The History of Creation, Vol. 2 (of 2) Or the Development of the Earth and its Inhabitants by the Action of Natural Causes

CHAPTER XXI.

Chapter 187,857 wordsPublic domain

PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.

IV. MAMMALS.

The System of Mammals according to Linnæus and Blainville.—Three Sub-classes of Mammals (Ornithodelphia, Didelphia, Monodelphia).—Ornithodelphia, or Monotrema.—Beaked Animals (Ornithostoma).—Didelphia, or Marsupials.—Herbivorous and Carnivorous Marsupials.—Monodelphia, or Placentalia (Placental Animals).—Meaning of the Placenta.—Tuft Placentalia.—Girdle Placentalia.—Disc Placentalia.—Non-deciduates, or Indeciduata.—Hoofed Animals.—Single and Double-hoofed Animals.—Whales.—Toothless Animals.—Deciduates, or Animals with Decidua.—Semi-apes.—Gnawing Animals.—Pseudo-hoofed Animals.—Insectivora.—Beasts of Prey.—Bats.—Apes.

There are only a few points in the classification of organisms upon which naturalists have always agreed. One of these few undisputed points is the privileged position of the class of Mammals at the head of the animal kingdom. The reason of this privilege consists partly in the special interest, also in the various uses and the many pleasures, which Mammals, more than all other animals, offer to man, and partly in the circumstance that man himself is a member of this class. For however differently in other respects man’s position in nature and in the system of animals may have been regarded, yet no naturalist has ever doubted that man, at least from a purely morphological point of view, belongs to the class of Mammals. From this there directly follows the exceedingly important inference that man, by consanguinity also, is a member of this class of animals, and has historically developed out of long since extinct forms of Mammals. This circumstance alone justifies us here in turning our especial attention to the history and the pedigree of Mammals. Let us, therefore, for this purpose first examine the groups of this class of animals.

Older naturalists, especially considering the formation of the jaw and feet, divided the class of Mammals into a series of from eight to sixteen orders. The lowest stage of the series was occupied by the whales, which seemed to differ most from man, who stands at the highest stage, by their fish-like form of body. Thus Linnæus distinguished the following eight orders: (1) Cetæ (whales); (2) Belluæ (hippopotami and horses); (3) Pecora (ruminating animals); (4) Glires (gnawing animals and rhinoceroses); (5) Bestiæ (insectivora, marsupials, and various others); (6) Feræ (beasts of prey); (7) Bruta (toothless animals and elephants); (8) Primates (bats, semi-apes, apes, and men). Cuvier’s classification, which became the standard of most subsequent zoologists, did not rise much above that of Linnæus. Cuvier distinguished the following eight orders: (1) Cetacea (whales); (2) Ruminantia (ruminating animals); (3) Pachyderma (hoofed animals, with the exclusion of ruminating animals); (4) Edentata (animals poor in teeth); (5) Rodentia (gnawing animals); (6) Carnassia (marsupials, beasts of prey, insectivora, and bats); (7) Quadrumana (semi-apes and apes); (8) Bimana (man).

The most important advance in the classification of Mammals was made as early as 1816 by the eminent anatomist Blainville, who has already been mentioned, and who first clearly recognised the three natural main groups or sub-classes of Mammals, and distinguished them according to the formation of their generative organs as _Ornithodelphia_, _Didelphia_, and _Monodelphia_. As this division is now justly considered by all scientific zoologists to be the best, on account of solid foundation on the history of development, let us here keep to it also.

The first sub-class consists of the _Cloacal Animals_, or _Breastless animals_, also called _Forked animals_ (Monotrema, or Ornithodelphia). This class is now represented only by two species of living mammals, both of which are confined to Australia and the neighbouring island of Van Diemen’s land, namely, the well-known Water Duck-bill (Ornithorhynchus paradoxus) with the beak of a bird, and the less known Beaked Mole (Echidna hystrix), resembling a hedgehog. Both of these curious animals, which are classed in the order of _Beaked Animals_ (Ornithostoma), are evidently the last surviving remnants of an animal group formerly rich in forms, which alone represented the Mammalia in the secondary epoch, and out of which the second sub-class, the Didelphia, developed later, probably in the Jurassic period. Unfortunately, we as yet do not know with certainty of any fossil remains of this most ancient primary group of Mammals, which we will call _Primary Mammals_ (Promammalia). Yet they possibly comprise the oldest of all the fossil Mammalia known, namely, the Microlestes antiquus, of which animals, however, we as yet only know some few small molar teeth. These have been found in the uppermost strata of the Trias, in the Keuper, first in Germany (at Degerloch, near Stuttgart, in 1847), later also in England (at Frome), in 1858. Similar teeth have lately been found also in the North American Trias, and have been described as Dromatherium sylvestre. These remarkable teeth, from the characteristic form of which we can conclude that they belonged to an insectivorous mammal, are the only remains of mammals as yet found in the older secondary strata, namely, in the Trias. It is possible, however, that besides these many of the other mammalian teeth found in the Jura and Chalk systems, which are still generally ascribed to Marsupials, in reality belong to Cloacal Animals. This cannot be decided with certainty owing to the absence of the characteristic soft parts. In any case, numerous Monotrema, with well-developed teeth and cloaca, must have preceded the advent of Marsupial animals.

The designation, “_Cloacal animals_” (Monotrema), has been given to the Ornithodelphia on account of the cloaca which distinguishes them from all other Mammals; but which on the other hand makes them agree with Birds, Reptiles, and Amphibia, in fact, with the lower Vertebrata. The formation of the cloaca consists in the last portion of the intestinal canal receiving the mouth of the urogenital apparatus, that is, the united urinary and genital organs, whereas in all other Mammals (Didelphia as well Monodelphia) these organs have an opening distinct from that of the rectum. However, in these latter also the cloaca formation exists during the first period of their embryonal life, and the separation of the two openings takes place only at a later date (in man about the twelfth week of development). The Cloacal animals have also been called “_Forked animals_,” because the collar-bones, by means of the breast bone, have become united into one piece, similar to the well-known fork-bone, or merry-thought, in birds. In all other Mammals the two collar-bones remain separated in front and do not fuse with the breast bone. Moreover, the coracoid bones are much more strongly developed in the Cloacal animals than in the other Mammalia, and are connected with the breast bone.

