general for walking on the earth, but with vigorous and continued
steps. The forms of the articulations of their skeleton are,
consequently, strictly defined, which determines all their motions
with the most rigorous precision.
Some of them, however, by means of limbs considerably elongated and
extended membranes, raise themselves in the air; others have them so
shortened that they can move with facility in water only, though this
does not deprive them of the general character of the class.
The upper jaw in all of these animals is fixed to the cranium; the
lower jaw is formed of two pieces only, articulated by a projecting
condyle to a fixed temporal bone; the neck consists of seven
vertebræ; one single species excepted, which has nine; the anterior
ribs are attached before, by cartilage to a sternum consisting of
several vertical pieces; their anterior extremity commences in a
shoulder-blade, that is not articulated, but simply suspended in the
flesh, often resting on the sternum by means of an intermediate bone,
called a clavicle. This extremity is continued by an arm, a forearm,
and a hand, the latter being composed of two ranges of small bones
called the carpus, of another range called the metacarpus, and of
the fingers, each of which consists of two or three bones, termed
phalanges.
With the exception of the cetacea, the first part of the posterior
extremity in all animals of this class is fixed to the spine,
forming a girdle or pelvis, which, in youth, consists of three
pairs of bones, the ilium which is attached to the spine, the pubis
which forms the anterior part of the girdle, and the ischium, the
posterior. At the point of union of these three bones is situated the
cavity with which the thigh is articulated, to which, in its turn,
is attached the leg, formed of two bones, the tibia and fibula; this
extremity is terminated by parts similar to those of the hand, _i.
e._, by a tarsus, metatarsus, and toes.
The head of the mammalia is always, articulated by two condyles, with
the atlas, the first vertebra of the neck.
The brain is always composed of two hemispheres, united by a
medullary layer, called the corpus callosum, containing the
ventricles, and enveloping four pairs of tubercles, named the corpora
striata, or striated bodies, the thalami nervorum opticorum, or beds
of the optic nerves, and the nates and testes. Between the optic beds
is a third ventricle, which communicates with a fourth under the
cerebellum, the crura of which always form a transverse prominence
under the medulla oblongata, called the pons Varolii, or bridge of
Varolius.
The eye, invariably lodged in its orbit, is protected by two lids and
a vestige of a third, and has its crystalline fixed by the ciliary
processes--its sclerotic is simply cellular.
The ear always contains a cavity called the tympanum, or drum, which
communicates with the mouth by the Eustachian tube; the cavity itself
is closed externally by a membrane called the membrana tympani, and
contains a chain of four little bones, named the incus or anvil,
malleus or hammer, the os orbiculare or circular bone, and the stapes
or stirrup; a vestibule, on the entrance of which rests the stapes,
and which communicates with three semicircular canals; and finally a
cochlea, which terminates by one canal in the vestibule and by the
other in the tympanum.
Their cranium is subdivided into three portions; the anterior is
formed by the two frontal and ethmoidal bones, the middle by the two
ossa parietalia and the os ethmoides, and the posterior by the os
occipitis. Between the ossa parietalia, the sphenoidalis, and the os
occipitis are interposed the two temporal bones, part of which belong
properly to the face.
Their face consists of two maxillary bones, between which pass the
nostrils; the two intermaxillaries are situated before and the two
ossa palati behind them; between these descends the vomer, a bony
process of the os ethmoides; at the entrance of the nasal canal are
placed the ossa nasi; to its external parietes adhere the inferior
turbinated bones, the superior ones which occupy its upper and
posterior portion belonging to the os ethmoides. The jugal or cheek
bone unites the maxillary to the temporal bone on each side, and
frequently to the os frontis; finally the os unguis and pars plana
of the ethmoid bone occupy the internal angle of the orbit, and
sometimes a part of the cheek. In the embryo state these bones also
are much more subdivided.
Their tongue is always fleshy, connected with a bone called the
hyoides, which is composed of several pieces, and suspended from the
cranium by ligaments.
