The Cambridge Natural History Part 6

(3) Arctogaea. This vast realm is clearly capable of subdivision into four regions, which will be considered in detail later. In the meantime the points of likeness between these subdivisions is more marked than are either the resemblances or the {86} differences of any one of them to either of the two realms which have just been defined. The two realms that have been discussed retain their distinctness from each other and from Arctogaea for a considerable way back into the Tertiary period. It is not until we reach very early Tertiary times that Edentates are met with in North America; and then it cannot be regarded as absolutely settled that the Ganodonta are really the forerunners of the Armadillos, Sloths, etc.

Nor do we find Marsupials in Europe until far back in time, and at a corresponding period in North America. Indeed the fauna of South America in late Tertiary times was even more distinct than it is now; for then we had confined to that region the Toxodonts, Glyptodonts, _Macrauchenia_, and other forms, while in Australia there were still Marsupials. In late Tertiary times Europe and India were by no means so distinct from Africa as they are to-day. North America does not resemble the Old World quite so much as the subdivisions of the Old World resemble each other; but, as will be pointed out later, there are and were very substantial agreements. The Elephants, Rhinoceroses, Giraffe, Hippopotamus, _Orycteropus_, are now distinctively African or Indian animals; but all these genera, or at least families (in the case of the Giraffe), have occurred in Europe during quite recent times. _Lycaon_ indeed, now confined to Africa, is thought to have had a European origin from its occurrence in caves there. The Hyaena and the Lion, certain members of the Horse tribe, Apes, and other animals, were also but are not now European.

India again, and the Oriental region generally, once possessed the Hippopotamus, the Chimpanzee, Giraffidae, the Antelopes, _Cobus_, _Hippotragus_, _Strepsiceros_, and _Orias_, which are now purely African animals. It shares at present with the Ethiopian region the Catarhines, including the Anthropoid Apes, the Lemurs, Tragulina (the genus _Dorcatherium_ is also known from fossils in India), _Manis_, _Hyaena_, the Cheetah, Elephant, Rhinoceros, and the Ratel. There is, in fact, no order of mammals which is now absent from one of these three regions though present in the others, save the Lemurs, and they occurred in past times in Europe. The Tapir of India is known fossil in Europe, and the latter continent had its Monkeys and even Anthropoids. On the other hand, North America is more distinct. It has no Lemurs, Apes, Elephants, Rhinoceroses, Tapirs, Old World Edentates {87} (Effodientia), Viverridae, Horses, or Antelopes, excepting _Antilocapra_, a type of a separate division of Bovidae. But since several of these groups have been represented in recent times, no primary line of division can be profitably drawn.

Arctogaea as a whole may be characterised by both negative and positive characters. As negative features may be mentioned;--the entire absence of Edentates (_Necrodasypus_ of Filhol is rather doubtful, see p. 164, n.), though a few crept up into the Nearctic region from Neogaea during past times; and of Hapalidae, Cebidae, and Marsupials, except an Opossum in North America. This realm has, on the other hand, all the Lemurs, all the Insectivores with the exception of the West Indian _Solenodon_, all the Proboscidea, Rhinoceroses, Horses, Deer, Antelopes, the last group including the Oxen and a variety of other important families. It is in fact the headquarters of all the Eutheria with the exception of the Edentata and Marsupials.

The subdivisions of this realm have been variously effected. The cla.s.sical subdivisions are of course those of Mr. Sclater, who would recognise (1) the Nearctic, North America; (2) the Palaearctic, including Europe, Northern Asia, and j.a.pan; (3) the Oriental, including Asia south of the Himalayas and the islands of the Malay Archipelago as far east as the Australian region; and (4) the Ethiopian, _i.e._ tropical Africa and Madagascar. Some would alter this by uniting America and the north of the Old World into a Holarctic region, separating off the southern parts of the North American continent into a Sonoran region. To some, the claims of Madagascar to form a separate region are convincing. To distinguish the boundaries of the several regions is a difficult task; they dovetail into each other on the frontiers with the complex curves of a puzzle-map. The difficulty has been grappled with by the suggestion of intermediate transitional areas; but this proceeding really doubles the difficulty, for there are then two frontiers to delimit in each case instead of only one.