In many other characteristics also—especially in the formation of their internal genital organs, their auricular labyrinth, and their brain—Beaked animals are more closely allied to the other Vertebrata than to Mammals, so that some naturalists have been inclined to separate them from the latter as a special class. However, like all other Mammals, they bring forth living young ones, which for a time are nourished with milk from the mother. But whereas in all other Mammals the milk issues through nipples, or teats, from the mammary glands, teats are completely wanting in beaked animals, and the milk comes simply out of a flat, sieve-like, perforated patch of the skin. Hence they may also be called _Breastless_ or _Teatless animals_ (Amasta).

The curious formation of the beak in the two still living Beaked animals, which is connected with the suppression of the teeth, must evidently not be looked upon as an essential feature of the whole sub-class of Cloacal animals, but as an accidental character of adaptation distinguishing the last remnant of the class as much from the extinct main group, as the formation of a similar toothless snout distinguishes many toothless animals (for instance, the ant-eater) from the other placental animals. The unknown, extinct Primary Mammals, or Promammalia—which lived during the Trias period, and of which the two still living orders of Beaked animals represent but a single degenerated branch developed on one side—probably possessed a very highly developed jaw like the marsupial animals, which developed from them.

_Marsupial_, or _Pouched Animals_ (Didelphia, or Marsupialia), the second of the three sub-classes of Mammals, form in every respect—both as regards their anatomy and embryology, as well as their genealogy and history—the transition between the other sub-classes—the Cloacal and Placental Animals. Numerous representatives of this group still exist, especially the well-known kangaroos, pouched rats, and pouched dogs; but on the whole this sub-class, like the preceding one, is evidently approaching its complete extinction, and the living members of the class are the last surviving remnants of a large group rich in forms, which represented the Mammalia during the more recent secondary and the earlier tertiary periods. The Marsupial Animals probably developed towards the middle of the Mesolithic epoch (during the Jura) out of a branch of the Cloacal Animals, and in the beginning of the Tertiary epoch again, the group of Placental Animals arose out of the Marsupials, and the latter then succumbed to the former in the struggle for life. All the fossil remains of Mammals known to us from the Secondary epoch, belong either exclusively to Marsupials, or partly perhaps to Cloacal animals. At that time Marsupials seem to have been distributed over the whole earth; even in Europe (France and England), well-preserved fossil remains of them have been found. On the other hand, the last off-shoots of the sub-class now living are confined to a very narrow tract of distribution, namely, to Australia, the Australasian, and a small part of the Asiatic Archipelago. There are also a few species still living in America, but at the present day not a single marsupial animal lives on the continent of Asia, Africa, or Europe.

The name of pouched animals is given to the class on account of the purse-shaped pouch (marsupium) existing in most instances on the abdominal side of the female animals, in which the mother carries about her young for a considerable time after their birth. This pouch is supported by two characteristic marsupial bones, also existing in Cloacal animals, but not in Placental animals. The young Marsupial animal is born in a much more imperfect form than the young Placental animal, and only attains the same degree of development which the latter possesses directly at its birth, after it has developed in the pouch for some time. In the case of the giant kangaroo, which attains the height of a man, the newly born young one, which has been carried in the maternal womb not much longer than five weeks, is not more than an inch in length, and only attains its essential development subsequently, in the pouch of the mother, where it remains about nine months attached to the nipple of the mammary gland.

The different divisions generally distinguished as families in the sub-class of Marsupial animals, deserve in reality the rank of independent orders, for they differ from one another in manifold differentiations of the jaw and limbs, in much the same manner, although not so sharply, as the various orders of Placental animals. In part they perfectly agree with the latter. It is evident that adaptation to similar conditions of life has effected entirely coincident or analogous transformations of the original fundamental form in the two sub-classes of Marsupials. According to this, about eight orders of Marsupial animals may be distinguished, the one half of the main group or legion of which are herbivorous, the other half carnivorous. The oldest fossil remains of the two legions (if the previously mentioned Microlestes and the Dromatherium are not included) occur in the Jurassic strata, namely, in the slates of Stonesfield, near Oxford. The slates belong to the Bath, or the Lower Oolite formation—strata which lie directly above the Lias, the oldest Jura formation. (Compare p. 15.) It is true that the remains of Marsupials found in the slates of Stonesfield, as well as those which were found later in the Purbeck strata, consist only of lower jaws. (Compare p. 29.) But fortunately the lower jaw is just one of the most characteristic parts of the skeleton of Marsupials. For it is distinguished by a hook-shaped process of the lower corner of the jaw turning downwards and backwards, which neither occurs in Placental nor in the (still living) Cloacal animals, and from the existence of this process on the lower jaws from Stonesfield, we may infer that they belonged to Marsupials.

SYSTEMATIC SURVEY OF CLOACAL AND MARSUPIAL MAMMALIA.

I. _First Sub-class of Mammalia:_

_Forked or Cloacal Animals (Monotrema, or Ornithodelphia)._

Mammals with Cloaca, without Placenta, with Marsupial Bones.

------------------------------------------------------------------------------------ I. } =Primary Mammals= } Unknown extinct Mammalia from the { (Microlestes?) +Promammalia+ } Trias Period { (Dromatherium?)

