<h2><SPAN name="CHAPTER_VI" id="CHAPTER_VI"></SPAN><small>CHAPTER VI</small><br/><br/> HOW ANIMALS ARE INTERPRETED BY THEIR BONES</h2>
<p>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.</p>
<p><span class="pagenum"><SPAN name="Page_38" id="Page_38">[Pg 38]</SPAN></span></p>
<h4>VARIATIONS OF BONES AMONG MAMMALIA</h4>
<div class="figcenter"> <SPAN name="Fig_14" id="Fig_14"></SPAN> <span class="caption">FIG. 14. THE FORE LIMB IN FOUR TYPES OF MAMMALS</span> <ANTIMG src="images/i_055.jpg" width-obs="640" height-obs="454" alt="FIG. 14." title="FIG. 14." /> <p class="center">Comparison of the fore limb in mammals, showing variation
of form of the bones with function</p>
</div>
<p>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<span class="pagenum"><SPAN name="Page_39" id="Page_39">[Pg 39]</SPAN></span>
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.</p>
<p>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 vertebræ are elongated
into a marvellous neck; so that in the technical
definition of a mammal both are said to have seven
neck vertebræ. Yet exceptions show a capacity for
variation. One of the Sloths reduces the number to
six, while another has nine vertebræ in the neck;
proving that there is no necessary difference between
a mammal and a reptile when judged by a character<span class="pagenum"><SPAN name="Page_40" id="Page_40">[Pg 40]</SPAN></span>
which is typically so distinctive of mammals as the
number of the neck bones.</p>
<p>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.</p>
<h4>VARIATION IN SKIN COVERING OF MAMMALS</h4>
<p>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<span class="pagenum"><SPAN name="Page_41" id="Page_41">[Pg 41]</SPAN></span>
state the hair must usually be lost beyond recognition
from its fineness and shortness of growth.</p>
<h4>VARIATION IN SKIN COVERING OF BIRDS</h4>
<p>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.</p>
<h4>VARIATION IN THE BONES OF BIRDS</h4>
<p>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 vertebræ 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<span class="pagenum"><SPAN name="Page_42" id="Page_42">[Pg 42]</SPAN></span>
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 vertebræ, 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.</p>
<h4>VARIATION IN THE BONES OF REPTILES</h4>
<p>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<span class="pagenum"><SPAN name="Page_43" id="Page_43">[Pg 43]</SPAN></span>
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.</p>
<p>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.</p>
<p>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.<span class="pagenum"><SPAN name="Page_44" id="Page_44">[Pg 44]</SPAN></span></p>
<p>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.</p>
<hr style="width: 65%;" />
<p><span class="pagenum"><SPAN name="Page_45" id="Page_45">[Pg 45]</SPAN></span></p>
<div style="break-after:column;"></div><br />
1 of 2
2 of 2
Previous
Next
Update Required
To play the media you will need to either update your browser to a recent version or update your Flash plugin.