The animal inhabitants must be expected to mingle somewhat at the lines of junction of one region with another.

The Sonoran region does not appear to us to have great claims to recognition. It shows a mingling of southern with northern forms exactly as might be expected. An Armadillo and _Didelphys_ have, as it is believed, invaded it from the Neogaeic realm; it possesses also the South American genera, _Dicotyles_, _Nasua_, {88} _Conepatus_, _Sigmodon_. On the other hand, the Sonoran genera _Antilocapra_, _Cynomys_, _Procyon_, and the Insectivora _Blarina_ and _Scapa.n.u.s_, extend further north. Peculiar to this region are only six genera of Rodents, which seems an insufficient reason for raising the Sonoran province to the dignity of a region.

Considered from the point of view of numbers of peculiar forms, the Thibetan subregion has more claims to distinction as a region; for confined to that area we have the genera _Nectogale_, _Aeluropus_, _Eupetaurus_, _Pantholops_, _Budorcas_; while by slightly extending its limits, a number of other peculiar forms might be added. Madagascar has distinctly more claims to regional division. Absolutely confined to it are eleven of the seventeen existing genera of Lemurs, the family Centetidae among the Insectivora, which contains seven genera, and another recently discovered and peculiar genus, _Geogale_; it has six peculiar genera of Viverridae; it has five peculiar genera of Rodents. In addition to this it is negatively characterised by the absence of the following typical African animals, Felidae, Proboscidea, Rhinocerotidae, Equidae, Monkeys, etc. It seems to be impossible to avoid allowing the rank of a region to this part of the world.

In separating the Nearctic from the Palaearctic region, stress must be laid rather upon the absence of Asiatic and European forms from North America than upon the existence in the northern half of the New World of many peculiar forms. Peculiar to the Nearctic are the Goat genus _Haploceros_, the Rodents _Erethizon_, _Zapus_, and the family Haplodontidae. The Mole genus _Condylura_ is also restricted to this part of the New World. Even so it has more peculiar forms than the Sonoran. If we add to this the absence of Horses, Antelopes except _Antilocapra_, Pigs, Hyaenas, etc., there are strong grounds for retaining this division. It must be agreed, however, that it comes rather nearer to the Eurasian district than the latter does to the Oriental.

The Oriental region has many characteristic animals. It has among the Anthropoid Apes the Orangs and Gibbons; of Old World Apes it has confined to its own area the genera _Semnopithecus_ and _Nasalis_. Of Lemurs there are _Loris_ and _Nycticebus_, and _Tarsius_, representing a family of that order, or even a sub-order. The Galeopithecidae are entirely Malayan. There are many Rodent, Carnivorous, and Insectivorous genera; the Rhinoceroses and the Elephant of this region differ from those of Africa. {89} _Tragulus_ concludes a sample from a very rich list of peculiar forms.

The Ethiopian region has also its Anthropoids, the Gorilla and the Chimpanzee, but they belong to genera or a genus different from those which include the Oriental forms. There are five peculiar genera of Cercopithecidae. The Lemurs restricted to this region are _Galago_, _Perodicticus_ and _Arctocebus_. The peculiar Insectivorous families Macroscelidae and Chrysochloridae are only found here, besides many other peculiar genera. Africa is especially the home of Antelopes, and the Giraffe is not found now outside its borders. The Elephant and the Rhinoceroses are of different species from those of India. There are many peculiar Rodents and Ungulates.