Their lungs, two in number, divided into lobes and composed of an
infinite number of cells, are always inclosed, without any adhesion,
in a cavity formed by the ribs and diaphragm and lined by the pleura;
the organ of voice is always at the upper extremity of the trachea;
a fleshy curtain, called the velum palati, establishes a direct
communication between their larynx and nasal canal.
Their residence on the surface of the earth rendering them less
exposed to the alternations of cold and heat, their tegument, the
hair, is but moderately thick, and in such as inhabit warm climates
even that is rare.
The Cetacea, which live exclusively in water, are the only ones that
are altogether deprived of it.
The young are nourished for some time after birth by a fluid (milk)
peculiar to animals of this class, which is produced by the mammæ at
the time of parturition, and continues to be so long as is necessary.
It is from the mammæ that this class derives its name, and being a
character peculiar to it, they distinguish it better than any other
that is external.
The variable characters which form essential differences among the
mammalia are taken from the organs of touch, on which depends their
degree of ability or address, and from the organs of manducation,
which determine the nature of their ailment, and are all closely
connected, not only with everything relative to the function of
digestion, but also with a multitude of other differences relating to
their intelligence.
The degree of perfection of the organs of touch is estimated by the
number and the pliability of the fingers, and from the greater or
less extent to which their extremities are enveloped by the nail or
the hoof.
A hoof which completely envelops the end of the toe blunts its
sensibility, and renders the foot incapable of seizing.
The opposite extreme is when a nail, formed of one single lamina,
covers only one of the faces of the extremity of the finger, leaving
the other possessed of all its delicacy.
The nature of the food is known by the grinders, to the form of which
the articulation of the jaws universally corresponds.
To cut flesh, grinders are required as trenchant as a saw and jaws
fitted like scissors, having no other motion than a vertical one.
For bruising roots or grains flat-crowned grinders are necessary,
and jaws that have a lateral motion; in order that inequalities may
always exist on the crown of these teeth, it is also requisite that
their substance be composed of parts of unequal hardness, so that one
may wear away faster than others.
Hoofed animals are necessarily herbivorous, and have flat-crowned
grinders, inasmuch as their feet preclude the possibility of their
seizing a living prey.
Animals with unguiculated fingers were susceptible of more variety;
their food is of all kinds, and independently of the form of their
grinders, they differ greatly from each other in the pliability
and delicacy of their fingers. There is one character with respect
to this which has immense influence on their dexterity and greatly
multiplies its powers; it is the faculty of opposing the thumb to the
finger for the purpose of seizing minute objects, constituting what
is properly called a hand, a faculty which is carried to its highest
perfection in man, in whom the whole anterior extremity is free and
capable of prehension.
These various combinations which strictly determine the nature of the
different mammalia have given rise to the following orders:
Among the unguiculated animals, the first is Man, who, in addition
to privileges of other descriptions, possesses hands at the anterior
extremities only, the posterior being designed to support him in an
erect position.
In the order next to man, that of the Quadrumana, we find hands at
the four extremities.
In another order, that of the Carnivora, the thumb is not free, and
can not be opposed to the anterior extremities.
Each of these orders has the three sorts of teeth, grinders, canini,
and incisors, or cutting teeth.
In the fourth order, that of the Rodentia, the toes differ but little
from those of the Carnivora, but there are no canine teeth, and the
incisors are placed in front of the mouth, and adapted to a very
peculiar sort of manducation.
Then come those animals whose toes are much cramped and deeply sunk
in large nails, which are generally curved; they have no incisors,
and in some the canines disappear, while others have none of any
description. We comprise them all under the title of Edentata.
This distribution of the unguiculated animals would be perfect and
form a very regular series were it not that New Holland has lately
furnished us with a little collateral one, consisting of animals with
pouches, the different genera of which are connected by a general
similarity of organization; some of them, however, in the teeth and
nature of their diet corresponding to the Carnivora and others to the
Rodentia, and a third to the Edentata.
