the emerging young one, which has an egg-breaking tubercle on its snout for this purpose; the mother removes the shell. When the young has attained a certain size, the mother removes it from the pouch, but takes it in from time to time to suckle it. When on her nightly rambles the young one is left in a burrow dug for the purpose. Dr. Semon was able, from his own observations, to substantiate this act of intelligence on the part of the Echidna. It is well known that the temperature of the Monotremes is less than that of higher mammals; in addition to this fact Dr. Semon found that the range of variation of temperature in the Echidna was as much as 13 degrees or more. It is thus intermediate between the "poikilothermal" reptiles and the "homoeothermal" mammals.
Fam. 2. Ornithorhynchidae.—There is no need to attempt to define this family, since it contains but one genusOrnithorhynchus, with but one species,O. anatinus. The general aspect of the animal is well known. It is covered with dense fur of a blackish brown colour; the limbs are short and five-toed, the toes being webbed. The tail is longish and broad, being flattened from above downwards. The webbing on the anterior toes considerably outdistances the tips of the claws, as in the Seals. But this is not the case with the hind-feet. The "beak," which is broad and flat, and does actually suggest that of a duck, is not covered with horn, as is often stated, but with a fine, soft, sensitive, naked skin, which abounds in sense-organs of a tactile nature. As to characters derived from the skeleton,Ornithorhynchushasseventeen pairs of ribs and only two lumbar vertebrae. The skull is expanded in front, and the bill is supported by two, at first diverging, and then converging, premaxillae. Between them is the famous "dumb-bell shaped bone," which is believed to be the representative of the reptilian prevomer. The pterygoids are smaller than inEchidna, and the hard palate does not extend so far back as in that genus. The brain of this genus is smooth.
Fig. 55.
Fig.55.—Duck-billed Platypus.Ornithorhynchus anatinus.×1⁄6.
The discovery of the real teeth ofOrnithorhynchusonly dates from the year 1888, when they were found by Professor Poulton[60]in an embryo. Later Mr. Thomas found[61]that the teeth persist for a considerable portion of the animal's life, and are only shed, like milk teeth, "after being worn down by friction with food and sand." We have already (p.98) called attention to the general similarity of these teeth to those of certain of the earliest Mammalia and of mammal-like reptiles. The teeth are all molars, and they are either eight or ten in number. They are replaced by the horny plates of the adult animal; but the mode of replacement is curious. The plates are developed from the epithelium of the mouth, but round and under the true teeth; the epithelium of the mouth grows gradually under the calcified teeth, a method of growth which has possibly something to do with the shedding of the latter. The hollows andgrooves in the plates are the remains of the original alveoli of the teeth.
Fig. 56.
Fig.56.—Skeleton of maleOrnithorhynchus. Ventral view. The right fore-limb has been separated and turned round so as to bring into view the dorsal surface of the manus. The lower jaw is removed.acc.tars, Accessory tarsal bone supporting the spur;ant.pal.for, anterior palatine foramen;ast, astragalus;atl, atlas;ax, axis;bs.oc, basi-occipital;bs.sph, basi-sphenoid;calc, calcaneum;cbd, cuboid;cerv.rb, cervical rib;clac, clavicle;cond.for, foramen above inner condyle of humerus;cor, coracoid;cun, cuneiform of carpus;dent, horny dental plate;ect.cun, ecto-cuneiform;ent.cun, ento-cuneiform;ep.co, epicoracoid;epist, episternum;ep.pb, epipubis;fb, fibula;fem, femur;for.mag, foramen magnum;glen, glenoid cavity of shoulder-joint;glen, glenoid cavity for mandible;hum, humerus;in.cond, inner condyle of humerus;inf.orb.for, points to position of infra-orbital foramen;infr.proc, inferior processes of caudal vertebrae;int.rbs, intermediate ribs;isch, ischium;mag, magnum of carpus;max, maxilla;max.for, maxillary foramen;metat.I, first metatarsal;metat.V, fifth metatarsal;nas.cart, nasal cartilage;obt, obturator foramen;ol, olecranon;out.cond, outer condyle of humerus;pal, palatine;pat, patella;post.pal.for, posterior palatine foramen;pr.max, premaxilla;pr.st, presternum;pter, pterygoid;pub, pubis;rad, radius;scap, scapula;scaph, scaphoid of tarsus;scaph.lun, scapho-lunar;ses, sesamoid bones of wrist and ankle;sp, tarsal horny spur;sq, squamosal;tib, tibia;trd, trapezoid;trm, trapezium;tym.c, tympanic cavity;uln, ulna;unc, unciform;vom, vomer;x, dumb-bell shaped bone;zyg, zygomatic arch;I-V, digits of manus;V, foramen for fifth nerve. (From Parker'sZoology.)