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CHAPTER III

THE POSSIBLE FORERUNNERS OF THE MAMMALIA

The relationship of Mammals to Vertebrates lying below them in the scale, their origin in fact, is a much-debated question, with many attempted solutions. To enter into this large question in detail would involve a great deal of useless statement of arguments founded upon misleading or upon quite inaccurate "facts." It will perhaps be sufficient if we reflect here the current view most in vogue at the present, _i.e._ that which would refer the Mammalia to reptiles belonging to the extinct Permian and Tria.s.sic group of the Theromorpha (also called Anomodontia). These have been explored lately to a very large extent, and chiefly by Professor Seeley.[45] The very fact that a genus _Tritylodon_, only known by the forepart of the skull, has been called Mammalian and Anomodont by various authors, shows at least the difficulty of differentiating the two groups when the material for study is imperfect. As a matter of fact these Theromorpha are without doubt reptiles; they show, for example, a lower jaw formed out of several distinct pieces, of which the articular articulates with a fixed quadrate on the skull. They possess the characteristic reptilian bones, the "transverse," the pre- and post-frontals, and there are various other points of structure which leave no room for doubt as to their truly reptilian nature. There are, however, numerous indications of an evolution in the mammalian direction in all parts of the skeleton, to the more important of which some reference will be made here. It may be as well to clear the {91} ground by mentioning the fact that among the Theromorpha four distinct types of reptiles are included, which are considered to form four orders, _i.e._ the Pareiasauri, the Theriodontia, the Anomodontia (Dicynodontia), and the Placodontia.

The first of these divisions includes what seem to be basal forms. These reptiles show numerous points of likeness to the Amphibian Labyrinthodonts.[46] On the other hand the third division, that of the Dicynodontia, are highly-specialised Theromorpha, from which no further evolution would appear to have been possible. Thus the dent.i.tion was either completely lost, or reduced to tusks as in _Dicynodon_. We need not therefore concern ourselves in the present volume with these Anomodonts. It is with the Theriodonts that our business lies. The very name, be it observed, is aptly chosen on the hypothesis to be explained here; but it is not only in the teeth that these reptiles show likenesses to the Theria or Mammals, but in almost every feature of their organisation. Unlike other reptiles, the Theromorpha in general were lifted comparatively high above the ground on legs of fair length and of mammalian relationship in the position of the segments of the limbs. The typical reptile grovels upon the earth with legs sprawling out, as indeed the very name suggests. One bar to the Theriodonts being on the direct line of mammalian ancestry has been urged as a preliminary difficulty, and that is their large size. The earliest undoubted mammals were small creatures, comparable to a Rat or a Mouse in size; whereas a good-sized Bear or a Wolf is a better standard of size for some of the best-known genera of Theriodonts. It has, however, been quite permissibly suggested that living in company with these large Theriodonts were less obtrusive genera, from which the mammals might have sprung. It is so familiar a fact that a given group of animals generally contains giants, dwarfs, and members of intermediate size, that this suggestion may almost be accepted as a fact. It need at least present no difficulties to us in our comparisons.

The most salient "mammalian" feature of the Theriodonts is the heterodonty of the teeth, the pattern of the "molars," and the limited number which const.i.tute the series. The fact, too, that they are limited to the dentary bones below and to the {92} maxillae and the premaxillae above, is a _sine qua non_ for mammalian comparison. In the more basal Theromorpha the teeth are not so limited in position. Finally, to complete the remarkable mammalian resemblance of the teeth of these reptiles, it must be mentioned that in _Tritylodon_ and _Diademodon_ the roots of the molars, as we may fairly term them, though not actually divided after the mammalian fashion, were deeply marked by a groove, which suggests an incipient division or a fusion of two distinct roots. Some of these facts of structure may now be considered in further detail. As to the incisors and canines, it is sufficient to say that the numbers of the former, and the shape of the latter, are in perfect consonance with a derivation of the Mammalia from this group. The molar series can be divided into premolars and molars, at least in so far as regards their shape; for the anterior teeth are often smaller and less complicated than those which follow, as is often the case with the two series in mammals. The molar series also consist of teeth in close apposition to each other and separated from the canines by a diastema, which is a character of mammalian teeth. The fact that in the reptile _Cynognathus_ and the mammal _Myrmecobius_ there are nine of these molar teeth in each half of each jaw is perhaps not a point upon which it is desirable to dwell with too much weight; but the general fact that the molars are further reduced in some genera of Theriodontia than in that which has been mentioned, is clearly a matter of significance when the ancestry of the mammals is under consideration.

The most interesting fact about the molar series in the Theriodontia is that we meet with the two types of molars that occur in the mammals.

