The Cambridge Natural History Part 8

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 separate _Ornithorhynchus_ and _Echidna_ from all other mammals. In his well-known memoir upon the arrangement of the Mammalia,[64] 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 epip.u.b.es; (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 in _Perameles_. The first character is apparently a valid distinction between the Marsupials {117} and 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.[65] 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 dent.i.tion 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 dent.i.tion.[66] There still remains, of course, the actual fact that the milk dent.i.tion 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 dent.i.tion were to be found in many other mammals admittedly Eutherian.

[Ill.u.s.tration]

FIG. 57.--Brain of _Echidna 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's _Zoology_.)

Huxley regarded the peculiarities in the reproductive organs {118} of 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).

[Ill.u.s.tration]

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's _Zoology_.)

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 dent.i.tion 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 Armadillo _Priodontes_, and the Manatee; or better, because free from the suspicion of secondary multiplication, _Otocyon_ and 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; {119} in _Myrmecobius_ (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 genera _Phascologale_, _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 surpa.s.sed by the ancient characteristics of the Monotremata. Again it has been pointed out that in _Phascologale_ and _Perameles_, the epistropheus (axis vertebra) has a separate rib as in _Ornithorhynchus_. In the third place, the likeness of the teeth of _Myrmecobius_ to those of _Ornithorhynchus_ is 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" of _Ornithorhynchus_ and _Echidna_.[67] 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 dent.i.tion), 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 princ.i.p.al 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 princ.i.p.al difficulties to the student {120} of this group. A short resume 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.[68] 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 const.i.tution of the sternum; (3) the long and simple lungs; (4) the retention of the testes within the body-cavity; (5) the occasional presence (in _Balaenoptera_) 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 a {121} primary 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.

{122}

CHAPTER VII

EUTHERIA--MARSUPIALIA

ORDER I. MARSUPIALIA[69]

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 p.u.b.es are well developed and ossified epipubic bones.

Fourth toe usually the most p.r.o.nounced. 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 p.e.n.i.s. 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.

[Ill.u.s.tration]

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 developed {123} on 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 a [70]-shaped manner; these are cartilaginous and vestigial in the Thylacine. They {124} are 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 a.s.serted 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.

[Ill.u.s.tration]

FIG. 60.--Ventral surface of innominate bone of Kangaroo (_Macropus major_). 1/3. _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's _Osteology_.)

[Ill.u.s.tration]

FIG. 61.--Mammary foetus of Kangaroo attached to the teat. (Nat. size.) (From Parker and Haswell's _Zoology_.)

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 of {125} a 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 in _Tarsipes_ are even raised into a wall, which projects from the body.

The tooth series of the Marsupials was once held to consist of one dent.i.tion 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 dent.i.tion, 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 dent.i.tion. 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 dent.i.tion 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 in _Perameles_ of a true allantoic placenta. The Marsupials have been sometimes called the Didelphia. This is on account of the fact that the uterus and the v.a.g.i.n.a are double. Very frequently the two uteri fuse above, and from the point of junction an unpaired descending pa.s.sage 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[71] seems to have shown that {126} the 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 of _Tarsipes_. On the other hand, the inflection is nearly as great in certain Insectivores, in _Otocyon_, 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.

[Ill.u.s.tration]

FIG. 62.--Skull of Rock Wallaby (_Petrogale penicillata_). (Ventral view.) _ali_, Alisphenoid; _bas.oc_, basi-occipital; _bas.sph_, basi-sphenoid; _ex.oc_, ex-occipital; _ju_, jugal; _max_, maxilla; _pal_, palatine; _par.oc_, paroccipital; _p.max_, premaxilla; _pr.sph_, presphenoid; _pt_, pterygoid; _sq_, squamosal; _ty_, tympanic. (From Parker and Haswell's _Zoology_.)

The Marsupials cannot be regarded as an intermediate stage in the origin of the Eutheria for a number of reasons. In the first place, the nature of their teeth shows them to be degenerate animals; one set, whether we regard it as the milk or permanent dent.i.tion, has become vestigial. The recent discovery of a true allantoic placenta in _Perameles_ removes one reason for regarding {127} the Marsupials as primitive creatures. It implies on the whole that the Marsupials have sprung from a stock with an allantoic placenta. The alternative is to a.s.sume the independent development of an allantoic placenta in both groups of the Mammalia; unless indeed the genus _Perameles_ is to be held to be the most primitive race of Marsupials living, a hypothesis which does not appear on the face of it likely. So long as it was believed that the mammary pouch of the Monotremes was the equivalent of the marsupium of the Marsupials, the persistence of this structure seemed to be a bond of union between the groups. But it is now known that the marsupium is a special organ confined to the Marsupials, an argument which is rather in favour of their being a lateral development of the mammalian stem. It is to be remarked also that the marsupium is feeblest in the Polyprotodonts, which may perhaps be looked upon as the most primitive of the Marsupials, owing to their more numerous teeth and other points to be referred to immediately.

