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.
[Illustration: OPOSSUM MOTHER AND YOUNG
Photo, American Museum of Natural History]
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.
[Illustration: ANTEATER, WHICH LIVES ON INSECTS CAUGHT IN THE STICKY
SALIVA OF ITS LONG TONGUE]
[Illustration: SLOTH, AN ANIMAL WHICH KEEPS TO TREES AND IS ALMOST
HELPLESS ON THE GROUND]
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.
[Illustration: A KANGAROO MOTHER
Showing young carried in the abdominal pouch]
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.