The Duck-billed Platypus is, as every one knows, an aquatic animal. It is not found all over Australia, but is limited to the southern and eastern parts of that continent, and to Tasmania. The animal excavates a burrow for itself in the bank of the slow streams which it frequents. The burrow has one opening below the water and one above; and it is of some length, twenty to fifty feet. The Platypus feeds upon animal food, chiefly "grubs, worms, snails, and, most of all, mussels." These it stows away when captured into its capacious cheek-pouches. The food is then chewed and swallowed above the surface as the animal drifts slowly along. Dr. Semon, from whose work,In the Australian Bush, this account of the animal's habits is quoted, thinks that in the nature of the food of the creature the explanation of the loss of the teeth is to be found. He is of opinion that for cracking the hard shells of the molluscCorbicula nepeanensis, upon whichOrnithorhynchusmainly feeds, the horny plates are preferable to brittle teeth.Ornithorhynchusis apparently not eaten by the natives by reason of its ancient and fish-like smell. Besides, it is hard to catch on account of its diving capacities, which are aided by an acute sense of sight and of hearing. When the Duck-bill was first brought to this country it was believed to be a deliberate fraud, analogous to the mermaids produced by neatly stitching together the forepart of a monkey and the tail of a salmon.
Definition.—Mammalia with teats. Mammary glands of sebaceous type. Heart with entirely membranous and complete right auriculo-ventricular valve. Brain generally with a corpus callosum. Coracoid much reduced and not reaching sternum. No interclavicle. Vertebrae with epiphyses. Ribs double-headed. Viviparous, with a small ovum.
In this group are included not only the Eutheria in the sense of Huxley, but also his Metatheria. Though the Metatheria, or Marsupials as we shall term them, undoubtedly form a most distinct order of mammals, perhaps even a trifle more distinct than most others, their differences from the remaining tribes are not by any means so great as those which separateOrnithorhynchusandEchidnafrom all other mammals. In his well-known memoir upon the arrangement of the Mammalia,[62]Professor Huxley enumerated eleven characters as distinguishing the Metatheria either from the Prototheria or from the Eutheria. Of these only three were characters in which they approach the lower mammals. According to his showing, therefore, the preponderance of marsupial features are Eutherian. The three characters of Prototherian type are (1) the presence of epipubes; (2) the small corpus callosum; (3) the absence of an allantoic placenta.
The last of these can be dismissed, in consequence of the recent discovery of an allantoic placenta inPerameles. The first character is apparently a valid distinction between the Marsupialsand their mammalian relatives higher in the series; but it is not a character that should have been made use of by Huxley, since he believed in the existence of a corresponding element in the Dog. As to the corpus callosum (Fig. 50, p.77) being small, that seems to be not more than a slight difference of degree.[63]A number of other characters of secondary importance were added by Huxley to the weight of evidence which led him to form a group Metatheria for the Marsupials. Some of these, however, are now known to be not evidence in that direction. For instance he observed that no Marsupial had more than a single successional tooth. It seems at the present moment to be fairly clear that Marsupials have a milk dentition like other Eutherians, but that only one of these teeth, the fourth premolar, comes to functional maturity. That it is really one of a complete milk series is evidenced by the fact that this tooth is differentiated contemporaneously with another series formerly held to belong to the so-called prelacteal dentition.[64]There still remains, of course, the actual fact that the milk dentition is not for the most part functional, but its significance breaks down with these fresh discoveries. Of this Professor Osborn has remarked: "The discovery of the complete double series seems to have removed the last straw from the theory of the marsupial ancestry of the Placentals." But Huxley did not lay much stress upon this matter of the teeth, since he observed that similar suppressions of the milk dentition were to be found in many other mammals admittedly Eutherian.
Fig. 57.
Fig.57.—Brain ofEchidna aculeata; sagittal section.ant.com, Anterior commissure;cbl, cerebellum;c.mam, corpus mammillare;col.forn, column of the fornix;c.qu, corpora quadrigemina;gang.hab, ganglion habenulare;hip.com, hippocampal commissure;med, medulla oblongata;mid.com, middle commissure;olf, olfactory lobe;opt, optic chiasma;tub.olf, tuberculum olfactorium;vent. 3, third ventricle. (From Parker and Haswell'sZoology.)
