Class Mammalia

Mammals (mamma, breast), of which there are about 15,000 species, are perhaps less spectacular anatomically than reptiles or birds, but in spite of this fact they have come to occupy a dominant place among the vertebrates. Mammals emphasize a new note of cooperation not discoverable to any great extent in the universal competition that elsewhere characterizes creation.

The first mammals to appear on the earth were small and insignificant contemporaries of the gigantic reptiles of the Mesozoic Age. As long as huge carnivorous dinosaurs held sway, the small ancestral mammals, which were probably largely arboreal in habit, kept out of the way and bided their time. Perhaps they hastened that time somewhat by feeding upon the eggs of their terrifying enemies while eluding capture themselves. At any rate it is certain that in the long struggle for a “place in the sun,” it has been wits rather than brute force that has enabled mammals to out-distance competitors.

No doubt the mechanism which insures “warm-bloodedness,” that is, a constant bodily temperature independent of changes in the surrounding atmosphere, has much to do with the conquest of the earth by mammals. By reason of this important characteristic they have been able to establish themselves not only throughout temperate areas but even in dry deserts and frigid polar regions.

While the highly specialized birds have sacrificed everything to developing the power of locomotion by flight in air, mammals have chosen the better part of improvement along the line of the nervous system, especially in the considerable enlargement of the very important cerebral hemispheres. Achievement in this direction has undoubtedly been the greatest of all factors in determining the present supremacy of the mammalian type. An unusual amount of plasticity and versatility is exhibited among mammals. For example, they vary in size from a field mouse scarcely more than an inch in length, to whales which may attain a length of over 100 feet, or well over 1000 inches.

Mammals are variously fitted for successful life in such diverse habitats as on the land (deer); in water (otters); in burrows (gophers); underground (moles); in open oceans (whales); in forests (monkeys); and in the air (bats).

The most important characteristics of mammals are:

1. Milk Glands, providing food for the more or less helpless young for some time after birth. Masses of milk glands (breasts) have given us the name Mammalia.

2. Hair, almost as typical of mammals as feathers are of birds. Occurs at least in embryonic life.

3. Sweat Glands. One interpretation is that certain sweat glands became modified into milk glands.

4. Enucleate Erythrocytes. The red blood cells are without nuclei when freed into the blood stream. Nuclei, present as the individual cells form, are extruded from them while they are still in the blood-forming tissues.

5. Seven Cervical (Neck) Vertebrae, in all'except a few species.

6. Muscular Diaphragm, separating the thoracic cavity, containing lungs and heart, from abdominal cavity.

7. Differentiation of the Teeth into three major types (cutting incisors, piercing canines, and grinding molars and premolars), a condition foreshadowed in the therapsids, the early reptiles from which mammals arose.

Additional characteristics will be considered during the discussions of the various organ systems.

Living mammals may be arranged in seventeen orders, which fall into three subclasses: Prototheria, Metatheria, and Eutheria.

Subclass Prototheria

Order Monotremata. The prototheria (proto, first; ther, beast) comprise a single order, Monotremata, of which only three genera are living today, namely, Echidna, Proechidna, and Ornithorhynchus. The digestive, urinary, and genital systems empty into a cloaca through which all of them communicate with the outside by a single opening (mono, one; trema, opening).

Monotremes are curious exceptional mammals that lay relatively large yolk-laden eggs, from which the young are hatched instead of being born alive in ordinary mammalian fashion. Ornithorhynchus incubates its leathery-shelled eggs in a shallow nest of grasses, while Echidna forms a temporary pouch from a fold of the skin upon its belly. In this portable nest the newly-laid egg is placed and incubated until hatched and the helpless offspring kept through the precarious days of its early growth and development. The young Echidna is fed upon a nutritious substitute for true milk, secreted by the mother from modified sweat glands, which it licks up with its long tongue from tufts of hair on the belly of the mother. No nipples are present and if they were the baby monotreme would not be able to suck, since its lips are prolonged into a horny toothless beak, not at all fitted for the muscular operation of sucking, but useful later for poking into ant-hills after food.

In Ornithorhynchus (Fig. 52) the beak is large and flattened, giving rise to the name of “duckbill” for this creature, a name all the more appropriate because it lives much of the time in water and has feet with webbed toes like those of a duck. The “incredible duckbill” is a native of South Australia and Tasmania.

