CHAPTER XXVII
THE BEASTS OF THE FIELD--SOME PRIMITIVE TYPES
We have now arrived at the highest rank in the scale of animal life--the four-footed, hair-clad, milk-nursed denizens of our woods and fields--the subclass Mammalia, mammals.
These are the "animals" of popular speech, but accuracy requires a more distinctive expression, for every living thing not a plant is an "animal." Unfortunately no such distinctive term exists in our language, and hence we must borrow from the Latin the word "mammal" for this group. It is correct, easy to remember, and there is no reason why it should not be used popularly as well as scientifically. It is good, because it is exact, and expresses the one great distinction which separates mammals from all other animals--the feeding of the young on milk secreted by the mother. The milk-producing glands were called in Latin "mammæ," whence our word "mammal" and the technical term Mammalia--animals that suckle their young.
Another peculiarity of the group is the coat of hair--persistently growing threads of horny substance produced from the skin in greater or less abundance and of varying quality and color. Its chief purpose appears to be that of keeping the body warm; and, as in the case of the feathers clothing birds, it enables the blood to rise to and maintain a temperature much higher than that of the air; hence the mammals are "warm-blooded." This condition, gradually acquired, stimulated their activity and hence their brain development, the result of which is a higher degree of intelligence than is manifested, as a class, by any other animals, and a moving cause of their progress to the highest plane of organic evolution.
The history of the evolution of the Mammalia may be traced back to obscure beginnings in the Triassic, the oldest of the three divisions of the Secondary or Mesozoic era. Just preceding that time there flourished a group of reptiles, the Theromorpha, whose skull, teeth, and forelimbs were very like those of a modern beast of prey; and zoölogists consider it "altogether probable" that the origin of the mammalian branch must be looked for among their number. It is not doubted, however, that true mammals, although very small and inconspicuous, existed throughout the whole Mesozoic era, despite the fact that the world at that time was filled with ravenous reptiles. Indeed, it is believed that their steady development was an important agency in destroying the reptile population, largely by eating their eggs. At any rate, before the end of the Mesozoic era the two grand divisions of Mammalia, Prototheria and Eutheria, had become established; and also the two primary divisions of the latter, the Marsupials and the Placentals, had been separated. Then came that extraordinary change in the physiography of the globe that marked the end of Mesozoic conditions and introduced those of the succeeding era named Tertiary. In the broader and higher land areas and the drier and more invigorating climate that followed, producing a vegetation tending constantly to become like that of the present, mammals found increasingly favorable conditions, and became the dominant race of animals.
RELICS OF PRIMITIVE TYPES
There live in Australia and New Guinea two curious little animals that most nearly represent in their low and generalized organization the primitive mammals, and differ so essentially from all other mammals that they are classed by themselves as Prototheria ("first beasts"). They are the duckbill (Ornithorhynchus) and the spiny anteater (Echidna). The duckbill is a small, softly furred, web-footed creature, as aquatic in its habits as a beaver, which finds its food in the worms and other things that live in and on the mud of its chosen stream, and digs a burrow in the bank for its home, where it stays most of the daylight hours, and where its young are born. Its special peculiarity is that instead of the muzzle and mouth of an ordinary mammal, it is furnished with a bill like that of a duck, and each jaw is armed with horny plates to do the work of teeth; in the young ones true molar "milk" teeth are present, but are soon shed. The cheeks contain pouches in which a quantity of food can be stored, the animal carrying it in to the safety of its burrow to be eaten, and so avoiding the danger of being out for a long time of feeding.
The echidnas are equally small, about eighteen inches long, covered with a mingled coat of hair and strong spines, and mounted on short legs and feet armed with powerful claws, for this animal dwells on land, and not only burrows, but must tear to pieces the hills of the ants that form its only food. Its round little head terminates in a long, slender snout containing a ribbon-shaped tongue with which it licks up the ants from their ruined nests.
The striking peculiarity of both these queer creatures, however, is the fact that they lay eggs. These are few--sometimes only one--and recall those of reptiles in their relatively large size, parchmentlike shells, and abundance of food-yolk. The duckbill deposits her eggs in her grass-lined burrow nest and covers them with her body until they quickly hatch. The blind and naked young then apply their lips to the nearest part of the mother's abdomen, and suck milk through the pores of the skin. In the echidna one sees a little advance on this extremely simple beginning of nursing; for here, instead of being laid in a burrow nest, and covered by the mother, the echidna's egg is placed by the mother within two parallel folds of skin which at that season form a deep groove in the abdomen inclosing the nursing area, and is held there until it hatches. When the young has attained a certain size the mother removes it from the "pouch," but takes it in from time to time to suckle it.
