CHAPTER VI. How Animals Are Interpreted by Their Bones

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

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.