II. } 1. Aquatic beaked 1. Ornithorhynchida { 1. Ornithorhynchus =Beaked Animals= } animals { paradoxus +Ornithostoma+ } 2. Terrestrial 2. Echidnida { 2. Echidna hystrix } beaked animals { ------------------------------------------------------------------------------------ II. _Second Sub-class of Mammalia:_

_Pouched or Marsupial Animals (Marsupialia, or Didelphia)._

Mammals without Cloaca, without Placenta, with Marsupial Bones. ------------------------------------------------------------------------------------ _Legions_ | _Orders_ | _Systematic Name_ | _Families of the_ _of_ | _of_ | _of_ | _Marsupialia._ _Marsupialia._ | _Marsupialia._ | _the Orders._ | ------------------------------------------------------------------------------------ { 1. Hoofed 1. Barypoda { 1. Stereognathida { Marsupial animals { 2. Nototherida III. { { 3. Diprotodontia =Herbivorous= { =Marsupial= { 2. Kangaroo 2. Macropoda { 4. Plagiaulacida =Animals= { Marsupial animals { 5. Halmaturida { (Leaping pouched { 6. Dendrolagida +Marsupialia+ { animals) +Botanophaga+ { { 3. Root-eating 3. Rhizophaga { { Marsupial animals { { (Gnawing pouched { 7. Phascolomyida { animals) { { { 4. Fruit eating 4. Carpophaga { { Marsupial animals { 8. Phascolarctida { (Climbing pouched { 9. Phalangistida { animals) { 10. Petaurida

{ 5. Insectivorous 5. Cantharophaga { 11. Thylacotherida { Marsupial animals { 12. Spalacotherida { (Primæval pouched { 13. Myrmecobida { animals) { 14. Peramelida IV. { =Carnivorous= { 6. Marsupial animals 6. Edentula { =Marsupial= { poor in teeth { =Animals= { (Pouched animals { 15. Tarsipedina { with trunks) { { { 7. Rapacious marsupial 7. Creophaga { 16. Dasyurida { animals { 17. Thylacinida +Marsupialia+ { (Rapacious pouched { 18. Thylacoleonida +Zoophaga+ { animals) { { 8. Ape-footed 8. Pedimana { { Marsupial animals { 19. Chironectida { (Pouched animals { 20. Didelphyida { with hands)

SYSTEMATIC SURVEY OF PLACENTAL ANIMALS.

III. _Third Sub-class of Mammalia:_

_Placentalia, or Monodelphia (Placental Animals)._

Mammals without Cloaca, with Placenta, without Marsupial Bones.

----------------------------------------------------------------------------------- _Legions of_ | _Orders of_ | _Sub-orders of_ |_Systematic Name_ _the_ | _the_ | _the_ | _of_ _Placental Animals._|_Placental Animals._|_Placental Animals._ | _the Sub-orders._ ----------------------------------------------------------------------------------- III. 1. INDECIDUA. _Placental Animals without Decidua._ ----------------------------------------------------------------------------------- V. { I. Single-hoofed { 1. Tapirs 1. Tapiromorpha =Hoofed Animals= { _Perissodactyla_ { 2. Horses 2. Solidungula +Ungulata+ { { II. Double-hoofed { 3. Pigs 3. Choeromorpha { _Artiodactyla_ { 4. Ruminating 4. Ruminantia

{ III. Herbivorous { VI. { Whales { 5. Sea cows 5. Sirenia =Whales= { _Phycoceta_ { +Cetacea+ { { IV. Carnivorous { 6. Whales 6. Autoceta { Whales { 7. Zeuglodonta 7. Zeugloceta { _Sarcoceta_ {

VII. { V. Digging Animals { 8. Ant-eaters 8. Vermilinguia =Animals= { _Effodientia_ { 9. Armadilloes 9. Cingulata =poor in teeth= { +Edentata+ { VI. Sloths { 10. Giant Sloths 10. Gravigrada { _Bradypoda_ { 11. Dwarf Sloths 11. Tardigrada ----------------------------------------------------------------------------------- III. 2. DECIDUATA. _Placental Animals with Decidua._ ----------------------------------------------------------------------------------- VIII. { VII. Rapacious {12. Rapacious land 12. Carnivora =Placental Animals= { Animals { animals +Zonoplacentalia+ { _Carnaria_ {13. Rapacious sea 13. Pinnipedia { { animals { { VIII. False-hoofed {14. Hyrax 14. Lamnungia { Animals {15. Toxodonts 15. Toxodontia { _Chelophora_ {16. Dinotheria 16. Gonyognatha { {17. Elephants 17. Proboscidea

XI. { IX. Semi-apes {18. Fingered animals 18. Leptodactyla =Disc Placental= { _Prosimiæ_ {19. Flying lemur 19. Ptenopleura =Animals= { {20. Long-footed 20. Macrotarsi +Discoplacentalia+ { {21. Short-footed 21. Brachytarsi { { X. Gnawing Animals {22. Squirrel species 22. Sciuromorpha { _Rodentia_ {23. Mouse species 23. Myomorpha { {24. Porcupine species 24. Hystrichomorpha { {25. Hare species 25. Lagomorpha { { XI. Insect-eating {26. With a cœcum 26. Menotyphla { Animals {27. Without a 27. Lipotyphla { _Insectivora_ { cœcum { { XII. Flying Animals{28. Flying foxes 28. Pterocynes { _Chiroptera_ {29. Bats 29. Nycterides { { XIII. Apes { 30. Clawed apes 30. Arctopitheci { _Simiæ_ { 31. Flat-nosed 31. Platyrrhinæ { { 32. Narrow-nosed 32. Catarrhinæ