_Cynognathus_ and other genera have molars which consist of a main cusp, and of one cusp before and one after the main cusp; in fact these teeth are triconodont as in certain early mammals, a state of affairs which is believed by the "trituberculists" (see p. 56) to have preceded the tritubercular tooth. There are also "mult.i.tubercular" teeth, especially well developed in _Tritylodon_, where they exactly resemble those of certain Mult.i.tuberculata, and whose structure originally led to the placing of _Tritylodon_ among the mammals of that group. If there is any question about the mammalian nature of this fossil, there remain several other Theriodontia in which the mult.i.tuberculism is well marked. It is so in _Trirhackodon_ {93} and in _Diademodon_ for instance. This incidentally lends some support to the idea that the Mammalia have been evolved from two sources, a way of looking at the origin of the group that will coincide with the views of some authors like the late Dr. Mivart, and will at the same time reconcile the trituberculists and the mult.i.tuberculists. For we should then a.s.sume that the Eutheria and Triconodontia had originated from some such form as _Cynognathus_; and the Mult.i.tuberculata and the existing Monotremes from some form like _Diademodon_. It is not of great use to point out that _Diademodon_ is really of the trituberculate pattern, because in its molars, though mult.i.tuberculate, the trituberculate main cones can be recognised; for that state of affairs could just as well have been brought about by a reduction from the mult.i.tuberculate type. The skull of these Theriodonts shows some well-marked approximations to the mammalian type. There is in the first place a commencing consolidation and reduction of the individual bones, which is so distinguishing a feature of the mammalian skull as opposed to the skull of lower vertebrates. In _Cynognathus_ the pos...o...b..tal is fused with the jugal, and the supratemporal with the squamosal, forming apparently one bone. In the lower jaw the splenial is often reduced to the thinness of paper, thus indicating a commencing disappearance. In many Theromorpha the squamosal shares largely in the formation of the articular facet for the lower jaw, obviously an important mammalian characteristic; this is brought about by the reduction of the quadrate, which latter bone, moreover, acquires in certain particulars the appearance of the mammalian malleus, with which it is, according to many, h.o.m.ologous. But this subject has been already dealt with on page 26. A very p.r.o.nounced likeness to the mammalian skull is that there are two occipital condyles. That this has been brought about by the further development of a tripart.i.te condyle such as occurs in tortoises, and that by the suppression of the basi-occipital part, does not affect the resemblance to the mammalian skull; in fact it explains the origin of two condyles from the typical reptilian single condyle, and disposes of the necessity for believing, with Huxley and others, the Amphibia to be on the main line of mammalian evolution on account of their two condyles. The general aspect of the skull in _Cynognathus_ has been {94} compared to that "of _Thylacinus_ or _Dissacus_." No one can examine the actual sketches of the skull of that Theriodont without endorsing that opinion. As a curious detailed point of likeness to certain Mammalia may be mentioned "a small descending process of the malar bone, which may be a diminutive representative of the descending element of the malar seen in _Elotherium_, _Nototherium_, _Diprotodon_, _Macropus_, certain Edentata, such as _Glyptodon_, _Megatherium_, _Mylodon_, _Bradypus_, but unparalleled so far as I am aware in fossil reptiles." (Osborn.) The zoologist cannot help being impressed with the significance of small details of similarity, which do not seem to be due in any way to surrounding conditions of life, and thus referable to mere convergence, like the fish-like form of Whales and Seals.

The rest of the skeleton of the Theriodontia is by no means so well known as the skull and teeth. But from what is known, other mammalian characters can be pointed out. Perhaps the most striking mammalian feature is to be found in the scapula of _Cynognathus_. It is in this creature somewhat narrow and elongated; but it has a well-marked spine, ending in a hooked acromion. Now it is to be noted in support, so far, of the diphyletic origin of mammals, that in the Monotreme, as in Whales indeed, the spine forms the anterior border of the scapula, and is coincident with it, there being thus no prescapula at all in the Monotreme, and only a trace of it in certain Whales.[47] Whether the mult.i.tuberculate _Tritylodon_ or _Diademodon_ had a scapula after the Monotreme pattern is not known; but it is clear that the scapula of the triconodont _Cynognathus_ is quite after the pattern of the Eutherian scapula. Furthermore, Professor Seeley is of opinion that the coracoid was relatively small, and indeed smaller than the same bone in Edentates, and _a fortiori_ than in Monotremes. Another fact of structure which points also, possibly, in the direction of a diphyletic origin for the Mammalia, is the double-headed ribs of _Cynognathus_. As is well known, the ribs of the Monotremata have only the central head, the capitulum.

