HOW ANIMALS ARE INTERPRETED BY THEIR BONES
There is only one safe path which the naturalist may follow who would
tell the story of the meaning and nature of an extinct type of animal
life, and that is to compare it as fully as possible in its several
bones, and as a whole, with other animals, especially with those which
survive. It is easy to fix the place in nature of living animals and
determine their mutual relations to each other, because all the
organs--vital as well as locomotive--are available for comparison. On
such evidence they are grouped together into the large divisions of
Beasts, Birds, and Reptiles; as well as placed in smaller divisions
termed Orders, which are based upon less important modifications of
fundamental structures. All these characteristic organs have usually
disappeared in the fossil. Hence a new method of study of the hard parts
of the skeleton, which alone are preserved, is used in the endeavour to
discover how the Flying Reptile or other extinct animal is to be
classified, and how it acquired its characters or came into existence.
VARIATIONS OF BONES AMONG MAMMALIA
[Illustration: FIG. 14. THE FORE LIMB IN FOUR TYPES OF MAMMALS
Comparison of the fore limb in mammals, showing variation of form
of the bones with function]
Resemblances and differences in the bones are easily over-estimated in
importance as evidence of pedigree relationship. The Mammalia show, by
means of such skeletons as are exhibited in any Natural History Museum,
how small is the importance to be attached to even the existence of any
group of bones in determining its grade of organisation. The whole Whale
tribe suckle their young and conform to the distinctive characters in
brain and lungs which mark them as being mammals. But if there is one
part of the skeleton more than another which distinguishes the Mammalia,
it is the girdle of bones at the hips which supports the hind limbs. It
is characterised by the bone named the ilium being uniformly directed
forward. Yet in the Whale tribe the hip-girdle and the hind limb which
it usually supports are so faintly indicated as to be practically lost;
while the fore limb becomes a paddle without distinction of digits, and
is therefore devoid of hoofs or claws, which are usual terminations of
the extremities in mammals. Yet this swimming paddle, with its
ill-defined bones--sometimes astonishing in number, as well as in
fewness of the finger bones--is represented by the burrowing fore limb
of the Mole, which lives underground; by the elongated hoofed legs of
the Giraffe, which lives on plains; and the extended arm and finger
bones of the Bat, which are equally mammals with the Whale. From such
comparison it is seen that no proportion, or form, or length, or use of
the bones of the limbs, or even the presence of limbs, is necessarily
characteristic of a mammal. No limitation can be placed upon the
possible diversity of form or development of bones in unknown animals,
when they are considered in the light of such experience of varied
structural conditions in living members of a single class.
What is true for the limbs and the bony arches which support them is
true for the backbone also, for the ribs, and to some extent for the
skull. The neck in the Whale is shortened almost beyond recognition. In
the Giraffe the same seven vertebrae are elongated into a marvellous
neck; so that in the technical definition of a mammal both are said to
have seven neck vertebrae. Yet exceptions show a capacity for variation.
One of the Sloths reduces the number to six, while another has nine
vertebrae in the neck; proving that there is no necessary difference
between a mammal and a reptile when judged by a character which is
typically so distinctive of mammals as the number of the neck bones.
The skull varies too, though to a less extent. The Great Ant-eater of
South America is a mammal absolutely without teeth. The Porpoises have a
simple unvarying row of conical teeth with single roots extending along
the jaw. And the dental armature of the jaws, and relative dimensions of
the skull bones, exhibit such diversity, in evidence of what may be
parted with or acquired, that recognition of the many reptilian
structures and bones in the skull of Ornithorhynchus, the Australian
Duckbill, demonstrates that the difficulties in recognising an animal by
its bones are real, unless we can discover the Animal Type to which the
bones belong; and that there is very little in osteology which may not
be lost without affecting an animal's grade of organisation.
VARIATION IN SKIN COVERING OF MAMMALS
Even the covering of the body varies in the same class, or even order of
animals, so that the familiar growth on the skin is never its only
possible covering. The Indian ant-eater, named Manis, which looks like a
gigantic fir-cone, the Armadillo, which sheathes the body in rings of
bone, bearing only a scanty development of hair, are examples of
mammalian hair, as singular as the quills of a Porcupine, the horn of a
Rhinoceros, or the growth of hair of varying length and stoutness on
different parts of the body in various animals, or the imperfect
development of hair in the marine Cetacea. Among living animals it is
enough for practical purposes to say that a mammal is clothed with hair,
but in a fossil state the hair must usually be lost beyond recognition
from its fineness and shortness of growth.