----------------------------------------------------------------

Of _Herbivorous marsupials_ (Botanophaga), only two fossils are as yet known from the Jura, namely, the Stereognathus ooliticus, from the slates of Stonesfield (Lower Oolite), and the Plagiaulax Becklesii, from the middle Purbeck strata (Upper Oolite). But in Australia there are gigantic fossil remains of extinct herbivorous Marsupials from the diluvial period (Diprotodon and Nototherium) which were far larger than the largest of the still living Marsupials. The Diprotodon Australis, whose skull alone is three feet long, exceeded even the river-horse, or Hippopotamus, in size and upon the whole resembled it in the unwieldy and clumsy form of body. This extinct group, which probably corresponded with the gigantic placental hoofed animals of the present day—the hippopotami and rhinoceroses—may be called Hoofed Marsupials (Barypoda). Closely allied to them is the order of kangaroos, or Leaping Marsupials (Macropoda), which all have seen in zoological gardens. In their shortened fore legs, their very lengthened hind legs, and very strong tail, which serves as a jumping pole, they correspond with the leaping mice in the class of Rodents. Their jaw, however, resembles that of horses, and their complex stomach that of Ruminants. A third order of Herbivorous Marsupials corresponds in its jaws to Rodents, and in its subterranean mode of life, especially, to digging mice. Hence they may be termed Rodent Marsupials, or root-eating pouched animals (Rhizophaga). They are now represented only by the Australian wombat (Phascolomys). A fourth and last order of Herbivorous Marsupials is formed by the climbing or Fruit-eating Marsupials (Carpophaga), whose mode of life and structure resembles partly that of squirrels, partly that of apes (Phalangista, Phascolarctus).

The second legion of Marsupials, the _Carnivorous Marsupials_ (Zoophaga), is likewise divided into four main groups or orders. The most ancient of these is that of the primæval, or Insectivorous Marsupials (Cantharophaga). It probably includes the primary forms of the whole legion, and possibly also those of the whole sub-class. At least, all the lower jaws from Stonesfield (with the exception of the Stereognathus) belong to Insectivorous Marsupials, and the still living Myrmecobius is their nearest relative. But some of those oolitic Primæval Marsupials possessed a larger number of teeth than all the other known mammals, for each half of the lower jaw of the Thylacotherium contained sixteen teeth (three incisors, one canine tooth, six pseudo, and six genuine molars). If the upper jaw, which is unknown, had as many teeth, then the Thylacotherium had no less than sixty-four teeth, just double the number possessed by man. The Primæval Marsupials correspond, on the whole, with the Insectivora among Placental animals, which order includes hedgehogs, moles, and shrew-mice. A second order, which has probably developed out of a branch of the last, consists of the Snouted, or Toothless Marsupials (Edentula), which resemble the Toothless animals, or Edentata, among the Placental animals by their tube-shaped snout, their degenerated jaws, and their corresponding mode of life. On the other hand, the mode of life and formation of the jaws of Rapacious marsupials (Creophaga) correspond with those of the genuine Beasts of Prey, or Carnivora, among Placental animals. This order includes the pouched marten (Dasyurus) and the pouched wolf (Thylacinus) in Australia. Although the latter attains to the size of a wolf, it is but a dwarf in comparison with the extinct Australian pouched lions (Thylacoleo) which were at least as large as a lion, and possessed huge canine teeth more than two inches in length. Finally, the eighth and last order is formed by the marsupials with hands, or the Ape-footed Pouched animals (Pedimana), which live both in Australia and America. They are frequently kept in zoological gardens, especially the different species of the genus Didelphys, and are known by the name of pouched rats, bush rats, or opossums. The thumb on their hinder feet is opposable to the four other toes, as in a hand, and by this they are directly allied to the Semi-apes, or Prosimia, among Placental animals. It is possible that these latter are really next akin to the marsupials with hands, and that they have developed out of their long since extinct ancestors.

It is very difficult to discover the genealogy of Marsupials, and this more especially because we are but very imperfectly acquainted with the whole sub-class; and the Marsupials of the present day are evidently only the last remnants of a group that was at one time rich in forms. It is possible that Marsupials with hands, those with snouts, as well as rapacious Marsupials, developed as three diverging branches out of the common primary group of Primæval Marsupials. In a similar manner, on the other hand, the rodent, leaping, and hoofed Marsupials have perhaps arisen as three diverging branches out of the common herbivorous primary group, that is, out of the Climbing Marsupials. Climbing and Primæval Marsupials might, however, be two diverging branches of the common primary forms of all Marsupials, that is, of the _Primary Marsupials_ (Prodidelphia), which originated during the older secondary period out of Cloacal animals.

The third and last sub-class of mammals comprises the _Placental animals_, or _Placentals_ (Monodelphia, or Placentalia). It is by far the most important, comprehensive, and most perfect of the three sub-classes; for the class includes all the known mammalia, with the exception of Marsupials and Beaked animals. Man also belongs to this sub-class, and has developed out of its lower members.

Placental animals, as their name indicates, are distinguished from all other mammals, more especially by the formation of a so called _placenta_. This is a very peculiar and remarkable organ, which plays an exceedingly important part in nourishing the young one developing in the maternal body. The placenta (also called after-birth) is a soft, spongy, red body, which differs very much in form and size, but which consists for the most part of an intricate network of veins and blood vessels. Its importance lies in the exchange of substance between the nutritive blood of the maternal womb, or uterus, and the body of the germ, or embryo. (See vol. i. p. 298.) This very important organ is developed neither in marsupials nor in beaked animals. But placental animals are also distinguished from these two sub-classes by many other peculiarities, thus more especially by the absence of marsupial bones, by the higher development of the internal sexual organs, and by the more perfect development of the brain, especially of the so-called callous body or beam (_corpus callosum_), which, as the intermediate commissure, or transverse bridge, connects the two hemispheres of the large brain with each other. Placental animals also do not possess the peculiar hooked process of the lower jaw which characterizes Marsupials. The following classification (p. 246) of the most important characteristics of the three sub-classes will best explain how Marsupials, in these anatomical respects, stand midway between Cloacal and Placental animals.