As a general mark of affinity with mammals the reduction of the intercentra in _Cynognathus_ may be noted, and also the existence of a small though perfectly obvious obturator-foramen, separating the pubis from the ischium.

There are further details {95} which tend in the same direction. And we shall probably not go far wrong in the present state of our knowledge if we a.s.sign the origin of the mammals to some type which would be included in the order Theriodontia or at least in the sub-cla.s.s Theromorpha.

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CHAPTER IV

THE DAWN OF MAMMALIAN LIFE

The animals that we considered in the last chapter, though showing certain unmistakable likenesses to the mammals, are nevertheless unquestionably not mammals but reptiles. In the Tria.s.sic strata, however, we first meet with the remains of undoubted mammals. The Mammalia first appeared upon the earth in a tentative and hesitating way: they had not cast off many of the characters of their supposed reptilian forefathers; they shrank from observation and destruction by their small size, and apparently, so far at any rate as their teeth afford a clue, by an omnivorous diet. The world abounded at that period in large and carnivorous reptiles, which may indeed have been the princ.i.p.al enemies with which the first mammals had to cope.

These early mammals lingered on to so late a period as the Eocene; but the majority of the genera were Tria.s.sic, Jura.s.sic, and Cretaceous. Certain of the primitive mammalian forms have been referred to the Marsupials, and their resemblances to the Monotremata have also been pointed out. The current view of the present time, however, is that they form a special order, which may possibly have embraced the ancestors of both Marsupials and Monotremes; for it is reasonable to explain in this way the combination of characters of these two orders which they present. For this group the name Allotheria has been proposed by Marsh, and Mult.i.tuberculata by Cope; the latter term is the less suitable, in that the Monotremata (_Ornithorhynchus_) are also "mult.i.tuberculate." The group is known in a very imperfect fashion. The remains are but few and fragmentary; and for the most part we have only a few teeth to speculate upon. This is natural enough, for the harder teeth might easily be supposed to {97} have resisted the decay which would more readily affect the softer bones. Where there are bones it is frequently the lower jaw alone which has been preserved for us--a bone which has also been preserved in the case of some of the contemporary Marsupials.

It has been pointed out (from the observation of dead dogs floating in ca.n.a.ls) that the lower jaw is occasionally detached from the carcase. It is the most readily separable part which contains a skeleton. It may be, therefore, that the remains of these early mammals, floating down some river to the sea, may have lost their jaws while in the river, or at furthest in the shallow waters of the sea, the rest of the carcase floating out to a greater distance, and being finally entombed in the stomach of some carnivorous fish, or in the mud at the bottom of a deep ocean, which has never since seen the light.

The characters of this group are really more those of the Monotremata than of the Marsupials. The undoubted likeness which their molar teeth show to the temporary teeth of the Platypus have already been commented upon. Like the Monotremes the Allotheria appear to have possessed a large and independent coracoid; the evidence for this rests upon the discovery of the lower end of a scapula of _Camptomus_, a Cretaceous genus from North America upon which there is a distinct facet for the articulation of what can have been nothing else than a coracoid. On the other hand they differ from the Monotremata by the presence of incisor teeth which were Rodent-like in form, and not very different from those of certain Marsupials. This point of difference cannot be regarded as of very first-rate importance; no one would relegate the Sloth and the Armadillo to different orders on account of their tooth differences, which are about on a par with those to which we have just referred. It seems indeed likely that it will be ultimately necessary to rub out the boundary line which now divides the Allotheria and the Monotremata.