VARIATION IN SKIN COVERING OF BIRDS
No Class of living animals is more homogeneous than Birds; and
well-preserved remains prove that, at least as far back in time as the
Upper Oolites, birds were clothed with feathers of essentially the same
mode of growth and appearance as the feathers of living birds. There
may, therefore, be no ground for assuming that the covering was ever
different, though some regions of the skin are free from feathers. Yet
the variations from fine under-down to the scale-like feathers on the
wings of a Penguin, or the great feathers in the wings of birds of
flight, or the double quill of the Ostrich group, are calculated to
yield dissimilar impressions in a fossil state, even if the fine down
would be preserved in any stratum.
VARIATION IN THE BONES OF BIRDS
Osteologically there is less variety in the skeleton of birds than in
other great groups of animals. The existing representatives do not
exhaust its capability for modification. The few specimens of birds
hitherto found in the Secondary strata have rudely removed many
differences in the bones which separated living birds from reptiles; so
that if only the older fossil birds were known, and the Tertiary and
living birds had not existed, a bird might have been defined as an
animal having its jaw armed with teeth, instead of devoid of teeth; with
vertebrae cupped at both ends, instead of with a saddle-shaped
articulation which in front is concave from side from side, and convex
from above downwards; in which the bones of the hand are separate, so
that three digits terminating in claws can be applied to the ground,
instead of the metacarpal bones being united in a solid mass with
clawless digits; and in which the tail is elongated like the tail of a
lizard. Yet the limits to variation are not to be formulated till Nature
has exhausted all her resources in efforts to preserve organic types by
adapting them to changed circumstances. Birds may be regarded
theoretically as equally capable with mammals of parting with almost
every distinctive structure in the skeleton by which it is best known.
Even the living frigate bird blends the early joints of the backbone
into a compact mass like a sacrum. The Penguin has a cup-and-ball
articulation in the early dorsal vertebrae, with the ball in front. And
the genus Cypselus has the upper arm bone almost as broad as long,
unlike the bird type. Such examples prove that we are apt to accept the
predominant structures in an animal type as though they were universal,
and forget that inferences based, like those of early investigators, on
limited materials may be re-examined with advantage.
VARIATION IN THE BONES OF REPTILES
The true Reptilia, notwithstanding some strong resemblances to Birds in
technical characters of the skeleton, display among their surviving
representatives an astonishing diversity in the bony framework of the
body, exceeding that of the mammalia. This unlooked-for capacity for
varying the plan of construction of the skeleton is in harmony with the
diversity of structure in groups of extinct animals to which the name
reptiles has also been given. The interval in form is so vast between
Serpent and Tortoise, and so considerable in structure of the skeleton
between these and the several groups of Lizards, Crocodiles, and
Hatteria, that any other diversity could not be more surprising. And the
inference is reasonable that just as mammals live in the air, in the
sea, on the earth, and burrow under the earth, similar modes of
existence might be expected for birds and reptiles, though no bird is
yet known to have put on the aspect of a fish, and no reptiles have been
discovered which roamed in herds like antelopes, or lived in the air
like birds or bats, unless these fossil flying animals prove on
examination to justify the name by which they are known.
Comparative study of structure in this way demolishes the prejudice,
born of experience of the life which now remains on earth, that the
ideas of Reptile and of Flight are incongruous, and not to be combined
in one animal. The comparative study of the parts of animals does not
leave the student in a chaos of possibilities, but teaches us that
organic structures, which mark the grades of life, have only a limited
scope of change; while Nature flings away every part of the skeleton
which is not vital, or changes its form with altering circumstances of
existence, enforced by revolutions of the Earth's surface in geological
time, in her efforts to save organisms from extinction and pass the
grade of life onward to a later age.
The bones are only of value to the naturalist as symbols, inherited or
acquired, and vary in value as evidence of the nature and association of
those vital organs which differentiate the great groups of the
vertebrata.
These distinctive structures, which separate Mammals, Birds, and
Reptiles, are sometimes demonstrated by the impress of their existence
left on the bones; or sometimes they may be inferred from the characters
of the skeleton as a whole.