Placental animals are more variously differentiated and perfected, and this, moreover, in a far higher degree, than Marsupials, and they have, on this account, long since been arranged into a number of orders, differing principally in the formation of the jaws and feet. But what is even of more importance than these, is the different development of the placenta, and the manner of its connection with the maternal uterus. For in the three lower orders of Placental animals, in Hoofed animals, Whales, and Toothless animals, the peculiar spongy membrane, which is called the _deciduous membrane_, or _decidua_, and which connects the maternal and the fœtal portions of the placenta, does _not_ become developed. This takes place exclusively in the seven higher orders of Placental animals, and we may, therefore, according to Huxley, class them in the main group of _Deciduata_, or animals with _decidua_. They are contrasted with the three first-mentioned legions of indeciduous animals, or _Indeciduata_.

But in the various orders of Placental animals the placenta differs not only in important internal differences of structure, which are connected with the absence or the presence of a decidua, but also in the external form of the placenta itself. In the Indeciduata it consists, in most cases, of numerous, single, scattered bunches or tufts of vessels, and hence this group may be called _tufted placental animals_ (Villiplacentalia). In the Deciduata, however, the single tufts of vessels are united into a cake, which appears in two different forms. In the one case it surrounds the embryo in the form of a closed band or ring, so that only the two poles of the oval egg bladder are free of tufts; this is the case in animals of prey (Carnaria) and the pseudo-hoofed animals (Chelophora), which may consequently be comprised as _girdled-placental animals_ (Zonoplacentalia). In the other Deciduata, to which man also belongs, the placenta is a simple round disc, and we therefore call them _disc-placentals_ (Discoplacentalia). This group includes the five orders of Semi-apes, Gnawing animals, Insectivora, Bats, and Apes, from the latter of which, in the zoological system, man cannot be separated.

It may be considered as quite certain, from reasons based upon their comparative anatomy and their history of development, that Placental animals first developed out of Marsupials, and that this very important development—the first origin of the placenta—probably took place in the beginning of the tertiary epoch, during the eocene period. But one of the most difficult questions in the genealogy of animals is the important consideration whether all Placental animals have arisen out of one or out of several distinct branches of Marsupials; in other words, whether the origin of the placenta occurred but once, or several times.

When, in my General Morphology, I for the first time endeavoured to establish the pedigree of Mammals, I here, as in most cases, preferred the monophyletic, or one-rooted, to the polyphyletic, or many-rooted, hypothesis of descent. I assumed that all Placental animals were derived from a single form of Marsupial animal, which, for the first time, began to form a placenta. In this case the Villiplacentals, Zonoplacentals, and Discoplacentals would perhaps have to be considered as three diverging branches of the common primary form of Placentals, or it might also be conceived that the two latter, the Deciduata, had developed only at a later period out of the Indeciduata, which on their part had arisen directly out of the Marsupials. However, there are also important reasons for the alternative; namely, that several groups of Placentals, differing from the beginning, arose out of several distinct groups of Marsupials, so that the placenta itself was formed several times independently. This opinion is maintained by Huxley, the most eminent English zoologist, and by many others. In this case the Indeciduata and the Deciduata would perhaps have to be considered as two completely distinct groups; then the order of Hoofed animals, as the primary group of the Indeciduata, might be supposed to have originated out of the Marsupial hoofed animals (Barypoda). Among the Deciduata, on the other hand, the order of Semi-apes, as the common primary form of the other orders, might possibly have arisen out of Handed Marsupials (Pedimana). But it is also conceivable that the Deciduata themselves have arisen out of several different orders of Marsupials, Animals of Prey out of Rapacious Marsupials, Gnawing animals out of Gnawing Marsupials, Semi-apes out of Handed Marsupials, etc. As we do not at present possess sufficient empiric material to solve this most difficult question, we must leave it and turn our attention to the history of the different orders of Placental animals, whose pedigree can often be very accurately established in detail.

We must, as already remarked, consider the order of _Hoofed animals_ (Ungulata) as the primary group of the Indeciduata, or Tuft-placentals; the two other orders, Whales and Toothless animals, developed out of them, as two diverging groups, probably only at a later period, by adaptation to very different modes of life. But it is also possible that the animals poor in teeth (Edentata) may be of quite a different origin.

Hoofed animals are in many respects among the most important and the most interesting Mammals. They distinctly show that a true understanding of the natural relationship of animals can never be revealed to us merely by the study of living forms, but in all cases only by an equal consideration of their extinct and fossil blood-relations and ancestors. If, as is usually done, only the living Hoofed animals are taken into consideration, it seems quite natural to divide them into three entirely distinct orders, namely: (1) Horses, or _Single-hoofed animals_ (Solidungula, or Equina); (2) Ruminating animals, or _Double-hoofed_ (Bisulca, or Ruminantia); and (3) Thick-skinned, or _Many-hoofed_ (Multungula, or Pachyderma). But as soon as the extinct Hoofed animals of the tertiary period are taken into consideration—of which animals we possess very numerous and important remains—it is seen that this division, but more especially the limitation of the Thick-skinned animals, is completely artificial, and that these three groups are merely top branches lopped from the pedigree of Hoofed animals, which are most closely connected by extinct intermediate forms. The one half of the Thick-skinned animals—rhinoceroses, tapirs, and palæotheria—manifest the closest relationships to horses, and have like them odd-toed feet; whereas the other half of the Thick-skinned animals—pigs, hippopotami, and anoplotheria—on account of their double-toed feet are much more closely allied to ruminating animals than to the former. Hence we must, in the first place, among Hoofed animals distinguish the two orders of Paired-hoofs and Odd-hoofs, as two natural groups, which developed as diverging branches out of the old tertiary primary group of Primary Hoofed animals, or Prochela.