The Plagiaulacidae are unquestionably mammals, and they are placed by most naturalists in this at present uncertain group of Mult.i.tuberculata, which will be retained here in deference to the distinguished authorities who have inst.i.tuted the group, though there are but few characters by which it can be defined. This family though appearing in the Trias, extends down in time to the Eocene. The type-genus, that which has given its name to {98} the family, is _Plagiaulax_. As it is not Tria.s.sic, the consideration of its characters will be deferred until later. _Microlestes_ is a Rhaetic genus, known from rocks in Germany and England; but it is entirely based upon molar teeth. _M. antiquus_ has a two-rooted molar of an elongated form with a row of tubercles on either side of a median groove, which traverses the long axis of the tooth. To some extent the teeth of the ancient form resemble those of _Ornithorhynchus_. _Microlestes_ has been sometimes spoken of as a Marsupial, but Mr. Tomes[48] has found that it does not show one very universal character of the Marsupial teeth: it has not those continuations of the dentinal tubes which traverse the enamel in all Marsupials that have been examined with the sole exception of the Wombat.

The rarity of the remains of mammals in these earliest rocks of the Secondary epoch has been accounted for in another way from that which has been suggested above. It may be that the group Mammalia was not evolved in Europe at all, and that the stray remains which have been found in that continent represent the fragmentary remnants of a few scattered immigrants which heralded the later invasion of more numerous genera during the Jura.s.sic period.

THE MAMMALS OF THE JURa.s.sIC PERIOD.--Some of the Allotheria or Mult.i.tuberculata described in the last section occur in the rocks of this early part of the Secondary epoch. They are doubtful in position, as already stated; some of them indeed, as for instance _Tritylodon_ and _Dromatherium_, are possibly not mammals at all, while the remainder probably belong to a non-existent order of mammals. Along with these dubious creatures are the fragmentary remains of small animals which are not merely mammals, but in all probability definitely Marsupials. It is true that here again we have little beyond lower jaws and teeth to deal with; so that there may be less certainty in referring them to the Marsupials than appears to be the opinion of the majority of Palaeontologists.

Professor Osborn in fact considers that the Mesozoic mammals consist of three groups: (1) The Mult.i.tuberculata, including the Bolodontidae, Stereognathidae, Plagiaulacidae, Polymastodontidae, and possibly the Tritylodontidae (which, however, are regarded by him and by others as more probably reptiles of the {99} Theromorphous group). (2) The Triconodonta, which were Marsupials, though in all probability with a complete succession of teeth and with an allantoic placentation. This group will include the genera _Phascolotherium_ and _Amphilestes_, as well as _Triconodon_ and _Spalacotherium_. Finally we have (3) the Trituberculata (or Insectivora Primitiva) with the genera _Amphitherium_, _Peramus_, _Amblotherium_, _Stylacodon_, and _Dryolestes_.

We shall take these three groups in order. The Mult.i.tuberculata have already been to some extent defined, if such a word can be used to express the summation of the very scanty information at our disposal. Of this group, _Plagiaulax_ is a genus which occurs in the Purbeck beds; it is only known by lower jaws implying an animal of the size of a Rat or rather smaller. The jaws have in front a large incisor which looks Rodent-like, and also like those of the Diprotodont Marsupials; but it is held that these teeth did not grow from persistent pulps, and there is in any case no anterior thickened coating of enamel. Canines are absent; the diastema is followed by four premolars increasing progressively in size and possessing somewhat complicated grinding surfaces. These surfaces are formed by several obliquely-set ridges. The succeeding teeth are termed molars on account of their difference in structure, and there are but two of them on each side. The molars are of a pattern common in the Mult.i.tuberculata; the centre is hollowed, and the raised rim is beset by tubercles. Other Jura.s.sic genera of Mult.i.tuberculates are _Bolodon_, _Allodon_, and _Stereognathus_. All of these possess the same mult.i.tuberculate molars.

Of the Triconodonta the type-genus is _Triconodon_. This genus is better known than most Jura.s.sic mammals, since both the upper and the lower dent.i.tion have been described. It appears to have possessed the typical Eutherian dent.i.tion of forty-four teeth, to which a fourth molar is added in some species. The great difference between the molars and premolars argues a complete tooth-change. The genus is American as well as European.