The duckbill, Ornithorhynchus, an Australian monotreme. The spiny anteater, Echidna, a monotreme

Echidna, the spiny anteater (Fig. 53), is found in Australia, Tasmania, and New Guinea.

Proechidna, distinguished from Echidna by an unusually long snout that gives it a “ridiculous resemblance to a miniature elephant,” is confined to New Guinea.

Subclass Metatheria

Order Marsupialia. The metatheria (meta, after; ther, beast), comprising the single order marsupialia, are primitive, or possibly degenerate, mammals whose young, bom prematurely in an extremely helpless condition, are fed upon true milk and carried about in a permanent brood pouch, or marsupium.

At first the young vestpocketed marsupials are unable to exercise the necessary muscular effort involved in sucking, and are securely attached in a passive way to the nipple by means of a sphincter-like mouth (Fig. 54), while the female expresses milk from her mammary glands down the throat of the helpless fetus by the contraction of the abdominal muscles. Later on, as development advances, the young marsupial draws its milk in the orthodox way.

A young marsupial, Sminthopis, with its snout gripped by the cup-like marsupial pocket

Osborn, in The Age of Mammals, catalogues 76 genera of marsupials of which 37 are extinct. The living ones, excepting the opossums Didelphys of North, Central, and South America, and Caenolestes of Central America, are confined to the Australian region. Extinct Eocene genera ranged over what is now Europe, as well as both Americas and Australasia.

It is considered probable that the origin and spread of marsupials occurred before the ancient land bridge that joined Australia to South America had disappeared. Those forms which became isolated at that time in the marsupial Ark of Australia were afterwards able to continue their handicapped existence with comparative success, since they were not brought into competition with the true mammals that developed later on the other great continental areas. Man’s introduction of cats, dogs, and especially rabbits into Australia has probably doomed many, if not all, species of monotremes and marsupials of that continent to extinction. As in America, a few species may be able to survive.

It is a striking fact that not only all the native mammals of Australia were monotremes and marsupials, but also that the latter became diversified in much the same way as true mammals into different types adapted to various habitats. The “koala” is a bear-like form. Species resembling wolves, hyenas, cats, rabbits, jumping mice, woodchucks, moles, flying squirrels, and mice are also found in this Order. The kangaroos, Macropus; bandicoots, Perameles; and opossums, Didelphys and Coenolestes, are marsupials that suggest cousins among the true mammals to a less degree.

Some of these marsupials are represented in sketches in Figure 55.

Marsupials

Subclass Eutheria

The eutheria (eu, true; theria, beasts) include all the other mammals, frequently termed “placentals” because they are characterized by the presence of a placenta, formed where the capillary-laden allantois comes into intimate contact with the richly vascularized wall of the uterus. Here interdigitations of the allantois and uterine wall bring the embryonic and maternal blood streams close together. Across the thin membranes which separate the two blood streams oxygen and food pass into the blood of the embryo, and waste products from the blood of the embryo to that of the mother. In this manner the placenta forms a functional connection between the mother and offspring throughout the long preparatory life before birth.

Arranged according to the degree of specialization which they exhibit from the most generalized to the most aberrant forms, the orders of living placentals are: Insectivora, Dermoptera, Chiroptera, Carnivora, Rodentia, Primates, Edentata, Pholidota, Tubulidentata, Perissodactyla, Artiodactyla, Proboscidea, Hyracoidea, Sirenia, and Cetacea.

A word of identification and comment about each of these orders of true mammals, with examples of a few representative genera, is essential in rounding out a roll call of the vertebrates.

Insectivora

The insectivores (vorare, to eat) subsist largely upon insects, hence their name. They are mostly small, sharp-snouted animals with leanings towards nocturnal or subterranean life. They include among other genera: the European hedgehog, Erinaceus; the moles (Fig. 56b) of which the common mole of the eastern United States, Scalopus, and the peculiar star-nosed mole, Condylura, as well as Scapanus of the Pacific Coast, are American genera; also the shrews, Sorex, and the short-tailed Blarina, both North American representatives. A few, the tree shrews (Fig. 56a), have retained the primitive mammalian habit of life in the trees. In a number of respects the anatomy of these tree shrews indicates that this group is not far removed from the insectivore stock which was probably ancestral to the Primates. Osborn names a total of 45 fossil and 34 living genera of insectivores.

