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 ftal 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, cla.s.s 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_.

---------------------------+-------------------+-------------------+------------------- | _Cloacal Animals_ | _Pouched Animals_ |_Placental Animals_ _Three Sub-Cla.s.ses_ | MONOTREMA | MARSUPIALIA | PLACENTALIA _of_ | _or_ | _or_ | _or_ _Mammals._ | ORNITHODELPHIA | DIDELPHIA | MONODELPHIA ---------------------------+-------------------+-------------------+------------------- 1. Cloaca formation | Constant | Embryonal | Embryonal | | | 2. Nipples of the pectoral | Wanting | Existing | Existing glands, or milk | | | warts | | | | | | 3. Fore collar bones, | United | Not united | Not united or clavicles, grown | | | together in the middle, | | | with the breast bone, | | | and forming a forked | | | bone | | | | | | 4. Marsupial bones | Existing | Existing | Wanting | | | 5. _Corpus callosum_ of | Feebly | Feebly | Strongly developed the brain | developed | developed | | | | 6. Placenta | Wanting | Wanting | Existing ---------------------------+-------------------+-------------------+-------------------

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 a.s.sumed 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 palaeotheria-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 _Palaeotheria_, fossils of which occur in the upper Eocene and lower Miocene. Out of the Palaeotheria, at a later period, the rhinoceroses (Nasicornia) and rhinoceros-horses (Elasmotherida) on the one hand, and the tapirs, lama-tapirs, and primaeval horses, on the other, developed as two diverging branches. The long since extinct primaeval horses, or Anchitheria, formed the transition from the Palaeotheria 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 s.p.a.ce 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_ (Chromorpha) 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 primaeval 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 Primaeval 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. Palaeotherida*

{ { 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. Primaeval 8. Anchitherida*

{ _Solidungula_ { horses { { 9. Horses 9. Equina

{ { 10. Primary 10. Anoplotherida*

{ { Pair-hoofs { { 11. Primaeval 11. Anthracotherida*

{ IV. Pig-shaped { pigs { _Chromorpha_ { 12. Pigs 12. Setigera { { 13. River horses 13. Obesa { { 14. Primaeval 14. Xiphodontia*

{ { ruminants { { { { {15. Primaeval 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. Primaeval 19. Sivatherida*

+Artiodactyla+ { Ruminating { { { giraffes { animals { { {20. Giraffes 20. Devexa { _Ruminantia_ { { { { {21. Primaeval 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

Oxen Giraffes | | | Sheep Deer | | | | | | | | | --v---/ Goats ---v---/ Musk deer Horses | | | | _Equi_ | | | | | ---v-----/ Antelopes | | Camels | | | ----v-----/ and Lamas Intermediate horses | | Deer-shaped _Tylopoda_ _Hippariones_ -----v------/ _Elaphia_ | | Hollow-horned | | | _Cavicornia_ | | | | | | | | | | Primaeval horses -------v--------/ | _Anchitherida_ Primaeval deer | | _Dremotherida_ | | | | =Single Hoofers= | | +Solidungula+ --v------------------/ | =Ruminating Animals= | +Ruminantia+ | Sea-oxen | Tapirs | _Sirenia_ | _Tapirida_ | | River-horses | | Lama-tapirs | | _Obesa_ | | _Macrauchenida_| | | | | | | | | Pigs | | | | ----v-----/ _Setigera_ | --------------v--/ | | | | | | | | | Rhinoceros-horses | | | | | _Elasmotherida_ | | | | | | | | ------v-------/ | | | | Primaeval pigs | Rhinoceruses | | | _Anthracotherida_ | _Nasicornia_ | ---v-----/ | | | | | | Primaeval ruminants | | | | _Xiphodontia_ ---v---/ | | | | | | | | | ------v------/ --------v--------/ =Primary Pair-hoofs= =Primary Odd-hoofs= +Anoplotherida+ +Palaeotherida+ | | | | ------------v-------------/ _Prochela_ Primary-hoofed-animals (_Lophiodontia_ and _Pliophida_) | | (Hoofed marsupials? _Barypoda?_)

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) const.i.tutes 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 (Prosimiae); these curious animals are probably the but little changed descendants of the primaeval group of Placentalia which we have to consider as the common primary form of all Deciduata. They have hitherto been cla.s.sed 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 primaeval 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) const.i.tutes 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) const.i.tute 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 Prosimiae 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 cla.s.sed 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 cla.s.sed among Rodentia by some celebrated zoologists, as an especial sub-cla.s.s (Lamnungia). Elephants, on the other hand, when not cla.s.sed 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 cc.u.m, 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 _a.n.a.logies_ (caused by the same kinds of adaptation), not as _h.o.m.ologies_ (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 (Simiae) 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.

CHAPTER XXII.

ORIGIN AND PEDIGREE OF MAN.

The Application of the Theory of Descent to Man.-Its Immense Importance and Logical Necessity.-Man's Position in the Natural System of Animals, among Disco-placental Animals.-Incorrect Separation of the Bimana and Quadrumana.-Correct Separation of Semi-apes from Apes.-Man's Position in the Order of Apes.-Narrow-nosed Apes (of the Old World) and Flat-nosed Apes (of America).-Difference of the two Groups.-Origin of Man from Narrow-nosed Apes.-Human Apes, or Anthropoides.-African Human Apes (Gorilla and Chimpanzee).-Asiatic Human Apes (Orang and Gibbon).-Comparison between the different Human Apes and the different Races of Men.-Survey of the Series of the Progenitors of Man.-Invertebrate Progenitors (Prochordata) and Vertebrate Progenitors.