Such are the Prototheria--one of the grand divisions of Mammalia, set apart by reason of their laying the eggs from which the young will afterward be born, whereas in the other division or Eutheria ("proper mammals") the "embryos," or unborn young, escape from the eggs in a less or greater degree of development before their birth from the mother. This period between the conception of life in the egg and its emergence at birth is called the period of gestation, and is much longer in large animals than in small ones. Fundamental differences in method of birth divide the Eutheria into two groups, designated as Nonplacentals and Placentals.
THE MARSUPIALS
The word marsupial means "pouched," and refers to the most characteristic peculiarity of the nonplacental division (order Marsupialia), which is the possession of a more or less pocketlike fold in the skin of the abdomen of the females within which the extremely immature young are nourished.
The egg-laying mammals, also nonplacental, have the young inclosed in a protective shell that they keep warm, as do the birds, until the embryo is sufficiently matured to be safely born. In the marsupials nature meets the difficulty another way. The embryo is but little advanced when born, in fact it is utterly helpless and minute, being, even in the case of the largest kangaroos, hardly as big as a mouse. It would be fatal, of course, to turn it loose upon the world; and therefore the mother is provided with the pouch already described.
The instant an embryo is born the mother picks it up and places it within the pouch, where it crawls about until it touches and instinctively takes hold of one of the threadlike teats. As it gets stronger it leaves the pouch now and then, but returns to it for nursing, sleeping, and protection when alarmed, until finally it departs altogether.
This description applies to the most advanced families of the order. In the oldest and most generalized families of marsupials, such as the banded anteaters, there is virtually no pouch at all.
As almost the whole marsupial tribe are natives of Australasia, it is odd that the family with which we must begin a list of them--the true opossums--should be American, and quite unknown in Australia. This is explainable when it is known that this family (Didelphidæ) is the most archaic of this ancient tribe, and was well established in Cretaceous times, and then and later was widely distributed in Europe and on this continent; yet so little change has occurred in the race that teeth from the Laramie formations of Wyoming are hardly distinguishable from those in the jaws of our 'possum-up-a-gum-tree to-day. No wonder the quaint creature is hoary and wrinkled; he is a very Methuselah among mammals, and looks it! All opossums seem to have disappeared from Europe before the close of the Miocene, but continued to survive numerously in South America. They probably owe their long career, in competition with animals of so much higher grade, to their small size, forest life, nocturnal habits, ability to eat all sorts of food, and, most of all, to their great fecundity. Our common opossum is the most northern of its kind, and ranges over the whole country as far north as the latitude of Lake Erie; it appears never to have crossed the Hudson River until comparatively recent times, but is now frequently met with in New England and on Long Island. It is at home in all sorts of places, except, perhaps, on the dry plains, for it is primarily an arboreal animal, aided in climbing about trees by its naked, prehensile tail, by which it may hang to a branch while using its forefeet to rob a bird's nest or gather fruit. It will eat anything it can get hold of, and with its sharp teeth, which number fifty, will kill animals as large as itself; hence it is a destructive raider of henroosts and sitting birds as well as a seeker of mouse nests and insects.
Opossums are amazingly prolific, and have broods of a dozen or more in many cases. These often crawl on the mother's back, and cling with claws and twisted tails to her fur and tail, and so are carried about. Burdened by these kittens she hunts daily--or rather at night, for the most part--and defends them savagely and bravely against foxes and other enemies, often successfully standing off the farmer's dogs. With a family to defend, or when faced by any foe that is at all equal to its powers, the opossum does not resort to "playing 'possum," for this is a last resource when surprised and "cornered" by an overwhelming danger that it can neither avoid nor cope with.
The proverbial feigning of death by this animal (many other small animals do the same) has excited much popular interest, and has received many explanations. I have suggested that it is a survival of a practice which in past ages had been an advantageous ruse of the ancestors of the opossums.
Several other species of opossums exist in Central and South America, some much smaller than ours and one hardly bigger than a mouse. One kind, the "yapock," is aquatic, dwelling on land only during the infancy of its progeny, and until they are old enough to be taught to swim. All the marsupials inhabiting the Americas (except a rare little molelike one in Patagonia), belong in the family Didelphidæ; but this family is not known in Australia, where the so-called "opossums" belong to a different tribe. They were named after our common northern opossum, which was known to science before Australia and its pouched fauna were discovered.