_Spalacotherium_ has more molars, five or six.

_Phascolotherium bucklandi_, on the other hand, is a much older type in the form of its teeth. There are, however, not so many of them as in _Amphitherium_; _Phascolotherium_ has but two premolars and five molars, making a total of forty-eight teeth. The teeth are of the triconodont form, the three cusps being in line, and the middle one the largest. {100}

_Amphilestes_ has teeth of the same pattern but has more of them, the premolars and molars being respectively four and five. All these animals had the lower jaw inflected. Whether they are all Marsupials or not, it is clear that _Phascolotherium_ and _Amphilestes_ should be united and placed away from _Amphitherium_ on account of the more primitive form of their teeth.

We next come to the Trituberculata.

Among the most celebrated of these remains are a few jaws discovered in the Stonesfield slates near Oxford, and examined by Buckland, Cuvier, and some of the most eminent naturalists of the beginning of the last century. These jaws have been lately submitted to a careful re-examination by Mr.

Goodrich,[49] who has increased our knowledge of the subject by exposing from the rocky matrix in which the jaws lie fresh details of their structure; it is probable therefore that now all that there is to be learnt from these specimens has been recorded.

_Amphitherium prevostii_ was a creature about the size of a Rat. Its jaw was first brought to Dean Buckland about the year 1814, and described six years later. Buckland thought the jaw to be that of an Opossum, an opinion in which Cuvier concurred. The jaw, however, is marked by a groove running along its length, and this groove was regarded by de Blainville as evidence of the composition of the jaw out of more than one element, which would naturally lead to its being regarded as the jaw of a reptile.[50] This species and another named after Sir Richard Owen have a dental formula which, like that of the Marsupials, is large as compared with that of the Placental mammals; it runs: I 4, C 1, Pm 5, M 6--_i.e._ 64 teeth altogether. This is a larger number than we find in any existing Marsupial.

But as in Marsupials, and in certain Insectivora also, the angle of the jaw is inflected. These teeth are of the tritubercular pattern with a "heel."

They are in fact closely like those of the living _Myrmecobius_; but not, it should be remarked, unlike those of certain Insectivora.

THE MAMMALS OF THE CRETACEOUS PERIOD.--At one time there was a totally inexplicable gap between the Jura.s.sic and the basal Eocene, a series of strata which occupy an enormous expanse of time in the history of the earth having appeared to {101} be devoid of mammalian remains. This gap, however, has been filled up by the discovery of mammalian remains in the North American Laramic formation, which seems to be clearly of Cretaceous age.

Furthermore, it is held by some that the Purbeck beds are more properly to be placed with the Cretaceous, which would then necessitate the consideration under the present heading of some of the types already dealt with; and if, as is suggested in the following section, the lowest so-called Eocene beds are really referable to the Cretaceous, there is no lack of mammalian remains in that period. And, moreover, it was in that case the Cretaceous period which witnessed the evolution of the existing orders of Placental mammals. Otherwise the mammalian remains of the Cretaceous agree with those of the Jura.s.sic. We find remains of the Mult.i.tuberculata in fragments of Plagiaulacidae and Polymastodontidae.

_Ptilodus_ is a genus which has two premolars; and _Meniscoessus_ is another mult.i.tuberculate from the same Laramic formation. The other detached fragments of mammals are thought by Osborn to represent both Placentals and Marsupials.

THE MAMMALS OF THE TERTIARY PERIOD.--Unless the lowest beds of the earliest Tertiary period, the Eocene, such as the Torrejon of North America, should be in reality referred to the Cretaceous, there is no evidence that the modern groups of Mammalia existed before the present epoch of the earth's history. It is probable, however, that the Eutheria as a group were Mesozoic. The fossil jaws that have been considered in the last chapter may quite probably be primitive Eutherians, or even divisible, as believed by Professor Osborn, into Marsupials and Insectivores. In the Tertiary, however, apart from the question as to the nature of the Puerco and Torrejon formations, and as to certain South American strata whose fossil contents have been investigated by Professor Ameghino, we find the first traces of mammals definitely referable to existing orders, or to be distinctly compared with existing orders. Since, however, representatives of types which have obvious relationships to modern types appear in considerable profusion in the very earliest strata of the Eocene, it seems clear that much remains to be discovered in beds earlier than these.