Of all the individual questions answered by the Theory of Descent, of all the special inferences drawn from it, there is none of such importance as the application of this doctrine to Man himself. As I remarked at the beginning of this treatise, the inexorable necessity of the strictest logic forces us to draw the special deductive conclusion from the general inductive law of the theory, that Man has developed gradually, and step by step, out of the lower Vertebrata, and more immediately out of Ape-like Mammals. That this doctrine is an inseparable part of the Theory of Descent, and hence also of the universal Theory of Development in general, is recognized by all thoughtful adherents of the theory, as well as by all its opponents who reason logically.

But if the doctrine be true, then the recognition of the animal origin and pedigree of the human race will necessarily affect more deeply than any other progress of the human mind the views we form of all human relations, and the aims of all human science. It must sooner or later produce a complete revolution in the conception entertained by man of the entire universe. I am firmly convinced that in future this immense advance in our knowledge will be regarded as the beginning of a new period of the development of Mankind. It can only be compared to the discovery made by Copernicus, who was the first who ventured distinctly to express the opinion, that it was not the sun which moved round the earth, but the earth round the sun. Just as the _geocentric conception_ of the universe-namely, the false opinion that the earth was the centre of the universe, and that all its other portions revolved round the earth-was overthrown by the system of the universe established by Copernicus and his followers, so the _anthropocentric conception_ of the universe-the vain delusion that Man is the centre of terrestrial nature, and that its whole aim is merely to serve him-is overthrown by the application (attempted long since by Lamarck) of the theory of descent to Man. As Copernicus' system of the universe was mechanically established by Newton's theory of gravitation, we see Lamarck's theory of descent attain its causal establishment by Darwin's theory of selection. This comparison, which is very interesting in many respects, I have discussed in detail elsewhere.

In order to carry out this extremely important application of the Theory of Descent to man, with the necessary impartiality and objectivity, I must above all beg the reader (at least for a short time) to lay aside all traditional and customary ideas on the "Creation of Man," and to divest himself of the deep-rooted prejudices concerning it, which are implanted in the mind in earliest youth. If he fail to do this, he cannot objectively estimate the weight of the scientific arguments which I shall bring forward in favour of the animal derivation of Man, that is, of his origin out of Ape-like Mammals. We cannot here do better than imagine ourselves with Huxley to be the inhabitants of another planet, who, taking the opportunity of a scientific journey through the universe, have arrived upon the earth and have there met with a peculiar two-legged mammal called Man, diffused over the whole earth in great numbers. In order to examine him zoologically, we should pack a number of the individuals of different ages and from different lands (as we should do with the other animals collected on the earth) into large vessels filled with spirits of wine, and on our return to our own planet we should commence the comparative anatomy of all these terrestrial animals quite objectively. As we should have no personal interest in Man, in a creature so entirely different from ourselves, we should examine and criticise him as impartially and objectively as we should the other terrestrial animals. In doing this we should, of course, in the first place refrain from all conjectures and speculations on the nature of his soul, or on the spiritual side of his nature, as it is usually called. We should occupy ourselves solely with his bodily structure, and with that natural conception of it which is offered by the history of his individual development.

It is evident that in order correctly to determine Man's position among the other terrestrial organisms we must, in the first place, follow the guidance of the natural system. We must endeavour to determine the position which belongs to Man in the natural system of animals as accurately and distinctly as possible. We shall then, if in fact the theory of descent be correct, be able from his position in the system to determine the real primary relationship, and the degree of consanguinity connecting Man with the animals most like him. The hypothetical pedigree of the human race will then follow naturally as the final result of this anatomical and systematic inquiry.

Now if, by means of comparative anatomy and ontogeny, we seek for man's position in that Natural System of animals which formed the subject of the last two chapters, the incontrovertible fact will at once present itself to us, that man belongs to the tribe, or phylum, of the Vertebrata. Every one of the characteristics, which so strikingly distinguish all the Vertebrata from all Invertebrata, is possessed by him. It has also never been doubted that of all the Vertebrata the Mammals are most closely allied to Man, and that he possesses all the characteristic features distinguishing them from all other Vertebrata.

If then we further carefully examine the three different main groups or sub-cla.s.ses of Mammals-the inter-connections of which were discussed in our last chapter-there cannot be the slightest doubt that Man belongs to the Placentals, and shares with all other Placentals, the important characteristics which distinguish them from Marsupials and from Cloacals. Finally, of the two main groups of placental Mammals, the Deciduata and the Indeciduata, the group of Deciduata doubtless includes Man. For the human embryo is developed with a genuine decidua, and is thus absolutely distinguished from all the Indeciduata. Among the Deciduata we distinguish two legions, the Zonoplacentalia, with girdle-shaped placenta (Beasts of Prey and Pseudo-hoofed animals), and the Discoplacentalia, with disc-shaped placenta (all the remaining Deciduata). Man possesses a disc-shaped placenta, like all Discoplacentalia; and thus our next question must be, What is man's position in this group?

In the last chapter we distinguished the following five orders of Discoplacentalia: (1) Semi-apes; (2) Rodents; (3) Insectivora; (4) Bats; (5) Apes. The last of these five orders, that of Apes, is, as every one knows, in every bodily feature far more closely allied to Man than the four others. Hence the only remaining question now is, whether, in the system of animals, Man is to be directly cla.s.sed in the order of genuine Apes, or whether he is to be considered as the representative of a special sixth order of Discoplacentalia, allied to, but more advanced than, that of the Apes.