One of the extraordinary things in zoölogy is that Australia, and the near-by islands that constitute with it a faunistic province, has no indigenous mammals (except a few mice and bats) other than marsupials, which have become so diversified as to represent the varied kinds of animals seen elsewhere; and no marsupials live anywhere else in the world except our single and primitive American family. This curious situation has caused much discussion. It is known that in late Mesozoic times marsupials were scattered all over the globe, but became exterminated everywhere outside of Australasia and America long before the present era. The Australian marsupials are supposed to be the survivors, flourishing in a favorable region; but why no other mammals survived there is still a puzzle. Another theory is that Australia, regarded as formerly a part of a much larger southern continent, is the original center from which the ancestors of the Marsupialia spread, but failed to maintain their race outside of their original home, with which South America was then connected.
The most archaic of these marsupials is the celebrated Tasmanian "wolf," or thylacine, which resembles in size and shape a pointer dog, but with a longer muzzle, and that long tail which seems to be a general characteristic of the Marsupialia. It is brownish gray, with a row of darker bands crossing the hinder half of the back, and is one of the most swift-footed and savage hunters in the world. It is confined to Tasmania, where it became so destructive to sheep when the island was settled that it was killed off until almost exterminated. This island was the home, also, of another smaller beast, looking somewhat like a wolverine with the head of a hyena, which was so morose, savage, and untamable that the settlers named it "Tasmanian devil," and destroyed it as rapidly as they could. It hid by day in some rock den and made its forays at night. This truly diabolic creature belonged to the family of dasyures, which is represented in Australia by several small, predatory beasts called "native cats." They fill the rôle there of our northern martens and weasels, and most of their time is passed in trees, although some are fond of hunting amid rocks and brush. They like to come about ranches and villages, where they are the pest of poultry keepers, but are rarely domesticated, even partly. Another carnivorous group (phascogales) contains the "pouched mice," which are not mouselike, except in size, but have more the nature of shrews that live in trees and hunt birds and any small creatures they can catch.
Of the phalangers a curious specimen is the wombat, named "native bear" by the early colonists--an animal about the size of our woodchuck, shaped like a miniature bear, and living mainly on roots, which it digs at night with its powerful claws; its thick fur makes its skin valuable in market. Related to it structurally, but much like our gray squirrel in shape, and having an even longer and more bushy tail, is the charming sugar squirrel, which dwells in trees, and sails in long flights from tree to tree in the twilights and on moonlight nights just as do our flying squirrels; there are also tree phalangers so small they are called "flying mice." Other tree-living phalangers are the "opossums" of Australia, whose soft gray pelts are exported in great numbers to foreign fur markets.
The kangaroos and wallabies (Macropodidæ) represent the highest development of the marsupial type, and number some fifty species spread over all Australia and New Guinea. While the majority inhabit open grassy plains, others brushy districts and rocks, and a few dwell in trees, the kangaroos proper include half a dozen of the largest kinds, the commonest of which is the great gray "boomer" or "forester," of the colonists, often seen in menageries. It stands four to five feet tall, with a tail thirty to thirty-six inches long; but this size is considerably exceeded by that of the red or woolly kangaroo, of eastern and southern Australia. Furthermore, fossil remains show that in the Pleistocene era kangaroos far bigger than even these existed there in numerous extinct species--one, for instance, whose skull alone measured nearly a yard in length. These animals take the place in Australia of the deer of northern countries. They are very gregarious, and are always to be met with in droves. Each drove frequents a certain district and has its particular camping and feeding grounds. The animal has a dreadful weapon of defense in the powerful hind claw, which it can use like the tusk of a boar.
The smaller kangaroos are called "wallabies," or brush kangaroos, and frequent scrub jungle and rocky places. These furnish most of the skins and leather sent to European markets and, like the big species of the plains, have been greatly reduced in numbers by hunters and sheep herders. Some of them are confined to the rough deserts and mountains, where they jump about the rocks with astonishing agility. One small genus includes the swift harelike species that resemble our jack rabbits in habits; and there are also the "dorca" kangaroos, which are arboreal in habit and handsomely colored. Another group are ratlike in form, colors, and manners, running rather than leaping, and dwelling among scrub and grass, scratching the ground all day in search of the roots upon which they feed, and making havoc in the frontiersman's potato patches. Several kinds have prehensile tails, which they use apparently only to carry to their underground homes the long grass of which they make their beds. They associate in connected burrows like a rabbit warren.
In the varied forms and functions they present, as beasts of prey, as grazers or root diggers, as ground-running, tree-climbing, burrowing or cave-haunting forms, some solitary and slow, others agile and gregarious, the marsupial tribe in its isolated corner of the earth exhibits an epitome of the whole mammalian world. It shows in a conspicuous way how the necessity and habit of making a living in varied circumstances, and exposed to lively competition, restricting every species to a particular manner, brings about a suitable modification of structure.
