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

by Ernst Haeckel

PEDIGREE OF THE MAMMALIA

					Man Homines
		Elephants Proboscidea			│ │		Bats Nycterides
	Rock Conies Lamnungia	│ │			Narrow-nosed Catarrhinæ		│ │
	│ │ │ │	│ │ │ │			│ │ │ │		│ │ │ │	Marine animals of prey Pinnipedia
	Pseudo-hoofed Chelophora │ │			Flat-nosed Platyrrhinæ │ │ │	│ │ │ │ │		Flying foxes Pterocynes Flying Animals Chiroptera	│ │ │ │ │
	│ │ │			│ │ │	│ │ │		│ │ │	│ │ │
	Gnawing Animals │ │ │ │ │			Apes Simiæ │ │ │			│ │ │ │ │ │	Land animals of prey Carnivora Animals of Prey Carnaria
	│ │ │		Fingered animals Leptodactyla	│ │ │		Lemurs Brachytarsi	│ │ │	│ │ │
	│		│	│		│
True whales Sarcoceta							Insect eaters Insectivora
│	│			│			│
Sea cows Sirenia Whales Cetacea		Semi-apes Deciduous Animals Deciduata
│ │	Poor in teeth Edentata	│ │
Hoofed Animals Ungulata	│ │ │ │	│ │ │ │
│	│	│
Indeciduous Indeciduata		│ │ │
│		│
Placental Animals Placentalia
		Herbivorous marsupials Marsupialia botanophaga	│ │ │ │	Carnivorous marsupials Marsupialia zoophaga
		│	│ │	│
		Marsupial Marsupialia
	Beaked animals Ornithostoma	│ │ │
	│	│
	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 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.

Three Sub-Classes of Mammals.		Cloacal Animals Monotrema or Ornithodelphia	Pouched Animals Marsupialia or Didelphia	Placental Animals Placentalia or Monodelphia
1.	Cloaca formation	Constant	Embryonal	Embryonal
2.	Nipples of the pectoral glands, or milk warts	Wanting	Existing	Existing
3.	Fore collar bones, or clavicles, grown together in the middle, with the breast bone, and forming a forked bone	United	Not united	Not united
4.	Marsupial bones	Existing	Existing	Wanting
5.	Corpus callosum of the brain	Feebly developed	Feebly developed	Strongly 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 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

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