Huxley regarded the peculiarities in the reproductive organsof the Marsupials as "singularly specialised characters," in no way intermediate in character. This view applies also to the pouch, which, as already stated, distinguishes the adults of that group. But the impossibility of using this last character as one of any importance has been shown by the discovery of rudiments of it in embryos of undoubtedly Eutherian mammals (see p.18).
Fig. 58.
Fig.58.—Sagittal section of brain of Rock Wallaby (Petrogale penicillata).ant.com, Anterior commissure;cbl, cerebellum;c.mam, corpus mammillare;c.qu, corpora quadrigemina;crur, crura cerebri;epi, epiphysis, with the posterior commissure immediately behind it;f.mon, position of foramen of Monro;hip.com, hippocampal commissure, consisting here of two layers continuous behind at the spleneium, somewhat divergent in front where the septum lucidum extends between them;hypo, hypophysis;med, medulla oblongata;mid.com, middle commissure;olf, olfactory lobe;opt, optic chiasma;vent. 3, third ventricle. (From Parker and Haswell'sZoology.)
Less stress is laid now upon the existence of four molars in the Marsupials as dividing them from the higher mammals than was formerly the case. The total dentition of the group is on the whole composed of more numerous individual teeth than in the typical Eutheria; but we have exceptions like the Whales, the ArmadilloPriodontes, and the Manatee; or better, because free from the suspicion of secondary multiplication,Otocyonand occasionally (according to Mr. Thomas)Centetes. In the last two there are at least sometimes four molars.
On the other hand, a few archaic characters of some importance crop up here and there among the Marsupials, which are sometimes held to point to a primitive ancestry. It has been remarked that in Marsupials it is the fourth toe which is dominant in size, whereas in Ungulates it is the third. An attempt has been made to explain this on the view (reasonable enough in itself) of a tree-living ancestry for the group. A greater development of the fourth toe is, however, by no means a necessary character of arboreal creatures; the Primates themselves are an exception. Nor is this prevalence universal among the Marsupials;inMyrmecobius(alone) is the third toe the longest; and no great difference can be detected between the third and fourth toes in the case of the generaPhascologale,Didelphys, and some others. Professor Leche compares the predominance of the fourth toe with the hyperphalangeal condition in the fourth toe of the embryo Crocodile, and considers it an archaic feature, not surpassed by the ancient characteristics of the Monotremata. Again it has been pointed out that inPhascologaleandPerameles, the epistropheus (axis vertebra) has a separate rib as inOrnithorhynchus. In the third place, the likeness of the teeth ofMyrmecobiusto those ofOrnithorhynchusis an argument in the same direction, which is furthermore supported by the great age (Mesozoic) of the Metatherian group, if we are right in regarding those extinct creatures as Marsupials.
We may now mention certain facts which are not so generally used. The partly primitive structure of the right auriculo-ventricular valve in the Monotremata has no counterpart in any Marsupial which has been dissected; but there are traces in the latter of the characteristic "ventral mesentery" ofOrnithorhynchusandEchidna.[65]Mr. Caldwell's interesting observation upon the segmenting egg of the Marsupial, the incompleteness of the first segmentation furrow (reminding us of the meroblastic ovum of the Monotreme), may possibly not turn out to be so exclusively Marsupial a feature as has been thought.
The balance of evidence thus points to the nearer relationship of the Marsupials to the Eutherian mammals; and their great specialisation combined with certain evidences of degeneration (disappearance in part of the milk dentition), and their age, point to the fact that they are, at any rate, the descendants of an early form of Eutherian. But they must have separated from the Eutherian stock after it had acquired a definite diphyodonty and the allantoic placenta, the two principal features of the Eutherian as opposed to the Prototherian mammals.
Nevertheless it seems probable that the Marsupial tribe is derived from some of the earliest Eutherians. And on this view may be explained the retention of Prototherian characters.
The remaining Eutheria are obviously all to be referred to one great division with the possible exception of the Whales, whose affinities form one of the principal difficulties to the studentof this group. A short résumé of what is at present thought of the systematic position of this anomalous order is appropriate here. Albrecht went so far as to regard the Cetacea as the nearest group of animals to the hypothetical Promammalia.[66]But discounting his arguments by the removal of such of them as relate to structure plainly altered by the singular mode of life of these creatures, there is really a great deal to be said in favour of his view.