The order of _Odd-hoofed animals_ (Perissodactyla) comprises those Ungulata in which the middle (or third) toe of the foot is much more strongly developed than the others, so that it forms the actual centre of the hoof. This order includes the very ancient, common, primary group of all Hoofed animals, that is, the _Primary-hoofed animals_ (Prochela), which are found in a fossil state in the oldest Eocene strata (Lophiodon, Coryphodon, Pliolophus). Directly allied to this group is that branch which is the actual primary form of the Odd-hoofed animals, namely, the _Palæotheria_, fossils of which occur in the upper Eocene and lower Miocene. Out of the Palæotheria, at a later period, the rhinoceroses (Nasicornia) and rhinoceros-horses (Elasmotherida) on the one hand, and the tapirs, lama-tapirs, and primæval horses, on the other, developed as two diverging branches. The long since extinct primæval horses, or Anchitheria, formed the transition from the Palæotheria and tapirs to the Miocene horses, or hipparions, which are closely allied to the genuine living horses.

The second main group of Hoofed animals, the order of _Pair-hoofed animals_ (Artiodactyla), comprises those hoofed animals in which the middle (third) and fourth toe of the foot are almost equally developed, so that the space between the two forms the central line of the entire foot. The order is divided into two sub-orders—the Pig-shaped and the Cud-chewing, or Ruminating. The _Pig-shaped_ (Chœromorpha) comprise in the first place the other branch of Primary-Hoofed-animals, the _Anoplotheria_, which we consider as the common primary form of all Pair-hoofed animals, or Artiodactyla (Dichobune, etc.). Out of the Anoplotheria arose, as two diverging branches, the primæval swine, or Anthracotheria, on the one hand, forming the transition to swine and river-horses, and the Xiphodonta on the other hand, forming the transition to Ruminating animals. The oldest _Ruminating animals_ (Ruminantia) are the Primæval Stags, or Dremotheria, out of which, possibly, the stag-shaped (Elaphia), the hollow-horned (Cavicornia), and camels (Tylopoda), have developed as three diverging branches. Yet these latter are, in many respects, more allied to the Odd-hoofs than to the genuine Pair-hoofs. The accompanying systematic survey on p. 252, will show how the numerous families of Hoofed animals are grouped, in correspondence with this genealogical hypothesis.

SYSTEMATIC SURVEY

_Of the Sections and Families of Hoofed Animals, or Ungulata._

(N.B. Those families that are extinct are marked with an asterisk.)

----------------+--------------------------------+------------------------+------------------- _Orders_ | | | _of_ | _Sections_ | _Families_ | _Systematic Name_ _Hoofed_ | _of_ | _of_ | _of_ _animals._ | _Hoofed Animals._ | _Hoofed Animals._ | _the Families._ ----------------+----------------------------=---+------------------------+------------------- I. { I. Primary Hoofed { 1. Lophiodonta 1. Lophiodontia* =Odd-toed= { Animals.* { 2. Pliolophida 2. Pliolophida* =Hoofed= { _Prochela_ =Animals= { { 3. Primary 3. Palæotherida* { { Odd-hoofs +Ungulata+ { II. Tapir-shaped { 4. Lama-tapirs 4. Macrauchenida* { _Tapiromorpha_ { 5. Tapirs 5. Tapirida +Perissodactyla+ { { 6. Rhinoceroses 6. Nasicornia { { 7. Rhinoceros-horses 7. Elasmotherida* { { III. Single-hoofs { 8. Primæval 8. Anchitherida* { _Solidungula_ { horses { { 9. Horses 9. Equina

{ { 10. Primary 10. Anoplotherida* { { Pair-hoofs { { 11. Primæval 11. Anthracotherida* { IV. Pig-shaped { pigs { _Chœromorpha_ { 12. Pigs 12. Setigera { { 13. River horses 13. Obesa { { 14. Primæval 14. Xiphodontia* { { ruminants { { { { {15. Primæval 15. Dremotherida* { { { { deer II. { { A. Stag-shaped { a. {16. Pseudo 16. Tragulida =Pair-toed= { { _Elaphia_ { { musk deer =Hoofed= { { { =Animals= { { { b. {17. Musk deer 17. Moschida { { { {18. Deer 18. Cervina +Ungulata+ { { { { V. { { c. {19. Primæval 19. Sivatherida* +Artiodactyla+ { Ruminating { { { giraffes { animals { { {20. Giraffes 20. Devexa { _Ruminantia_ { { { { {21. Primæval 21. Antilocaprina* { { { d. { gazelles { { { {22. Gazelles 22. Antilopina { { { { { B. Hollow-horned { {23. Goats 23. Caprina { { _Caricornia_ { e. {24. Sheep 24. Ovina { { { {25. Oxen 25. Bovina { { { { C. Pad-footed { 26. Lamas 26. Auchenida { { _Tylopoda_ { 27. Camels 27. Camelida

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It is probable that the remarkable legion of _Whales_ (Cetacea) originated out of Hoofed animals, which accustomed themselves exclusively to an aquatic life, and thereby became transformed into the shape of fish. Although these animals seem externally very like many genuine Fish, yet they are, as even Aristotle perceived, genuine Mammals. By their whole internal structure—in so far as it has not become changed by adaptation to an aquatic life—they, of all known Mammals, are most closely allied to Hoofed animals, and more especially agree with them in the absence of the decidua and in the tufted placenta. Even at the present day the river-horse (Hippopotamus) constitutes a kind of transition form to the Sea Cows (Sirenia), and from this it seems most probable that the extinct primary forms of the Cetacea are most closely allied to the Sea Cows of the present day, and that they developed out of Pair-hoofed animals, which were related to the hippopotamus. Out of the order of _Herbivorous whales_ (Phycoceta)—to which the sea cows belong, and which accordingly, very probably, contain the primary forms of the legion—the other order of _Carnivorous whales_ (Sarcoceta) appears to have developed at a later period. But Huxley thinks that these latter were of quite a different origin, and that they arose out of the Carnaria through the Seals. Among the Sarcoceta, the extinct gigantic Zeuglodonta (Zeugloceta)—whose fossil skeletons some time ago excited great interest, it being thought that they were “sea serpents”—are probably only a peculiarly developed lateral branch of genuine whales (Autoceta), which comprise, besides the colossal whalebone whales, the cachalot or spermaceti whales, dolphins, narwhals, porpoises, etc.