Insectivores

Dermoptera

The dermopterans, which are without fossil representation, are set aside in an independent Order, although it consists of but a single Genus, Galeopithecus (Fig. 57), the “flying lemur” of the Malay region that is an anatomical connecting link between insectivores and bats.

Flying lemur of Madagascar, Galeopithecus

Chiroptera

The chiropterans (chir, hand; pter, wing), or bats, are mammalian aviators, rather helpless when not on the wing, which fly about at twilight by means of enormously elongated webbed fingers. While the erratic flight of bats is by no means as sustained as the more powerful flight of birds, yet, aided by extremely responsive sense organs, these creatures are unsurpassed in avoiding obstacles while hawking insects in the crepuscular traffic of semi-darkness.

The food of bats in general, and of North American bats in particular, consists practically of insects caught on the wing (Fig. 58). In the Old World tropics the habit of eating fruit has developed on the part of certain large bats called “flying foxes,” that live upon figs, guavas, and similar soft fruits.

An insectivorous bat, Synotus

Another aberrant adaptation in the chiropteran type is presented by the blood-sucking vampires of Central and South America, that have a highly modified saclike stomach for the storage of blood which they gorge from some unwilling host. Aside from the unpleasantness of their blood-sucking habits, these bats are under suspicion because they may be agents for the transfer of vicious blood parasites.

Myotis is the common harmless brown bat of cosmopolitan distribution. Of 63 genera of bats in Osborn’s list only three are extinct.

Carnivora

The carnivores (cam, flesh) have specialized in alertness and brains. They are keen, swift, athletic, gladiatorial killers, feeding by preference upon the flesh of other animals. Their place in the general scheme of things seems to be to keep within limits the prolific rodents that in their absence tend to overrun everything. They also have by their aggressive ways served in the course of evolution as schoolmasters for other mammals, particularly the primates to which man belongs, by stimulating self reliance and resourcefulness. The carnivores are not particularly modified except that the clavicles (collar bones) are reduced or missing and the teeth are somewhat specialized.

They are represented by 73 living and 113 extinct genera in Osborn’s list. The suborder FISSIPEDIA (fissi, split; ped, foot) is made up principally of land animals, while the suborder Pinnipedia (pinna, fin) comprise fish-eating carnivores that have become secondarily modified for aquatic life.

Fissipede carnivores

Among the better known fissipede carnivores (Fig. 59) are the following: Canis, dogs, wolves, and coyotes; Ursus and Thalassarctos, bears; Felis, cats, lions, and their kind; Cynaelurus, the “cheetah” of India, a kind of cat without retractile claws; and the hyenas, foxes, raccoons, weasels, minks, ermines, skunks, and others.

Among pinnipede carnivora (Fig. 60) are the sea-lions, seals, and walrus.

Pinnipede carnivores

Rodentia

The rodents, or gnawing animals, with 101 living and 61 fossil genera, are the most numerous of all living mammals, particularly as they make up in number of individuals what they lack in size. Their strong, deep-set incisor teeth, which grow continuously throughout life, are self-sharpening chisels. The hard enamel of this type of tooth, confined to the anterior surface, does not wear away as readily as the softer dentine which makes up the bulk of the tooth. Thus a chisel-like, sharp cutting edge of enamel is always maintained. They are prevailingly plant eaters and form an important link in nature’s chain, since they hand on the sun’s energy, stored by green plants, to the carnivores by which they are devoured.

The rodent bloc (Fig. 61) in the Congress of Mammals is represented by the following: Lepus, hares; Sciurus, squirrels; Cavia, guinea pigs; Sciuropterus, flying squirrels; Mus, rats and mice; and the beavers, muskrats, porcupines, prairie dogs and woodchucks. By some authorities the rabbits and hares are placed in a separate Order because of their extra pair of incisors and other evidence that they have independently acquired the gnawing habit.

Representative rodents

This group is of great economic importance. On the negative side are their destructive habits and possibly disease transmission, such as the carrying of bubonic plague by fleas which may inhabit the fur of rats. On the other hand, several species furnish us with fur or meat and several have been of great value for laboratory experimentation and tests of serums and other material.