The chief facts which argue a primitive position among mammals for the Cetacea are perhaps: (1) the slight union of the rami of the lower jaw; (2) the occasionally rather marked traces of the double constitution of the sternum; (3) the long and simple lungs; (4) the retention of the testes within the body-cavity; (5) the occasional presence (inBalaenoptera) of a separate supra-angular bone. These points, however, are but few, and are not of such great weight as those which ought to be present to establish a claim to separate treatment for the Cetacea as opposed to the Eutheria. If this group of mammals can be tacked on anywhere, it appears to us that the nearest relatives are not, as is sometimes put forward, the Ungulata or the Carnivora, but the Edentata. There are quite a number of rather striking features in which a likeness is shown between these apparently diverse orders of mammals. The chief ones are these: (1) the existence of traces of a hard exoskeleton, of which vestiges remain in the Porpoise; (2) the double articulation of the rib of the Balaenopterids to the sternum, with which compare the conditions obtaining in the Great Anteater; (3) the concrescence of some of the cervical vertebrae; (4) the share which the pterygoids may take in the formation of the hard palate; (5) the fact that in the Porpoise, at any rate, as in many Edentates, the vena cava, instead of increasing in size as it approaches the liver, diminishes.
Another group which is perfectly isolated is that of the Sirenia. The alliance advocated by some with the Cetacea, and quite recently renewed by Professor Haeckel, is contradicted by so many important features that it seems necessary to abandon it. The recent discovery of a fossil Sirenian jaw by Dr. Lydekker with teeth highly suggestive of those of Artiodactyla, may prove a clue. A third group which is so isolated as to have been placed in aprimary division, proposed to be called Paratheria, is that of the Edentates. Probably the group so called should really be divided into the Edentata and the Effodientia, the latter containing the Old World forms. Whether or not it be ultimately shown that the Ganodonta are ancestral Edentates (sensu strictiori), the connexion of the group with others is not at present plain. The same is the case with the extensive order of Rodents. It is true that the extinct order of the Tillodontia shows certain Rodent-like characters on the one hand, and likenesses to Ungulates on the other. Certain likenesses shown by such apparently diverse animals as the Rabbit and the Elephant used to be insisted upon by Professor Huxley. For the present, however, the Rodents must remain as an isolated group with only very dubious affinities to others. The remaining groups of existing mammals are easier to connect. At first the differences between a Cat and a Horse seem to be quite as wide as those which separate any two of the higher Eutherian orders. But it seems to become clearer and clearer, as palaeontological investigation proceeds, that the bulk of the Ungulate and the Carnivorous, Insectivorous, and perhaps Lemuroid stocks converge into the early Eocene Creodonta. From the Lemuroid branch the higher Primates can be derived. The only "Ungulates" which cannot be fitted in with some reasonable probability is the group of the Proboscidea. But of the early forms of this division we have at present no knowledge.
The Marsupials may be thus defined:—Terrestrial, arboreal, or burrowing (rarely aquatic) mammals, with furry integuments; palate generally somewhat imperfectly ossified; jugal bone reaching as far as the glenoid cavity; angle of lower jaw nearly always inflected. The clavicle is developed. Arising from the pubes are well developed and ossified epipubic bones. Fourth toe usually the most pronounced. Teeth often exceed the typical Eutherian number of forty-four; molars generally four on each side of each jaw. As a rule but one tooth of the milk set is functional, which is (according to many) the fourth premolar. Teats lying within a pouch, in which the young are placed. Young born in an imperfect condition, and showing certain larval characters. There is a shallow cloaca. The testes are extra-abdominal, but hang in front of the penis. In the brain the cerebellum is completely exposed; the hemispheres are furrowed, but the corpus callosum is rudimentary. An allantoic placenta is rarely present.
Structurally the Marsupials are somewhat intermediate between the Prototheria and the more typical Eutheria, with a greater resemblance to the latter.
Fig. 59.
Fig.59.—Rock Wallaby (Petrogale xanthopus), with young in pouch. ×1⁄7. (After Vogt and Specht.)
The name Marsupial indicates what is perhaps the most salient character of this order. The pouch in which the young are carried is almost universally present. It is less developedon the whole in the Polyprotodont forms, such as the Thylacine, Dasyures, etc., but is found in so many of them that the two divisions of the Marsupials, the Diprotodonts and the Polyprotodonts, cannot be raised to distinct orders on this and other grounds. The marsupial pouch of the Marsupials must not, as has been already pointed out, be confounded with the pouch of the Monotreme mammals. Distinct teats are found in the marsupium of the Marsupials, while there are none in the mammary pouch of the Monotreme, the pouch itself indeed representing an undifferentiated teat, of which the walls have not closed up. The pouch opens forward in the Kangaroos, and backwards in the Phalangers and in the Polyprotodonts. Its walls are supported by a pair of bones diverging from each other in aV-shaped manner; these are cartilaginous and vestigial in the Thylacine. Theyare the precise equivalents of similar bones in the Monotremata. It has been held, but apparently erroneously, that these bones are mere ossifications in the tendons of the external oblique muscle of the abdomen, or of the pyramidalis of the same region; and vestiges have been asserted to exist in the Dog. Such bonelets are undoubtedly present in the Dog; but it seems clear from their development in Marsupials, as structures actually continuous with the median unossified portion of the symphysis pubis, that the "marsupial bones" belong to that part of the skeleton, and that they correspond with the epipubis of certain amphibians and reptiles. The pouch, it may be remarked, exists in a rudimentary form in the males of many Marsupials.
