THE SHAPES OF VERTEBRATE SKELETONS.
§ 254. When an elongated mass of any substance is transversely
strained, different parts of the mass are exposed to forces of opposite
kinds. If, for example, a bar of metal or wood is supported at its two
ends, as shown in Fig. 281, and has to bear a weight on its centre, its
lower part is thrown into a state of tension, while its upper part is
thrown into a state of compression. As will be manifest to any one who
observes what happens on breaking a stick across his knee, the greatest
degree of tension falls on the fibres forming the convex surface, while
the fibres forming the concave surface are subject to the greatest
degree of compression. Between these extremes the fibres at different
depths are subject to different forces. Progressing upwards from the
under surface of the bar shown in Fig. 281, the tension of the fibres
becomes less; and progressing downwards from the upper surface, the
compression of the fibres becomes less; until, at a certain distance
between the two surfaces, there is a place at which the fibres are
neither extended nor compressed. This, shown by the dotted line in the
figure, is called in mechanical language the “neutral axis.” It varies
in position with the nature of the substance strained: being, in common
pine-wood, at a distance of about five-eighths of the depth from the
upper surface, or three-eighths from the under surface. Clearly, if
such a piece of wood, instead of being subject to a downward force, is
secured at its ends and subject to an upward force, the distribution
of the compressions and tensions will be reversed, and the neutral
axis will be nearest to the upper surface. Fig. 282 represents these
opposite attitudes of the bar and the changed position of its neutral
axis: the arrow indicating the direction of the force producing the
upward bend, and the faint dotted line _a_, showing the previous
position of the neutral axis. Between the two neutral axes will be seen
a central space; and it is obvious that when the bar has its strain
from time to time reversed, the repeated changes of its molecular
condition must affect the central space in a way different from that
in which they affect the two outer spaces. Fig. 283 is a diagram
conveying some idea of these contrasts in molecular condition. If A B C
D be the middle part of a bar thus treated, while G H and K L are the
alternating neutral axes; then the forces to which the bar is in each
case subject, may be readily shown. Supposing the deflecting force to
be acting in the direction of the arrow E, then the tensions to which
the fibres between G and F are exposed, will be represented by a series
of lines increasing in length as the distance from G increases; so that
the triangle G F M, will express the amount and distribution of all
the molecular tensions. But the molecular compressions throughout the
space from G to E, must balance the molecular tensions; and hence, if
the triangle G E N be made equal to the triangle G F M, the parallel
lines of which it is composed (here dotted for the sake of distinction)
will express the amount and distribution of the compressions between
E and G. Similarly, when the deflecting force is in the direction of
the arrow F, the compressions and tensions will be quantitatively
symbolized by the triangles K F O, and K E P. And thus the several
spaces occupied by full lines and by dotted lines and by the two
together, will represent the different actions to which different
parts of the transverse section are subject by alternating transverse
strains. Here, then, it is made manifest to the eye that the central
space between G and K, is differently conditioned from the spaces above
and below it; and that the difference of condition is sharply marked
off. The fibres forming the outer surface C D, are subject to violent
tensions and violent compressions. Progressing inwards the tensions and
compressions decrease--the tensions the more rapidly. As we approach
the point G, the tensions to which the fibres are alternately subject,
bear smaller and smaller ratios to the compressions, and disappear
at the point G. Thence to the centre occur compressions only, of
alternating intensities, becoming at the centre small and equal; and
from the centre we advance, through a reverse series of changes, to the
other side.
[Illustration: Fig. 281.]
[Illustration: Fig. 282.]
[Illustration: Fig. 283.]
Thus it is demonstrable that any substance in which the power of
resisting compression is unequal to the power of resisting tension,
cannot be subject to alternating transverse strains, without having
a central portion differentiated in its conditions from the outer
portions, and consequently differentiated in its structure. This
conclusion may easily be verified by experiment. If something having
a certain toughness but not difficult to break, as a thick piece of
sheet lead, be bent from side to side till it is broken, the surface of
fracture will exhibit an unlikeness of texture between the inner and
outer parts.
§ 255. And now for the application of this seemingly-irrelevant
truth. Though it has no obvious connection with the interpretation of
vertebral structure, we shall soon see that it fundamentally concerns
us.
[Illustration: Fig. 284.]
The simplest type of vertebrate animal, the fish, has a mode of
locomotion which involves alternating transverse strains. It is not,
indeed, subjected to alternating transverse strains by some outer
agency, as in the case we have been investigating: it subjects itself
to them. But though the strains are here internally produced instead
of externally produced, the case is not therefore removed into a
wholly different category. For supposing Fig. 284 to represent the
outline of a fish when bent on one side (the dotted lines representing
its outline when the bend is reversed), it is clear that part of the
substance forming the convex half must be in a state of tension. This
state of tension implies the existence in the other half of some
counter-balancing compression. And between the two there must be a
neutral axis. The way in which this conclusion is reconcilable with the
fact that there is tension somewhere in the concave side of a fish,
since the curve is caused by muscular contractions on the concave
side, will be made clear by the rude illustration which a bow supplies.
A bow may be bent by a thrust against its middle (the two ends being
held back), or it may be bent by contracting a string that unites its
ends; but the distributions of mechanical forces within the wood of the
bow, though not quite alike in the two cases, will be very similar.
Now while the muscular action on the concave side of a fish differs
from that represented by the tightened string of a bow, the difference
is not such as to destroy the applicability of the illustration: the
parallel holds so far as this, that within that portion of the fish’s
body which is passively bent by the contracting muscles, there must be,
as in a strung bow, a part in compression, a part in tension, and an
intermediate part which is neutral.
