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 Hoofed animals. | Sections of Hoofed Animals. | Families of Hoofed Animals. | Systematic Name of the Families. | ||||||||
| I. Odd-toed Hoofed Animals Ungulata Perissodactyla |
| I. Primary Hoofed Animals.* Prochela |
| 1. | Lophiodonta | 1. | Lophiodontia* | ||||
| 2. | Pliolophida | 2. | Pliolophida* | ||||||||
| II. Tapir-shaped Tapiromorpha |
| 3. | Primary Odd-hoofs | 3. | Palæotherida* | ||||||
| 4. | Lama-tapirs | 4. | Macrauchenida* | ||||||||
| 5. | Tapirs | 5. | Tapirida | ||||||||
| 6. | Rhinoceroses | 6. | Nasicornia | ||||||||
| 7. | Rhinoceros-horses | 7. | Elasmotherida* | ||||||||
| III. Single-hoofs Solidungula |
| 8. | Primæval horses | 8. | Anchitherida* | ||||||
| 9. | Horses | 9. | Equina | ||||||||
| II. Pair-toed Hoofed Animals Ungulata Artiodactyla |
| IV. Pig-shaped Chœromorpha |
| 10. | Primary Pair-hoofs | 10. | Lophiodontia* | ||||
| 11. | Primæval pigs | 11. | Anthracotherida* | ||||||||
| 12. | Pigs | 12. | Setigera | ||||||||
| 13. | River horses | 13. | Obesa | ||||||||
| 14. | Primæval ruminants | 14. | Xiphodontia* | ||||||||
| V. Ruminating animals Ruminantia |
| A. Stag-shaped Elephia |
| a. |
| 15. | Primæval deer | 15. | Dremotherida* | ||
| 16. | Pseudo musk deer | 16. | Tragulida | ||||||||
| b. |
| 17. | Musk deer | 17. | Moschida | ||||||
| 18. | Deer | 18. | Cervina | ||||||||
| c. |
| 19. | Primæval giraffes | 19. | Sivatherida* | ||||||
| 20. | Giraffes | 20. | Devexa | ||||||||
| B. Hollow-horned Cavicornia |
| d. |
| 21. | Primæval gazelles | 21. | Antilocaprina* | ||||
| 22. | Gazelles | 22. | Antilopina | ||||||||
| e. |
| 23. | Goats | 23. | Caprina | ||||||
| 24. | Sheep | 24. | Ovina | ||||||||
| 25. | Oxen | 25. | Bovina | ||||||||
| C. Pad-footed Tylopoda |
| 26. | Lamas | 26. | Auchenida | ||||||
| 27. | Camels | 27. | Camelida | ||||||||