Primates

The primates, including 39 living genera of lemurs, monkeys, apes, and mankind, while not so highly specialized in many ways with regard to bodily structure as the several orders yet to be mentioned, stand first in the vertebrate class with respect to brain development. Most of the 23 fossil genera of this order are lemurs, which probably dwelt in trees just as their modern representatives in the forests of Madagascar do to this day. There is considerable evidence that the primates arose from a primitive, tree-dwelling Insectivora stock. Practically all species living today are arboreal, with skeletons not greatly modified from the condition of primitive mammals. Their teeth, also, are comparatively unspecialized but tend to be reduced in number from the 11 pairs of teeth on each jaw of primitive placentals. The brain and eyes become highly developed, the cerebral hemispheres increasing in size until they completely cover the rest of the brain in higher primates.

Although many travel on all fours, their arboreal habit has resulted in an upright sitting posture that made possible the development of a pair of handy hands and admission to the Manual Training School of the Treetops. This was the beginning of a wedgelike vista of possibilities at the broad end of which lies the intellectual life. Organs of defense, like scales, claws, horns, and hoofs, are not needed by primates since wits take their place.

The three suborders are: Lemuroidea, Tarsioidea and Anthropoidea.

The lemurs (Fig. 62), very primitive, arboreal primates not greatly different from the tree shrews, are found chiefly in Madagascar but with some species in Africa and Southern Asia. These small animals have an elongate snout and pointed ears. They are covered with a heavy coating of hair. Their big toes and thumbs are set apart from the other digits. Their long tail is not prehensile.

Smiths dwarf lemur, Microcebus smithii

Tarsius (Fig. 63), the only living tarsioid genus, is intermediate between the other two primate suborders. A reduction of the olfactory organs and shortening of the snout have permitted a shifting of the eyes forward into a position for binocular vision, although neither eyes nor cerebrum is as well developed as in the monkeys. Like the lemurs they have pointed ears.

Tarsius spectrum

In the Anthropoids (Fig. 64) the cerebral hemispheres are greatly enlarged and richly convoluted. The eyes are highly developed for increased clarity of vision. The New World monkeys, usually called Platyrrhines (platy, flat; rhin, nose) have broad, flat noses with the external openings directed somewhat laterally. Their thumbs are usually reduced. Their tails are in most cases long and prehensile. Probably the best known member of this group is Cebus, the Capuchin monkey, famous as the companion of the hand-organ man.

Anthropoid Primates

The Old World Anthropoids, known as the Catarrhines because the external nares are close together and usually open downward (cata, down), include three general types: the various African monkeys, macaques, and baboons; the anthropoid apes (belonging to the Family Simiidae); and man. These animals have opposable thumbs. Their teeth are reduced in number to 32. The tail is non-prehensile and frequently greatly reduced. Macacus is the lively little monkey so often seen serving time behind the bars in zoological gardens and menageries. The rhesus monkey, Macacus rhesus, is used extensively in experimental work. The importance of this animal in work on the so-called Rh factor of the blood led to the use of the first two letters of Rhesus as the symbol for the factor.

The tailless Simians include four genera (Fig. 64). The gibbon (Hylobates) is a long-armed, arboreal form of small size, found in the Malay region and neighboring islands. The orang-utan (Simia), a native of Sumatra and Borneo, is also a long-armed arboreal type but is considerably larger than the gibbons. The chimpanzee (Anthropopith ecus) and the gorilla (Gorilla), both African forms, have relatively shorter arms than the gibbon or orang. The chimpanzee, primarily an arboreal form, spends some of its time on the ground. The gorilla, essentially a ground dweller, is the largest and most powerful of all apes. Both chimpanzee and gorilla exhibit some degree of intelligence and reasoning power of the human type. Of the attempts to educate these apes, the most successful ones have been those using chimpanzees. They can be taught to wear clothing, dine at a table, ride a bicycle, act in moving pictures, smoke a pipe, expectorate with precision, and perform many other acts characteristically human.

Modern man of whatever race or color belongs zoologically to a single genus and species, Homo sapiens, Linn., although the name “wise man” is more appropriate for some individuals than for others. Although similar to Simiidae in most respects, man differs from them in some structural details. He is more erect, has shorter arms and relatively larger thumbs. The big toes of the apes are opposable but in man these toes are in line with the others. While the simians have powerful teeth set in heavy jaws, man has smaller front teeth and tooth-bearing regions of the jaws. Consequently man has a chin. The supraorbital ridges over the eyes are much less pronounced in man. The human cerebrum is distinctly larger, in part due to the considerable development of the frontal lobes ,where the speech centers are located. Room for these anterior lobes is provided by a raised front part of the skull which gives the higher forehead region. These differences are chiefly associated with man’s mental development, speech and ground-dwelling habits.