The third legion of the Indeciduata, or Sparsi-placentalia, comprises the strange group of the animals _poor in teeth_ (Edentata); it is composed of the two orders of burrowers and sloths. The order of _Burrowers_ (Effodientia) consists of the two sub-orders of _ant eaters_ (Vermilinguia), to which the scaled animals also belong, and the _girdle animals_ (Cingulata), which were formerly represented by the gigantic Glyptodons. The order of _Sloths_ (Tardigrada) consists of the two sub-orders of the small, still living _dwarf sloths_ (Bradypoda), and of the extinct unwieldy _giant sloths_ (Gravigrada). The enormous fossil remains of these colossal herbivora suggest that the whole legion is becoming extinct, and that the Edentata of the present day are but a poor remnant of the mighty order of the diluvial period. The close relations between the still living South American Edentata and the extinct gigantic forms which are found beside the latter on the same part of the globe, made such an impression upon Darwin on his first visit to South America, that they even then suggested to him the fundamental idea of the Theory of Descent. (See above, vol. i. p. 134.) But it is precisely the genealogy of this legion which is most difficult. The Edentata are perhaps nothing but a peculiarly developed lateral branch of the Ungulata; but it may also be that their root lies in quite another direction.

We now leave the first main group of Placental animals, the Indeciduata, and turn to the second main group, namely, the Deciduata, or animals with decidua, which are distinguished from the former by possessing a deciduous membrane, or decidua, during their embryonal life. We here meet with a very remarkable small group of animals, for the most part extinct, and which probably were the old tertiary (or eocene) ancestors of man. These are the Semi-apes, or Lemurs (Prosimiæ); these curious animals are probably the but little changed descendants of the primæval group of Placentalia which we have to consider as the common primary form of all Deciduata. They have hitherto been classed together in the same order with Apes which Blumenbach called Quadrumana (four-handed). However, I regard them as entirely distinct from these, not merely because they differ from all Apes, much more than do the most different Apes from one another, but also because they comprise most interesting transitional forms leading to the other orders of Deciduata. I conclude from this that the few still living Semi-apes, which moreover differ very much among one another, are the last surviving remnants of a primary group now almost extinct, but which was at one time rich in forms, and out of which all the other Deciduata (possibly with the single exception of Beasts of Prey, and Pseudo-hoofed animals) have developed as diverging branches. The old primary group of Semi-apes has probably developed out of Handed or Ape-footed Marsupials (Pedimana), which are surprisingly like them in the transformation of their hinder feet into grasping hands. The primæval primary forms themselves (which probably originated in the eocene period) are of course long since extinct, as are also the greater portion of the transition-forms between them and all the other orders of Deciduata. However, individual remnants of the latter are preserved among the Semi-apes of the present day. Among these, the remarkable Finger-animal of Madagascar (Chiromys madagascariensis) constitutes the remnant of the group of the Leptodactyla and the transition to Rodents. The strange flying lemur in the South Sea and Sunda islands (Galeopithecus), the only remnant of the group of Pteropleura, forms a perfect intermediate stage between Semi-apes and Bats. The long-footed Semi-apes (Tarsius, Otolicnus) constitute the last remnant of that primary branch (Macrotarsi) out of which the Insectivora developed. The short-footed forms (Brachytarsi) are the medium of connection between them and genuine Apes. The Short-footed Semi-apes comprise the long-tailed Lemur, the short-tailed Lichanotus, and the Stenops, the latter of which seems to be very closely allied to the probable ancestors of man among the Semi-apes. The short-footed as well as the long-footed Prosimiæ live widely distributed over the islands of southern Asia and Africa, more especially in Madagascar; some live also on the continent of Africa. No Semi-ape, either living or in a fossil state, has as yet been found in America. They all lead a solitary, nocturnal kind of life, and climb about on trees. (Compare vol. i. p. 361.)

Among the six remaining orders of Deciduata, all of which are probably derived from long since extinct Semi-apes, the order of _Gnawing animals_ (Rodentia), which is rich in forms, has remained at the lowest stage. Among these the _squirrel-like_ animals (Sciuromorpha) stand nearest akin to the Pedimanous Marsupials. Out of this primary group the _mouse-like_ animals (Myomorpha) and the _porcupine-like_ animals (Hystricomorpha) developed probably as two diverging branches, the former of which are directly connected with the squirrel-like animals, by the eocene Myoxida, the latter by the eocene Psammoryctida. The fourth sub-order, the _hare-like_ animals (Lagomorpha), probably developed only at a later period out of one of the other three sub-orders.