The hoofed animals are less numerous, and have likewise fewer
irregularities.
The Ruminantia, by their cloven foot, the absence of true incisors in
their upper jaw, and their four stomachs, form an order that is very
distinct.
The remaining hoofed animals may all be united in a single order,
which I shall call Pachydermata or Jumenta, the elephant excepted,
which might constitute a separate one, and which is remotely
connected with that of the Rodentia.
In the last place we find those of the mammalia which have no
posterior extremities, whose piscatory form and aquatic mode of life
would induce us to form them into a particular class, were it not
that in everything else their economy is similar to that in which
we have them. These are the hot-blooded fishes of the ancients, or
the Cetacea, which, uniting to the vigor of the other mammalia the
advantage of being sustained by the watery element, present to our
wondering sight the most gigantic of animals.
ZOOLOGICAL ZONES
--SIR RICHARD OWEN
Organic life in its animal form is much more developed, and more
variously, in the sea than in its vegetable form.
Observations of marine animals have led to attempts at generalizing
the results; and the modes of enunciating these generalizations or
laws of geographical distribution are very analogous to those which
have been applied to the vegetable kingdom, which is as diversely
developed on land as is the animal kingdom in the sea. Certain
horizontal areas, or provinces, have been characterized by the entire
assemblage of animals and plants constituting their population, of
which a considerable proportion is peculiar to each province, and
the majority of the species have their areas of maximum development
within it.
Of such provinces of marine life, that much-lamented, far-seeing,
and genial philosopher, Edward Forbes, has provisionally defined
twenty-five.
The same physical conditions are associated with a certain similarity
between the animals of different provinces. Where those provinces are
proximate, such likeness is due to the identity or close affinity of
the species; but where the provinces are remote the resemblance is
one of analogy, and species of different genera or families represent
each other.
A second mode of expressing the ascertained facts of the geographical
distribution of marine animals is by tracts called Homoiozoic
Belts, bounded by climatal lines; which are not, however, parallel
with lines of latitude, but undulate in subordination to climatal
influences of warm or cold oceanic currents, relations of land to
water, etc. Of these belts Professor E. Forbes has defined nine: one
equatorial, with four to the north and four to the south, which are
mutually representative.
But the most interesting form of expression of the distribution of
marine life is that which parallels the perpendicular distribution of
plants. Edward Forbes, availing himself of the valuable results of a
systematic use of the dredge, first showed that marine animals and
plants varied according to the depth at which they lived in a manner
very analogous to the changes in the forms and species of vegetation
observed in the ascent of a tropical mountain. He has expressed these
facts by defining five bathymetrical zones, or belts of depth, which
he calls: 1. Littoral; 2. Circumlittoral; 3. Median; 4. Infra-median;
5. Abyssal.
The life-forms of these zones vary, of course, according to the
nature of the sea bottom; and are modified by those primitive or
creative laws that have caused representative species in distant
localities under like physical conditions--species related by analogy.
Very much remains to be observed and studied by naturalists
in different parts of the globe, under the guidance of the
generalizations thus sketched out, to the completion of a perfect
theory. But in the progress to this, the results can not fail to be
practically most valuable. A shell or a sea-weed, whose relations
to depth are thus understood, may afford important information or
warning to the navigator. To the geologist the distribution of marine
life according to the zones of depth has given the clew to the
determination of the depth of the seas in which certain formations
have been deposited.
By the light of these laws of geographical distribution we view
with quite a new interest the shells, corals, and sea-weeds of our
own shores. We trace the regions whence they have been invaded by
races not aboriginally belonging to our seas; we obtain indications
of irruptions of sea-currents of dates anterior to the present
arrangements of land and water. Thus, part of our marine fauna has
been traced back to the old Pliocene period, part to the somewhat
newer period of the red-crag, part to the still more recent glacial
period--all these being anterior to the constitution of the Celtic
Province, as it is now displayed.