The fossil representatives of man will be considered in a later chapter. Together with modern man all are included in the family Hominidae.

Edentata

The edendates (Fig. 65c and d) are rather degenerate mammals, either toothless as the name implies, or with poor chalky teeth. Their center of distribution is South America, for all of the 15 living genera are found there, although the anteaters, Tamandua, Myrmecophaga, and Cyclopes, extend as far north as Central America and Mexico, and the nine-banded armadillo, Dasypus novemcincta, even into Texas. Of the 34 genera of fossil edentates, four are from North America, three from both North and South America, and twenty-five exclusively from South America. Thus it is evident that the center of distribution of this order is South America.

Representatives of the edentate Orders

The topsy-turvy sloths, Brady pus (three-toed), and Choelepus (twotoed), are well named because of their sluggish habits. They are awkward defenseless creatures clothed with coarse gray hair and equipped with long hooklike claws which enable them to hang upside down in the branches of tropical trees, where they depend upon their resemblance to motionless masses of “gray-beard mosses” for protection from their carnivorous foes. Sidney Smith said of them: “Sloths live suspended, and sleep suspended, and in fact, pass their whole lives in a state of suspense, like a young curate when he is distantly related to a bishop.”

Of the armadillos, besides the nine-banded one already mentioned, there is the six-banded Dasypus sexcinctus of Paraguay and Brazil, and the three-banded Tolypeutes. Some of these can roll up in their scaly armor like “pill bugs” when danger threatens, presenting a hard nut for any predaceous foe to crack.

The extinct giant armadillo, Glyptodon, which was encased in an armor without bands that would enable it to roll up, was, however, so well protected that, like a war tank, it did not need to imitate a pill bug. In spite of their armor these giants became extinct and their fossil remains are an eloquent memorial to the fact that something more than passive resistance is necessary in order to maintain a species on the earth.

Pholidota

The pholidotes include two genera, Manis and Pholidotus, the pangolins or scaly anteaters (Fig. 65a) of Africa and Southern Asia. They have elongate heads with long, very protrusible tongues with which they pick up termites. No teeth are present. Their bodies are covered with large, horny, overlapping scales. Powerful, sharp claws are present, especially on the front feet. Although long grouped with the edentates, these animals are more properly placed in a separate order for geographical as well as anatomical reasons.

Tubulidentata

The tubulidendates, formerly placed among the edentates, include a single species of the genus Orycteropus, the practically hairless aardvark of the South African Boers (Fig. 65B). This animal has the long snout, long tongue and powerful claws of a termite eater. Its teeth, without enamel, have a perforated dentine.

Perissodactyla

The perissodactyls (perissos, odd-numbered; dactyl, finger, toe) are large, herbivorous ungulates (ungula, hoof) which walk on the hoofed tips of an odd number of digits. Their molariform teeth have large chewing surfaces usually bearing high cross-ridges which greatly increase the effectiveness of the teeth in grinding grass, leaves and other vegetable matter. In all of these animals, the main axis of each appendage passes through the third digit, which therefore carries most of the weight.

The tapirs (Fig. 66b) of South America and the Malay region have probably retained more of the general features of the ancestral stock of the Order than have any other members. They have four toes on the front feet (thumbs missing) and three on the hind feet (big and little toes missing). Their practically complete dentition includes relatively generalized molariform teeth. Among their specializations is a short proboscis formed by a slight elongation of upper lip and nose.

Perissodactyl ungulates

The rhinoceros (rhino, nose; ceros, horn) group (Fig. 66c and d) includes several species of large, awkward animals with three toes on the hind feet and three or four on the front ones. On the top of the snout, in the mid-line, they have either a single horn or two, one behind the other. Each horn is really a mass of hairs stuck together.

The horses (Fig. 66a) are characterized by the reduction of their digits until only one (the third) is in contact with the ground while two others (the second and fourth) are small splints at the base of the functional toe. All living species of horses, including their close relatives the asses and zebras, belong to the genus Equus. These animals have perhaps the most definitely traced pedigree of all mammals. Their record goes all the way back by successive links to Eohippus, a small Eocene ancestor about the size of a fox that had four toes on the front feet and three on the hind feet, as do present-day tapirs and some rhinoceroses.

