The Cambridge Natural History Part 7

It is often said, but with some looseness of statement, that ancient quadrupeds are huger than their modern representatives. This statement is partly true in fact, but largely wrong in implication. For it suggests that--and the suggestion is often expressed in books that are not authoritative--huge animals {104} have left a dwarfish offspring; that there were giants of old, and that there is a puny race to-day. As a matter of fact, the study of the gradual evolution of the early Tertiary Mammalia into their descendants of later times shows very plainly the truth of this interesting generalisation: That the primitive types were all small creatures, and that in those instances where we can trace a pedigree, there was a gradual increase in size up to a point where greater increase led to extinction. We point out later on a number of facts ill.u.s.trating this matter in detail. It has been ascertained, for instance, that the pedigree of the Horses, the Camels, the Rhinoceroses, and many other groups, commences with small forms and culminates in large ones. It may be urged that such animals as the Tapir are to-day smallish forms, and that related to them in the past were the gigantic t.i.tanotheres; but in this and similar cases it will be found that the extinct giants were not in the direct line of pedigree, but represented side-branches which waxed huge on their own account and then disappeared.

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CHAPTER V

THE EXISTING ORDERS OF MAMMALS

PROTOTHERIA--MONOTREMATA

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-cla.s.ses; the Mammalia as a whole being termed a cla.s.s of the Vertebrata comparable with the cla.s.s 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 Mult.i.tuberculata, Trituberculata, and Triconodonta, considered in the last chapter, are really to be distributed among these two sub-cla.s.ses 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

SUB-CLa.s.s I.--_PROTOTHERIA._

To this group belongs the order Monotremata, and possibly also the so-called Allotheria or Mult.i.tuberculata. As, however, {106} 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 Mult.i.tuberculata. I have introduced a short account of the princ.i.p.al 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 ent.i.tled to rank in a group equivalent to that including all other mammals of which we have sufficient knowledge to construct a cla.s.sificatory 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.[52] 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 laying {107} of 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[53] for _Ornithorhynchus_, and his results were confirmed later for _Echidna_.[54] 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.[55] 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 aquatic _Ornithorhynchus_. The ears are absolutely dest.i.tute 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 of _Ornithorhynchus_, is a structure which argues the specialised condition of these two modern representatives of what must have been a large order in the past.

{108} [Ill.u.s.tration]

FIG. 51.--Ventral view of skull of _Echidna 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 in _Echidna_, and only to be seen in the tail-region in _Ornithorhynchus_. 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, {109} 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 h.o.m.ologue of the spine and the acromion of other mammals.

Here, again, we have a point of likeness to the Cetacea.[56] In the pelvis the acetabulum is perforate (in _Echidna_), as in Sauropsida.

[Ill.u.s.tration]

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's _Osteology_.)

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 cla.s.s, is perfectly normal in the Monotremata. The alimentary ca.n.a.l shows no great divergences from the normal structure. The stomach is almost globular, with a projecting pyloric region in _Ornithorhynchus_; the intestine is divided into a "small" and "large" intestine by a slender caec.u.m. 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 in _Echidna_ and _Ornithorhynchus_ has 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,[57] is of opinion that the size of the cerebral hemispheres is not at all reptilian; indeed, it "greatly exceeds that of {110} many other mammals." In _Echidna_, too, but not in _Ornithorhynchus_, 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 in _Echidna_, 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,[58] and showed that _Echidna_ was in this respect typically mammalian. The absence of the corpus callosum is one of the princ.i.p.al features separating the Monotremes from other mammals.

The Monotremata are represented to-day by two types, _Ornithorhynchus_ and _Echidna_, which are no doubt worthy of being placed in separate families.

Fossil remains of the group (apart from the problematical Mult.i.tuberculata) are only known from Pleistocene times in Australia, and consist of the bones of a large species of _Echidna_, and some fragments of _Ornithorhynchus_, indicating a smaller animal than the living Platypus.

[Ill.u.s.tration]

FIG. 53.--Brain of _Echidna aculeata_, dorsal view. (Nat. size.) (From Parker and Haswell's _Zoology_.)

FAM. 1. ECHIDNIDAE.--This family contains two genera, of which _Echidna_ is 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 in _Ornithorhynchus_. 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 of _Myrmecophaga_. In relation to this habit the salivary glands are enormously developed, and indeed the animal has been confounded with _Myrmecophaga_,[59] as the vernacular name "Australian Anteater"

exemplifies. {111}

In the skull the Echidna differs from _Ornithorhynchus_ in 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. _Echidna_ has no trace of teeth, and there are no h.o.r.n.y 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 in _Ornithorhynchus_. 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 true _Echidna_, a New Guinea species must clearly be referred to a distinct genus _Proechidna_.[60] 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 named _P. bruijnii_. The Hon. W. Rothschild[61] distinguishes a form _P.

nigroaculeata_, which is allowed by Mr. Lydekker.

[Ill.u.s.tration]

FIG. 54.--Australian Anteater. _Echidna aculeata._ 1/6.

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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 in _Ornithorhynchus_, 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 sh.e.l.l is broken by the emerging young one, which has an egg-breaking tubercle on its snout for this purpose; the mother removes the sh.e.l.l. 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 "h.o.m.oeothermal"

mammals.

FAM. 2. ORNITHORHYNCHIDAE.--There is no need to attempt to define this family, since it contains but one genus _Ornithorhynchus_, 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, _Ornithorhynchus_ has {113} seventeen 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 in _Echidna_, and the hard palate does not extend so far back as in that genus. The brain of this genus is smooth.

[Ill.u.s.tration]

FIG. 55.--Duck-billed Platypus. _Ornithorhynchus anatinus._ 1/6.

The discovery of the real teeth of _Ornithorhynchus_ only dates from the year 1888, when they were found by Professor Poulton[62] in an embryo.

Later Mr. Thomas found[63] 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 h.o.r.n.y 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 and {114} grooves in the plates are the remains of the original alveoli of the teeth.

[Ill.u.s.tration]

FIG. 56.--Skeleton of male _Ornithorhynchus_. Ventral view. The right fore-limb has been separated and turned round so as to bring into view the dorsal surface of the ma.n.u.s. 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_, h.o.r.n.y 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 h.o.r.n.y 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 ma.n.u.s; _V_, foramen for fifth nerve. (From Parker's _Zoology_.)

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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 sh.e.l.ls of the mollusc _Corbicula nepeanensis_, upon which _Ornithorhynchus_ mainly feeds, the h.o.r.n.y plates are preferable to brittle teeth. _Ornithorhynchus_ is 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, a.n.a.logous to the mermaids produced by neatly st.i.tching together the forepart of a monkey and the tail of a salmon.

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CHAPTER VI

INTRODUCTION TO THE SUB-CLa.s.s EUTHERIA

SUB-CLa.s.s II.--_EUTHERIA_

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.