THE EDENTATA--ANTEATERS, SLOTHS, AND ARMADILLOS
At the base of the great division of Eutherian mammals, to which belong all that remain to be described, is found the order Edentata ("toothless"), whose modern representatives are few and unimportant in comparison with those of past ages, when gigantic ground sloths, armored glyptodons, and other fossil species flourished in a luxuriant world. The name is not well chosen, for many of these animals possess at least a few teeth, but always composed of vasodentine and not coated with enamel. Although the origin of this race is obscure, it was certainly far in the past, for its characters are archaic in many particulars, and its members are often far separated in structure, and also in their geographical distribution. Two families belong to the Old World, one in the Orient and another in South Africa, but all the other edentates are American. The Oriental one includes most of the "pangolins," or scaly anteaters, which are covered from head to foot in a coat of mail formed of overlapping horny plates, and can roll themselves into a ball that will defy any jaws not big enough to tear them to pieces; while the African family consists of the naked, long-nosed aard-vark ("ant bear"), which burrows in the ground, and cuts its way at night into the mud forts of termites and other ants in search of its favorite food. These two ancient creatures differ so much in their anatomy from the American edentates that they are classified by some naturalists in a separate order (Fodentia); and they differ almost as radically from one another.
It should not be surprising to find most of the modern edentates in South America, since that is the most ancient and unchanged of all the continents; but a few sorts of anteaters, sloths, and armadillos alone remain where once their race, in its heroic age, dominated the world of its time. The puny survivors look and act like the relics they are. The "great" anteater, or tamandua, standing eighteen inches or more in height, has flatfooted, bearlike hind feet, and short forelegs that end in huge claws bent under, or backward, so that the animal walks on the outer face of its toes. Its tail is a great bushy mass of hair with which the animal may cover itself as with a blanket, and its long neck tapers off into a head with a very long nose and little room for brains. The big claws are not used for burrowing an underground home, but for digging up the nests of ants and termites which it licks up with its long, sticky tongue. When one realizes the enormous colonies of ants in the tropics it is not amazing that so large an animal should subsist exclusively on these minute creatures. The claws are formidable weapons of defense also, the animal throwing itself on its back and defying the foe, or rising on its hind legs and giving a tearing, bearlike hug that even a man might well fear. This is a slow-moving creature, more fond of open country than forests; but a smaller tamandua belongs wholly to the woods and spends both days and nights in the tree tops, tearing open the burrows and nests of arboreal insects and devouring their inhabitants and their stores of honey and young. A third species is the rare little yellow two-toed anteater of the Isthmus region, which appears to live almost wholly on wasp grubs.
Much like these in organization are the two species of sloth, hairy creatures that hang all day long by their long, muscular limbs and two or three curved claws, underneath a branch of the tree through whose top they slowly creep about at night, collecting, crushing with their peglike teeth, and swallowing the leaves that constitute their fare. Their long hair, naturally gray, becomes green by accumulating a coating of minute plants that thrive on it, and this helps to conceal the sloths amid the foliage, yet they are killed by eagles and by all sorts of beasts of prey, against which they have no means of defense. These listless creatures are the degenerate descendants of a very long ancestry. The early Tertiary rocks of Argentina contain the bones of small slothlike animals that apparently were ground dwellers and must have been active diggers. Later that region became filled with larger ground sloths, apparently their descendants, that are believed to have browsed on bushes and trees; and some of these became the megatheres of the late Tertiary, which were as big as elephants. Similar giants inhabited North America.
Even in the earliest days known to paleontologists the anteater-sloth group had become well separated from their fellow edentates, the armadillos, arguing a far-preceding origin. In the later Tertiary the latter type developed such huge and heavily armored forms as the glyptodon, on whose bony shell the teeth of even the great saber-toothed tigers of the time could make little impression. These grotesque tortoiselike glyptodons, of which there was a great variety, were vegetable eaters, and some survived to a time so recent that there is evidence that they were finally killed off by human hunters. Beside them were smaller armadillos, more like the modern ones, which are armored with overlapping belts of horny material between which coarse hairs sprout; but the amount of this armor varies greatly among the several species scattered from Patagonia to northern Mexico. In some it is a continuous shell, in others it consists of several belts, in still others is nearly absent. Armadillos are carnivorous, digging out worms, grubs and the underground nests of wasps, catching insects of all sorts, stealing eggs and young from ground-nesting birds, killing serpents by leaping on them and sawing their bodies in two by means of the rough edges of their plates. In some places on the pampas armadillo burrows are so numerous as to make riding dangerous.