Artiodactyla

The artiodactyls (artios, even-numbered) are large, herbivorous ungulates with molariform teeth similar to those of perissodactyls. In these animals, however, the main axis of the leg passes between the third and fourth digits which usually bear equally the weight of the animal. The first digit (thumb or big toe) has disappeared and usually the second and fifth digits are considerably reduced.

Non-ruminant artiodactyls

The pigs (genus, Sus), peccaries, wart hogs of Africa, and hippopotami use all four toes and have simple stomachs and no horns (Fig. 67). They are not limited to a diet of plants but will also eat flesh.

Representative ruminants

All the other artiodactyla are ruminants (ruminare, to chew over again), so called because they first swallow their food whole, then regurgitate it at their leisure, and thoroughly chew it (Figs. 68 and 69). The small balls of food returned to the mouth are known as cuds. Strictly herbivorous, they have three- or four-chambered stomachs adapted to their cud-chewing habits. They use only their third and fourth digits. Many are provided with defensive horns, either hollow and permanent, like those of the cow, or solid and periodically shed and renewed like the antlers of a stag. Among the better known members of this group are: the camels and dromedaries (Camelus); the South American llamas; the giraffes (Giraffa); the deer, elks, moose, and similar forms; sheep (Olds); goats (Capra); and domestic cattle (Bos).

Horned ruminants

Ogden Nash has described the common cow as follows:

“The cow is of the bovine ilk,
One end is moo, the other milk.”

The artiodactyls and perissodactyls are of great economic importance to man. They furnish us with portions of our clothing, transportation, food - including milk and much of our meat-and serums so important in fighting disease.

Proboscidea

The proboscideans are the elephants, largest of living land animals, which are so bulky that they are obliged to walk stiff-legged in order to support their tremendous weight (Fig. 70). The heavy head is sustained horizontally by a short stout neck, and the rigidity brought about by this arrangement, as well as by the stiff uncompromising pillar-like legs, is compensated by the development of a “trunk,” a combination of the nose and the upper lip enormously drawn out into a flexible prehensile organ (proboscis). Each of their five digits terminates in a hoof-like structure. Their molar if orm teeth reach the extreme of development of the grinding cross-ridges. Their upper incisors are greatly enlarged into long tusks.

Proboscidea

Some proboscideans, such as the mastodons and the hairy mammoths of the Ice Ages, became extinct in comparatively recent times, geologically speaking, while other less specialized ancestors, as Dinotherium of Europe and Asia, and Palaeomastodon of Egypt, are considerably more ancient.

There are two genera of living proboscideans: Elephas, the small-eared Asiatic elephant; and Loxodonta, the large-eared African variety. The former species has been domesticated in India for a long time. One famous individual, “Jumbo,” weighed 6,5 tons and was 11 feet high. For years this gigantic beast entertained thousands of children, young and old, under Barnum as impressario. Jumbo’s monumental skeleton stands in the American Museum of Natural History in New York.

Hyracoidea

The hyracoideans are coneys or “rock rabbits,” which, according to the Book of Proverbs “are but a feeble folk, yet they make their homes in the rocks.” They are small cud-chewing animals, superficially resembling guinea pigs, with hooflike tips to their toes, of which there are four on each front foot and three on each hind foot. They were probably derived from the same basic stock as the proboscideans and the sirenians. They include two genera: Dendrohyrax (Fig. 71) of Africa, and Procavia of Syria and Arabia.

African coney, Dendrohyrax

Because of certain anatomical similarities the last four Orders just mentioned (Perissodactyla, Artiodactyla, Proboscidea, and Hyracoidea) may be artificially grouped together as the ungulates or hoofed mammals. Hoofs have probably appeared independently in the different orders. For the most part they are large, rather stolid, plant-feeding creatures, most at ease when standing up on their highly specialized feet which are adapted for bearing continuous weight by being encased in shoelike hoofs. Unlike the soft-footed carnivores that collapse into a reposeful recumbent posture at every opportunity only to spring into alert activity upon the slightest incentive, ungulates never sit down and do not lie down without considerable deliberation and effort.

The 73 genera of ungulates include many kinds of great utility to man. They have also played a notable role in the past history of the world, as evidenced by the fact that 204 genera of fossil ungulates are known, many more than of any other order of mammals. These numbers are no doubt due in part to the readiness with which this group of animals has left fossil evidence of a former existence.