The class of animals to which the restrictive laws of geographical
distribution might seem least applicable is that of birds: their
peculiar power of locomotion, associated in numerous species with
migratory habits, might seem to render them independent of every
influence, save those of climate and of food, which directly affects
the conditions of their existence. Yet the long-winged albatross
is never met with north of the equator; nor does the condor soar
above other mountains than the Andes. The geographical range of its
European representative, the strong-winged lammergeyer, is similarly
restricted. The Asiatic Phasianidæ and Pavonidæ are represented
by turkeys (Meleagris) in America; by the guinea-fowl (Numida
Agelastus, Phasidus) in Africa, and by the Megapodiæ or mound birds
in Australia. Several genera of finches are peculiar to the Galapagos
Islands; the richly and fantastically ornate birds of paradise are
restricted to New Guinea and some neighboring isles. Mr. Sclater, who
has contributed the latest summary of facts on the distribution of
birds, reckons seventeen families as peculiar to America and sixteen
families as peculiar to Europe, Asia, and Africa. Some species have a
singularly restricted locality, as the red-grouse (Tetrao scoticus)
to the British Isles; the owl-parrot (Nestor productus) to Philip
Island, a small spot near New Zealand.
When birds have wings too short for flight, we marvel less at their
restricted range; and particular genera of brevipennate birds have
their peculiar continents and islands. The long and strong-limbed
ostrich courses over the whole continent of Africa and conterminous
Arabia. The genus of three-toed ostriches (rhea) is similarly
restricted to South America. The emeu (Dromaius) has Australia
assigned to it. The continent of the cassowary (Casaurius) has been
broken up into Sumatra, Java, Banda, and other islands, extending
from the southeastern peninsula of Asia to New Guinea. A second
species of cassowary has recently been imported to our zoological
gardens from the more southern island of New Britain. The singular
nocturnal wingless kivi (apteryx) is peculiar to the island of New
Zealand.
Other species and genera which seem to be like the apteryx, as it
were mocked with feathers and rudiments of wings, have wholly ceased
to exist within the memory of man in the islands to which they were
respectively restricted. The dodo (Didus ineptus) of the Mauritius,
and the solitaire (Pezophops solitaria) are instances.
In New Zealand also there existed, within the memory of the Maori
ancestry, huge birds having their nearest affinities to the still
existing apteryx of that island, but generically distinct from that
and all other known birds. I have proposed the name of Dinornis for
this now extinct genus, of which more than a dozen well-defined
species have come to my knowledge, all peculiar to New Zealand, and
the last discovered the strangest, by reason of the elephantine
proportions of its feet. A tridactyle wingless bird of another genus,
Æpyornis, second only to the gigantic Dinornis in size, appears to
have only recently become extinct--if it be extinct--in the island
of Madagascar. The egg of this bird, which may have suggested to the
Arabian voyagers, attaining Madagascar from the Red Sea, the idea
of the Roc of their romances, would hold the contents of six eggs
of the ostrich, sixteen eggs of the cassowary, and one hundred and
forty-eight eggs of the common fowl.
Had all the terrestrial animals that now exist diverged from one
common centre within the limited period of a few thousand years,
it might have been expected that the remoteness of their actual
localities from such ideal centre would bear a certain ratio with
their respective powers of locomotion. With regard to the class
of birds, one might have expected to find that those which were
deprived of the power of flight, and were adapted to subsist on the
vegetation of a warm or temperate latitude, would still be met with
more or less associated together, and least distant from the original
centre of dispersion situated in such a latitude. But what is the
fact? The species of no one order of birds is more widely dispersed
over the earth than the wingless or struthious kind. Assuming that
the original centre has been somewhere in the southwestern mountain
range of Asia, there is but one of the species of flightless birds
whose habitat can be reconciled with the hypothesis. By the neck of
land still uniting Asia with Africa, the progeny of the primary pair
created or liberated at the hypothetical centre might have traveled
to the latter continent, and there have propagated and dispersed
themselves southward to the Cape of Good Hope. It is remarkable,
however, that the ostrich should not have migrated eastward over
the vast plains or steppes which extend along the warmer temperate
zone of Asia, or have reached the southern tropical regions; it is
in fact scarcely known in the Asiatic continent, being restricted to
the Arabian deserts, and being rare even in those parts which are
most contiguous to what I have called its proper continent, Africa.