Fig. 60.
Fig.60.—Ventral surface of innominate bone of Kangaroo (Macropus major). × ⅓.a, Acetabulum;ab, acetabular border of ilium;is, iliac surface;m, "marsupial" bone;pb, pubic border;pt, pectineal tubercle;s, symphysis;si, supra-iliac border;ss, sacral surface;thf, thyroid foramen;ti, tuberosity of ischium. (From Flower'sOsteology.)
Fig. 61.
Fig.61.—Mammary foetus of Kangaroo attached to the teat. (Nat. size.) (From Parker and Haswell'sZoology.)
The most salient feature in the life-history of the Marsupials is the imperfect condition in which the young are born. The egg is no longer laid, as in the Monotremes; but curiously enough the ovum, which has the small size of that of the Eutheria, divides incompletely at the first division (as Mr. Caldwell has shown), and this developmental feature may perhaps be looked upon as a reminiscence of a former large-yolked condition. The young when born are small and nude; the newly born young of a large Kangaroo is perhaps as large as the little finger. The young are transferred by the lips of the mother to the pouch, where they are placed upon a teat. It is an interesting fact that they are not merely imperfect foetuses, but that they are actual larvae. They possess in fact at any rate one larval organ in the shape ofa special sucking mouth. This sucking mouth is an extra-uterine production, and is of course an adaptation to the particular needs of the young, just as are other larval organs, such as the chin-suckers of the tadpole, or the regular ciliated bands of the larvae of various marine invertebrate organisms.
There are a number of other features which distinguish the Marsupials from other mammals.
The cloaca of the Marsupials is somewhat reduced, but is still recognisable. Its margins inTarsipesare even raised into a wall, which projects from the body.
The tooth series of the Marsupials was once held to consist of one dentition only, with the exception of the last premolar, which has a forerunner. The interpretation of the teeth of Marsupials are various. Perhaps most authorities regard the teeth as being of the milk dentition, with the exception of course of the single tooth that has an obvious forerunner. But there are some who hold that the teeth are of the permanent dentition. In any case it is proved that a set of rudimentary teeth are developed before those which persist. Those who believe in the persisting milk dentition describe these as prelacteal. Another matter of importance about the teeth of this order of mammals is that their numbers are sometimes in excess of the typical Eutherian 44. This, however, holds good of the Polyprotodonts only.
It was for a long time held that the Marsupials differed from all other mammals in having no allantoic placenta. But quite recently this supposed difference has been proved to be not universal by the discovery inPeramelesof a true allantoic placenta. The Marsupials have been sometimes called the Didelphia. This is on account of the fact that the uterus and the vagina are double. Very frequently the two uteri fuse above, and from the point of junction an unpaired descending passage is formed (see Fig. 48 on p.74).
A character of the brain of Marsupials has been the subject of some controversy. Sir Richard Owen stated many years ago that they were to be distinguished from the higher mammals by the absence of the corpus callosum. Later still it was urged that a true corpus callosum, though a small one, was present; while, finally, Professor Symington[68]seems to have shown thatthe original statement of Owen was correct, at least in part. It is at most feebly developed (see Fig. 58, p.118).
As to skeletal characters, the Marsupial skull has on the whole a tendency towards a permanent separation of bones usually firmly ankylosed. Thus the orbitosphenoids remain distinct from the presphenoid. The palate is largely fenestrated, a return as it were—says Professor Parker—to the Schizognathous palate of the bird. The mandible is inflected; this familiar character of the Marsupials goes back to the earliest representatives of the order in Mesozoic times (see p.96); but it is not absolutely universal, being absent from the much weakened skull ofTarsipes. On the other hand, the inflection is nearly as great in certain Insectivores, inOtocyon, etc. The malar always extends back to form part of the glenoid cavity. The shoulder girdle has lost the large coracoid of Monotremes; this bone has the vestigial character that it possesses in other Eutheria. The clavicle is present except in the Peramelidae. A third trochanter upon the femur seems to be never present.