Sirenia

The sirenians, although of very different external appearance, have certain unmistakable anatomical affiliations with elephants and vegetarian ungulates. They are large clumsy water animals having a broad snout covered with sparse coarse bristles and an otherwise hairless skin. The anterior legs are modified into swimming flippers, while the hind legs are entirely absent.

They are perhaps the animals that have furnished the slender basis of fact from which imaginative sailors from time immemorial have spun tales of mythical mermaids and sirens. A less romantic but more apt common name for them is “sea cows.”

Atlantic sea cow or manatee, Trichechus. Dugong, or Indian Ocean sea cow, Halicore

Of this order only two genera (Figs. 72 and 73) are represented by living animals, that are separated from each other on the globe about as far as it is possible, since “manatees” representing the genus Trichechus inhabit the rivers of the northeastern coast of South America and beyond as far north as the Everglades of Florida; while the “dugong,” Halicore, lives in the Red Sea and Indian Ocean.

Of the 7 fossil genera, one, Rhytina stelleri, or Steller’s sea cow, has been extinct less than 200 years. This species first became known in 1741 when Steller, a Russian whaler, was shipwrecked upon a small group of islands in Bering Sea. He was saved from starvation by finding a rookery of these large sea cows upon which he and his crew fed until rescued. During the following quarter of a century Russian whalers with human greed and stupidity hunted these valuable food animals to extinction, for Nordenskiold, who visited the islands in 1768, reported that the last individual of the colony had been killed. This species of sea cow has never been found elsewhere.

Cetacea

The cetaceans, or whales and their allies, among which are to be found the largest animals that ever lived, include the leviathans of the oceans. The ancestry of the cetacea is a puzzle for the solution of which fossils give scanty aid. Comparative anatomy shows that they bear unmistakable hallmarks of mammalian forbears, such as breathing air by means of lungs and feeding the young upon milk. Since the mammalian plan undoubtedly originated with land forms, cetaceans must have undergone profound modih cation in order to become adapted secondarily to a marine existence where their great weight could be supported in water. By reason of their warm bloodedness and a thick blanket of heat-retaining blubber under the skin, these gigantic animals are able to pursue their activities even in Arctic waters.

While in the act of nursing, which is obviously accomplished under difficulties, the young whale presents a curious resemblance to a small tug attached to the side of an ocean liner (Fig. 74).

Humpback whale suckling her young

Whales may be grouped into two suborders; odontoceti, or toothed whales that feed primarily upon fishes, and mystacoceti, or whalebone whales which, by means of a peculiar brushlike device of “whalebone” in the cavernous mouth cavity, strain out and swallow countless myriads of microscopic ocean inhabitants, that constitute for them a nutritious “sea soup” of unlimited supply.

The toothed whales (Fig. 75) are usually not of extraordinary size and frequently forage about in their watery hunting grounds in schools. Some of them are: Delphinus, the dolphin; Phocaena, the porpoise; Grampus, the grampus; Orca, the killer; Monodon, the narwhal, with a single enormous twisted tooth projecting horizontally in front like a pikestaff; Physeter, the sperm whale, with teeth only in the lower jaw; and Hyperoodon, the bottlenosed whale. The last two attain considerable size.

Odontoceti, or toothed whales

Examples of the giant whalebone whales are: Rachianectes, the gray whale; Balaenoptera, the blue or sulphur-bottomed whale; Megaptera, the humpbacked whale; and Balaena, the right whale (Fig. 76). The blue whale is reputed to reach a length of 100 feet and a weight of perhaps 150 tons.

Right whale, Balaena

There are 9 genera of fossil cetaceans, and 27 genera of living ones, some of which are becoming scarce because they have been so relentlessly hunted by man.

Speaking of whales, John Godman a century ago wrote the following perfect apology for those travelers who return from foreign lands with tall stories to tell to those who stay at home: “Large as the size of the whale certainly is, it has been much overrated; for such is the avidity with which the human mind receives communications of the marvellous, and such the interest attached to those researches which describe any remote and extraoidinary production of nature, that the judgment of the traveller 1 eceives a bias, which, in case of doubt, induces him to fix upon that extreme point in his opinion which is calculated to afford the greatest surprise and interest.”