If we next consider the locality of the cassowary, we find great
difficulty in conceiving how such a bird could have migrated to the
islands of Java, the Moluccas, or New Guinea, from the continent
of Asia. The cassowary is not web-footed like the swimming birds;
for wings it has only a few short and strong quills. How could it
have overcome the obstacles which some hundreds of miles of ocean
would present to its passage from the continent of Asia to those
islands? If the difficulty already be felt to be great in regard to
the insular position of the cassowary, it is still greater when we
come to apply the hypothesis of dispersion from a single centre to
the dodo of the island of Mauritius, or the solitaire of the island
of Rodriguez. How, again, could the emeu have overcome the natural
obstacles to the migration of a wingless terrestrial bird from Asia
to Australia? and why should not the great continent of Asia have
offered in its fertile plains a locality suited to its existence, if
it ever at any period had existed on that continent? A bird of the
nature of the emeu was hardly less likely to have escaped the notice
of scientific travelers than the ostrich itself; but, save in the
Arabian deserts, the ostrich has not been found in any part of Asia,
and no other species of wingless birds has ever been met with on
that continent; the evidence in regard to such large and conspicuous
birds is conclusive to that fact. Again, in order that the rhea, or
three-toed ostrich, should reach South America by traveling along
that element on which alone it is organized and adapted to make
progress, it must, on the hypothesis of dispersion from a single
Asiatic centre, have traveled northward into the inhospitable wilds
of Siberia; it must have braved and overcome the severer regions
of the Arctic zone; it must have maintained its life with strength
adequate to the extraordinary power of walking and running more
than a thousand miles of land or frozen ocean utterly devoid of the
vegetables that now constitute its food before it could gain the
northern division of America, to the southern division of which it
is at present and seems ever to have been confined. The migration in
this case could not have been gradual and accomplished by successive
generations. No individual of the large vegetable-feeding wingless
bird that now subsists in South America could have maintained its
existence, much less hatched its eggs, in Arctic latitudes, where
the food of the species is wholly absent. If we are still to apply
the current hypothesis to this problem in natural history, we must
suppose that the pair or pairs of the rhea that started from the
highest temperate zone in Asia capable of maintaining their life
must have also been the same individuals which began to propagate
their kind when they reached the corresponding temperate latitude
of America. But no individuals of the rhea have remained in the
prairies or in any part of North America--they are limited to the
middle and southern division of the South American continent. And
now, finally, consider the abode of the little apteryx at the
Antipodes, in the comparatively small insulated patch of dry land
formed by New Zealand. Let us call to mind its very restricted means
of migration--the wings reduced to the minutest rudiments, the feet
webless like the common fowls, its power of swimming as feeble!
How could it ever have traversed six hundred miles of sea that
separate it from the nearest land intervening between New Zealand
and Asia? How pass from the southern extremity of that continent to
the nearest island of the Indian Archipelago, and so from member to
member of that group to Australia--and yet leave no trace behind of
such migration by the arrest of any descendants of the migratory
generations in Asia itself, or in any island between Asia and New
Zealand?
If these facts are inexplicable on the hypothesis of the dispersion
of the species of the air-breathing animals from a single Asiatic
centre, we must next endeavor to collect analogous facts and classify
them, and so try to explain intelligibly, that is agreeably with the
facts, the true law or cause of the actual geographical distribution
of animals.