Not only are the Marsupials interesting from the point of view of their structure; their present and past distribution is of equal interest. During the Mesozoic epoch they occurred in Europe and North America; but not, so far as negative evidence means anything, in Australia, which is now their headquarters. In Europe Marsupials lingered on into the Tertiary period, when they finally became extinct. In America, of course, the group has persisted to the present day. Now it is important to notice that the two main subdivisions of the Marsupials, the Polyprotodontia and the Diprotodontia, exist to-day in both Australia and South America. These two divisions, it should be explained, differ princ.i.p.ally in that one has numerous, the other rarely more than two,[72] incisors in the lower jaw. It is perhaps the more widely distributed opinion that the Polyprotodontia are the more archaic group; this opinion rests upon one or two facts in addition to the absence of specialisation in the incisor teeth. Among the Polyprotodontia the total number of teeth is greater--a clearly primitive character; secondly, the general form of the body of these animals, with four subequal limbs and carnivorous or omnivorous diet, contrasts with the purely vegetarian and much specialised Kangaroos at any rate. Finally--and sufficient stress {128} has perhaps not been laid upon this matter--the brain among the Polyprotodonts is less convoluted than among the genera of the other division. This statement is of course made with due regard to parallelism in size (see p. 77). It is well known that the complexity of a brain bears a distinct relation to the size of its possessor within the group. Now the most ancient Marsupials are decidedly more Polyprotodont-like. No European form from the earlier periods is distinctly to be referred to the Diprotodonts. But both divisions now exist in America and Australia.

We must a.s.sume, therefore, one of three hypotheses. Either the differentiation into the two great divisions occurred in Jura.s.sic or Cretaceous times before the migration of the order southwards; or the Diprotodont type is only a type, and not a natural group, _i.e._ it has been separately evolved in America and Australia; or, finally, there was formerly a land-connexion in the Antarctic hemisphere, along which the Diprotodonts of Australia wandered into South America. The middle hypothesis has this to commend it, that syndactylism occurs in both divisions, and that in some Diprotodonts the pouch opens backwards as it does in the Polyprotodonts. So great are the resemblances that but little difference is really left--of great importance that is to say. Hence it is not difficult to imagine the reduction of the incisors having taken place twice. In favour of the first hypothesis there are no positive facts.

Finally, in favour of the last, which is so strongly supported by the facts of distribution derived from the study of other groups of animals,[73]

there is at least this striking fact or rather series of facts: that some of the South American fossil Polyprotodonts have a "strictly Dasyurine relationship."[74] If there has not been a direct migration, then the Dasyurine type has been twice evolved, an improbability that few will attempt to explain away. In any case we shall adopt here the usual division of the Marsupials into Diprotodontia and Polyprotodontia.

SUB-ORDER 1. DIPROTODONTIA.

This group includes the herbivorous Marsupials. The incisors are as a rule three above, but one only in the Wombats. Below {129} is one strong pair, with occasionally one or two rudimentary incisors. The upper canines, if present, are not large. The molars are tuberculate or ridged. All Marsupials (except the Wombats) to some extent, and the Macropods especially, are characterised by the prolongation of the tubes of the dentine into the clear enamel. The significance of this fact is, however, lessened by the fact that the same penetration of the enamel by dentinal tubes occurs in the Jerboa, the Hyrax, and some Shrews. The feet have two syndactylous toes,[75] less marked in the Wombats than in the Kangaroos and Phalangers.

[Ill.u.s.tration]

FIG. 63.--Skull of Wombat (_Phascolomys wombat_). (Lateral view.) _ang_, Angular process; _cond_, condyle of mandible; _ext.aud_, opening of bony auditory meatus; _ex.oc_, exoccipital; _ju_, jugal; _lcr_, lachrymal; _max_, maxilla; _nas_, nasal; _p.max_, premaxilla; _sq_, squamosal; _ty_, tympanic. (From Parker and Haswell's _Zoology_.)