Very closely allied to the Rodentia is the remarkable order of _Pseudo-hoofed animals_ (Chelophora). Of these there now live but two genera, indigenous to Asia and Africa, namely, Elephants (Elephas), and Rock Conies (Hyrax). Both have hitherto generally been classed among real Hoofed animals, or Ungulata, with which they agree in the formation of the feet. But an identical transformation of nails or claws into hoofs occurs also in genuine Rodentia and in certain hoofed Rodentia (Subungulata) which live exclusively in South America. Beside smaller forms (for example, guinea pigs and gold hares) the Subungulata also include the largest of all Rodentia, namely, the Capybara Rats, which are about four feet in length. The Rock Conies, which are externally very nearly akin to Rodents, especially to the hoofed Rodents, were formerly classed among Rodentia by some celebrated zoologists, as an especial sub-class (Lamnungia). Elephants, on the other hand, when not classed among Hoofed animals, were generally considered as the representatives of a special order which were called Trunked animals (Proboscidea). But the formation of the placentas of Elephants and of Hyrax agree in a remarkable manner, and are entirely distinct from those of Hoofed animals. These latter never possess a decidua, whereas Elephants and Hyrax are genuine Deciduata. Their placenta is indeed not of the form of a disc, but of a girdle, as in the case of Animals of Prey; it is very possible that the girdle-shaped placenta is but a secondary development of the discoplacenta. Thus, then, it might be thought that the Pseudo-hoofed animals have developed out of a branch of the Rodentia, and in a similar manner perhaps the Carnivora out of a branch of the Insectivora. At all events, Elephants and Hyrax in many respects, especially in the formation of important skeletal parts, of the limbs, etc., are more closely allied to the Rodentia, and more especially to hoofed Rodentia, than to genuine Hoofed animals. Moreover several extinct forms, especially the remarkable South American Arrow-toothed animals (Toxodontia), stand in many respects midway between Elephants and Rodentia. That the still living Elephants and Hyrax are but the last survivors of a group of Pseudo-hoofed animals, which was once rich in forms, is proved not only by the very numerous fossil species of Elephants and Mastodon (some of which are even larger, others also much smaller than the Elephants of the present day), but also by the remarkable miocene _Dinotheria_ (Gonyognatha), between which and their next kindred, the Elephants, there must be a long series of unknown connecting intermediate forms. Taking all things into consideration, the most probable hypothesis which can be established at present as to the origin and the relationship of Elephants, Dinotheria, Toxodon, and Hyrax is, that they are the last survivors of a group of Pseudo-hoofed animals rich in forms, which developed out of the Rodentia, and probably out of relatives of the Subungulata.

The order of _Insect Eaters_ (Insectivora) is a very ancient group, and is next akin to the common extinct primary form of the Deciduata, as well as to the Semi-apes of the present day. It has probably developed out of Semi-apes which were closely allied to the Long-footed Lemurs (Macrotarsi) of the present day. It is separated into two orders, Menotyphla and Lipotyphla; the Menotyphla are probably the older of the two, and are distinguished from the Lipotyphla by possessing an intestinal cœcum, or typhlon. The Menotyphla include the climbing Tupajas of the Sunda Isles, and the leaping Macroscelides of Africa. The Lipotyphla are represented in our country by shrew mice, moles, and hedgehogs. The Insectivora, in the formation of their jaws and their mode of life, are nearly akin to Carnivora, but are, on the other hand, by their discoplacentas and by their large seminal vesicles, allied to Rodents.

It is probable that the order of _Rapacious animals_ (Carnaria) developed out of a long since extinct branch of Insectivora, at the beginning of the Eocene period. It is a natural group, very rich in forms, but still of very uniform organization. The Rapacious animals are sometimes also called Girdle-placentals (Zonoplacentals), although the Pseudo-hoofed animals (Chelophora), in the same way, also deserve this designation. But as the latter, in other respects, are more closely allied to the Rodentia than to Carnaria, we have already discussed them in connection with the former. Animals of prey are divided into two, externally very different, but internally very closely related, sub-orders, namely, Land animals of prey and Marine animals of prey. The _Land animals of prey_ (Carnivora) comprise bears, dogs, cats, etc., whose pedigree can be approximately guessed at by means of many extinct intermediate forms. The _Marine animals of prey_, or _Seals_ (Pinnipedia), comprise sea bears, sea dogs, sea lions, and walruses. Although marine animals of prey appear externally very unlike land animals of prey, yet by their internal structure, their jaw and their peculiar girdle-shaped placenta, they are very nearly akin to them, and have evidently originated out of a branch of them, probably out of a kind of weasel (Mustelina). Even at the present day the fish otters (Lutra), and still more so the sea otters (Enhydris), present a direct form of transition to Seals, and clearly show how the bodies of land Carnivora are transformed into the shape of a Seal, by adaptation to an aquatic life, and how the steering fins of marine rapacious animals have arisen out of the legs of the former. The latter consequently stand in the same relation to the former as do the Whales to Hoofed animals among the Indeciduata. In the same way as the river-horse at present stands midway between the extreme branches of oxen and sea oxen, the sea otter still forms a surviving intermediate stage between the widely separated branches of dogs and sea dogs. In both cases the complete transformation of the external form, consequent upon adaptation to entirely different conditions of life, has not been able to efface the solid foundation of the inherited internal peculiarities.

According to Huxley’s opinion, which has already been quoted, only the Herbivorous Whales (Sirenia) are derived from Hoofed animals; on the other hand, the Carnivorous Cetacea (Sarcoceta) are derived from the marine animals of prey; the Zeuglodonts would form a transition between the two latter. But in this case it would be difficult to understand the close anatomical relations which exist between the Herbivorous and Carnivorous Cetacea. The strange peculiarities in the internal and external structure which so strikingly distinguish the two groups from all other mammals would then have to be regarded only as _analogies_ (caused by the same kinds of adaptation), not as _homologies_ (transmitted from a common primary form). The latter, however, strikes me as being by far the more probable, and hence I have left all the Cetacea among the Indeciduata as one group of kindred origin.

The remarkable order of _Flying Mammals_, or _Bats_ (Chiroptera), stands near to the Carnaria as well as to the Insectivora. It has become strikingly transformed by adaptation to a flying mode of life, just as marine animals of prey have become modified by adaptation to a swimming mode of life. This order probably also originated out of the Semi-apes, with which it is even at present closely allied, through the flying lemurs (Galeopithecus). Of the two orders of flying animals, the insect-eating forms, or _flying mice_ (Nycterides), probably developed out of those eating fruits, or _flying foxes_ (Pterocynes); for the latter are, in many ways, more closely allied to Semi-apes than are the former.

We have now still to discuss the genuine Apes (Simiæ) as the last order of Mammals; but as, according to the zoological system, the human race belongs to this order, and as it undoubtedly developed historically out of a branch of this order, we shall devote a special chapter to a more careful examination of its pedigree and history.