Confining ourselves, however, to facts and to comparisons which can be made on more than a few lower jaws and scattered teeth, which is practically all that we {102} possess of earlier mammals, we must arrive at the general conclusion that two of the existing larger groups of the Eutherian, non-Marsupial, mammals were differentiated at quite the beginning of the Eocene, and were represented by forms from which it is possible to derive at least the existing Carnivora, Insectivora, Artiodactyla, and Perissodactyla. These were the Creodonta and the Ungulate Condylarthra. In addition to these we may enumerate as very early types the Lemuroidea, represented by such forms as _Indrodon_ in the New World, and (though later) by _Necrolemur_, etc., in the Old World, and the Edentata, if we are to allow as their ancestors the Ganodonta.

The early Eocene strata also contain representatives of at least one order, the Amblypoda, which increased subsequently, but has died out without descendants, unless we are to believe with some that the Elephants are to be derived from these Eocene "pachyderms." In later Eocene times the great majority of the existing orders, and even subdivisions of orders, are to be met with; and there are in addition such totally extinct orders as the Typotheria, Ancylopoda, and Tillodontia. Coupled with this gradual specialisation in the orders of Eutherian mammals, there is naturally a vast increase in the number of generic and family types. This culminates perhaps in the Miocene, from which time there has been a gradual decline in mammalian variety, so that it is justly said that we live now in an epoch which is impoverished of mammals. This gradual decay has persisted until to-day, as is witnessed by the extinction of the Rhytina and the Quagga, and the growing rarity of the White Rhinoceros and the American Bison.

The early Eutherian stock consisted of small mammals with small heads and slender, long tails. The limbs were pentadactyle, ensheathed in claws or broader hoofs. The fore-limbs may have been partly prehensile. The teeth were forty-four, completely differentiated into incisors, canines, molars, and premolars; and there appears to have been a complete diphyodontism. The canines were not greatly enlarged, and no diastema separated any of the teeth. The molars were bunodont or of a more cutting pattern, with some five or six tubercles. These animals were, moreover, very small-brained.

This early stock is represented by Creodont and Condylarthrous animals, the exact boundaries between which are hardly marked in the {103} very early types. Professor Osborn has argued that from this early Eutherian stock there were two waves of progress, or, as he expresses it, "two great centres of functional radiation."[51]

The first was largely ineffective, the second has produced all the Eutherian orders of to-day. These two divisions are termed by him "Mesoplacentalia" and "Cenoplacentalia." The first division embraces the Amblypoda and their descendants the Coryphodonts and Dinocerata, many of the Condylarthra, the bulk of the Creodonts and the Tillodonts. These creatures persisted for a time, but died out in the Miocene. They were mainly distinguished by the smallness of their brain; the great specialisation of structure which they exhibit having left that organ unaffected, and therefore tending in the long run to render them unable to cope with changes in the inorganic and organic world. The successful division of the primitive Eutheria comprises the groups which exist at the present day, and is not connected directly with those small-brained Mesoplacentals; it has apparently originated, however, from the least specialised of their ancestors. Professor Osborn thinks, moreover, that the Lemurs and the Insectivores are persistent descendants of the earlier wave of Eutherian life. It appears in fact as if Nature had created the existing Ungulate, Unguiculate, and other types on a defective plan, and, instead of mending them to suit more modern requirements, had evolved an entirely new set of similarly-organised types from some of the more ancient and plastic forms remaining over. The Marsupials may be the only group of the early wave remaining, and they have been able to hold their own for the geological reason that Australia was early cut off from communication with the rest of the world. That they are disappearing seems to be shown by their gradual diminution as we pa.s.s from Australia towards the continent of Asia, through the islands of the Malay Archipelago. Compet.i.tion has here decimated them, as it may do in the remote future in Australia.