The _Fan-tailed_, or _Keel-breasted birds_ (Carinatae), which form the second sub-cla.s.s, comprise all living Birds of the present day, with the exception of those of the ostrich kind, or Rat.i.tae. They probably developed out of Feather-tailed Birds during the first half of the secondary period, namely, in the Jura or Chalk period, by the hinder tail vertebrae growing together, and by the tail becoming shortened. Only very few remains of them are known from the secondary period, and these moreover only out of the last section of it, namely, from the Chalk.

These remains belong to a swimming bird of the albatross species, and a wading bird like a snipe. All the other fossil remains of birds as yet known have been found in the tertiary strata.

The _Bushy-tailed_, or _Ostrich-like Birds_ (Rat.i.tae), also called _Running Birds_ (Cursores), the third and last sub-cla.s.s, is now represented only by a few living species, by the African ostrich with two toes, the American and Australian ostrich with three toes, by the Indian ca.s.sowary and the four-toed kiwi, or Apteryx, in New Zealand. The extinct giant birds of Madagascar (aepyornis) and the New Zealand Dinornis, which were much larger than the still living ostriches, also belong to this group. The Birds of the ostrich kind-by giving up the habit of flying, by the degeneration of the muscles for flying resulting from this, and of the breast bone which serves as their support, and by the corresponding stronger development of the hinder legs for running-have probably arisen out of a branch of the Keel-breasted birds.

But possibly, as Huxley thinks, they may be the nearest relations of the Dinosauria and of the Reptiles akin to them, especially of the Compsognathus; at all events, the common primary form of all Birds must be looked for among the extinct Reptiles.

CHAPTER XXI.

PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.

IV. MAMMALS.

The System of Mammals according to Linnaeus and Blainville.-Three Sub-cla.s.ses 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 cla.s.sification of organisms upon which naturalists have always agreed. One of these few undisputed points is the privileged position of the cla.s.s 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 circ.u.mstance that man himself is a member of this cla.s.s. 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 cla.s.s of Mammals. From this there directly follows the exceedingly important inference that man, by consanguinity also, is a member of this cla.s.s of animals, and has historically developed out of long since extinct forms of Mammals. This circ.u.mstance 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 cla.s.s of animals.

Older naturalists, especially considering the formation of the jaw and feet, divided the cla.s.s 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 Linnaeus distinguished the following eight orders: (1) Cetae (whales); (2) Belluae (hippopotami and horses); (3) Pecora (ruminating animals); (4) Glires (gnawing animals and rhinoceroses); (5) Bestiae (insectivora, marsupials, and various others); (6) Ferae (beasts of prey); (7) Bruta (toothless animals and elephants); (8) Primates (bats, semi-apes, apes, and men). Cuvier's cla.s.sification, which became the standard of most subsequent zoologists, did not rise much above that of Linnaeus. 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) Carna.s.sia (marsupials, beasts of prey, insectivora, and bats); (7) Quadrumana (semi-apes and apes); (8) Bimana (man).

The most important advance in the cla.s.sification 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-cla.s.ses 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-cla.s.s consists of the _Cloacal Animals_, or _Breastless animals_, also called _Forked animals_ (Monotrema, or Ornithodelphia).

This cla.s.s 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 cla.s.sed 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-cla.s.s, the Didelphia, developed later, probably in the Jura.s.sic 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 ca.n.a.l 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 r.e.c.t.u.m. 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 cla.s.s. 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-cla.s.s of Cloacal animals, but as an accidental character of adaptation distinguishing the last remnant of the cla.s.s 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-cla.s.ses 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-cla.s.ses-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-cla.s.s, like the preceding one, is evidently approaching its complete extinction, and the living members of the cla.s.s 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 succ.u.mbed 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-cla.s.s 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 cla.s.s 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-cla.s.s 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 a.n.a.logous transformations of the original fundamental form in the two sub-cla.s.ses 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 Jura.s.sic 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-cla.s.s 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-cla.s.s 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 { (Primaeval 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-cla.s.s 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. Ch.o.e.romorpha { _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= { _Prosimiae_ {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. Hystrich.o.m.orpha { {25. Hare species 25. Lagomorpha { { XI. Insect-eating {26. With a cc.u.m 26. Menotyphla { Animals {27. Without a 27. Lipotyphla { _Insectivora_ { cc.u.m { { XII. Flying Animals{28. Flying foxes 28. Pterocynes { _Chiroptera_ {29. Bats 29. Nycterides { { XIII. Apes { 30. Clawed apes 30. Arctopitheci { _Simiae_ { 31. Flat-nosed 31. Platyrrhinae { { 32. Narrow-nosed 32. Catarrhinae

=Man= +Homines+ Elephants | _Proboscidea_ | Bats | | _Nycterides_ Rock Conies | | | Marine animals of prey _Lamnungia_ | Narrow-nosed | _Pinnipedia_ | | _Catarrhinae_ | | | | | Flying foxes | ------v-----/ Flat-nosed | _Pterocynes_ | Pseudo-hoofed _Platyrrhinae_ | =Flying Animals= | _Chelophora_ | | +Chiroptera+ | | | | | | | ----v---/ | Land animals of prey | =Apes= | _Carnivora_ =Gnawing Animals= +Simiae+ | =Animals of Prey= +Rodentia+ | | +Carnaria+ | Fingered animals | Lemurs | | | _Leptodactyla_ | _Brachytarsi_ | | True | | | | -------v---------/ whales | | | | Insect eaters _Sarcoceta_ -----v----/ ---v----/ _Insectivora_ | | | | | ------------------v---------------------/ Sea cows Semi-apes _Sirenia_ _Prosimiae_ =Whales= =Deciduous Animals= +Cetacea+ +Deciduata+ | Poor in teeth | =Hoofed Animals= _Edentata_ | +Ungulata+ | | | | | -------v--------/ | =Indeciduous= | +Indeciduata+ | | | -------------v-------------/ =Placental Animals= +Placentalia+ Herbivorous marsupials | Carnivorous marsupials _Marsupialia botanophaga_ | _Marsupialia zoophaga_ | | | --------------v-----------------/ =Marsupial= +Marsupialia+ Beaked animals | _Ornithostoma_ | | | ---------v--------/ Primary mammals _Promammalia_ =Cloacal Animals= +Monotrema+

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 cla.s.s 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 primaeval, or Insectivorous Marsupials (Cantharophaga). It probably includes the primary forms of the whole legion, and possibly also those of the whole sub-cla.s.s. 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 Primaeval 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 Primaeval 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-cla.s.s; 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 Primaeval 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 Primaeval 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-cla.s.s 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-cla.s.ses; for the cla.s.s includes all the known mammalia, with the exception of Marsupials and Beaked animals. Man also belongs to this sub-cla.s.s, 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-cla.s.ses by many other peculiarities, thus more especially by the absence of marsupial bones, by the higher development of the internal s.e.xual 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 cla.s.sification (p. 246) of the most important characteristics of the three sub-cla.s.ses 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 princ.i.p.ally 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.