After thus seeing that even in the developed fish with its complex
locomotive apparatus, this law of the transverse strain holds in a
qualified way, we shall understand how much more it must hold in any
form that may be supposed to initiate the vertebrate type--a form
devoid of that segmentation by which the vertebrate type is more or
less characterized. We shall see that assuming a rudimentary animal,
still simpler than the _Amphioxus_, to have a feeble power of moving
itself through the water by the undulations of its body, or some part
of its body, there will necessarily come into play certain reactions
which must affect the median portion of the undulating mass in a way
unlike that in which they affect its lateral portions. And if there
exists in this median portion a tissue which keeps its place with any
constancy, we may expect that the differential conditions produced in
it by the transverse strain, will initiate a differentiation. It is
true that the distribution of the viscera in the _Amphioxus_, Fig. 191,
and in the type from which we may suppose it to have arisen, is such as
to interfere with this process. It is also true that the actions and
reactions described would not of themselves give to the median portion
a cylindrical shape, like that of the cartilaginous rod running along
the back of the _Amphioxus_. But what we have here to note in the
first place is, that these habitual alternate flexions have a tendency
to mark off from the outer parts an unlike inner part, which may be
seized hold of, maintained, and further modified, by natural selection,
should any advantage thereby result. And we have to note in the second
place, that an advantage _is_ likely to result. The contractions
cannot be effective in producing undulations, unless the general shape
of the body is maintained. External muscular fibres unopposed by an
internal resistant mass, would cause collapse of the body. To meet
the requirements there must be a means of maintaining longitudinal
rigidity without preventing bends from side to side; and such a means
is presented by a structure initiated as described. In brief, whether
we have or have not the actual cause, we have here at any rate “a true
cause.” Though there are difficulties in tracing out the process in a
definite way, it may at least be said that the mechanical genesis of
this rudimentary vertebrate axis is quite conceivable. And even the
difficulties may, I think, be more fully met than at first sight seems
possible.
[Illustration: Fig. 191.]
What is to be said of the other leading trait which the simplest
vertebrate animal has in common with all higher vertebrate animals--the
segmentation of its lateral muscular masses? Is this, too, explicable
on the mechanical hypothesis? Have we, in the alternating transverse
strains, a cause for the fact that while the rudimentary vertebrate
axis is without any divisions, there are definite divisions of the
substance forming the animal’s sides? I think we have. A glance at the
distribution of forces under the transverse strain, as represented in
the foregoing diagrams, will show how much more severe is the strain
on the outer parts than on the inner parts; and how, consequently,
any modifications of structure eventually necessitated, will arise
peripherally before they arise centrally. The perception of this may be
enforced by a simple experiment. Take a stick of sealing-wax and warm
it slowly and moderately before the fire, so as to give it a little
flexibility. Then bend it gently until it is curved into a semi-circle.
On the convex surface small cracks will be seen, and on the concave
surface wrinkles; while between the two the substance remains
undistorted. If the bend be reversed and re-reversed, time after time,
these cracks and wrinkles will become fissures which gradually deepen.
But now, if changes of this class, entailed by alternating transverse
strains, commence superficially, as they manifestly must; there arise
the further questions--What will be the special modifications produced
under these special conditions? and through what stages will these
modifications progress? Every one has literally at hand an example
of the way in which a flexible external layer that is now extended
and now compressed, by the bending of the mass it covers, becomes
creased; and a glance at the palms and the fingers will show that the
creases are near one another where the skin is thin, and far apart
where the skin is thick. Between this familiar case and the case of
the rhinoceros-hide, in which there are but a few large folds, various
gradations may be traced. Now the like must happen with the increasing
layers of contractile fibres forming the sides of the muscular tunic
in such a type as that supposed. The bendings will produce in them
small wrinkles while they are thin, but more decided and comparatively
distant fissures as they become thick. Fig. 289, which is a horizontal
longitudinal section, shows how these thickening layers will adjust
themselves on the convex and the concave surfaces, supposing the fibres
of which they are composed to be oblique, as their function requires;
and it is not difficult to see that when once definite divisions have
been established, they will advance inwards as the layers develop;
and will so produce a series of muscular bundles. Here then we have
something like the _myocommata_ [or myotomes as now called] which are
traceable in the _Amphioxus_, and are conspicuous in all superior
fishes.
[Illustration: Fig. 289.]
§ 256. These are highly speculative conceptions. I have ventured to
present them with the view of implying that the hypothesis of the
mechanical genesis of vertebrate structure is not wholly at fault when
applied to the most rudimentary vertebrate animal. Lest it should be
alleged that the question is begged if we set out with a type which,
like the _Amphioxus_, already displays segmentation throughout
its muscular system, it seemed needful to indicate conceivable modes
in which there may have been mechanically produced those leading
traits that distinguish the _Amphioxus_. All I intend to suggest
is that mechanical actions have been at work, and that probably they
have operated in the manner alleged: so preparing the way for natural
selection.
But now let us return to the region of established fact, and consider
whether such actions and reactions as we actually witness, are adequate
causes of those observed differentiations and integrations which
distinguish the more-developed vertebrate animals. Let us see whether
the theory of mechanical genesis affords us a deductive interpretation
of the inductive generalizations.
Before proceeding, we must note a process of functional adaptation
which here co-operates with natural selection. I refer to the usual
formation of denser tissues at those parts of an organism which are
exposed to the greatest strains--either compressions or tensions.
Instances of hardening under compression are made familiar to us by
the skin. We have the general contrast between the soft skin covering
the body at large, and the indurated skin covering the inner surfaces
of the hands and the soles of the feet. We have the fact that even
within these areas the parts on which the pressure is habitually
greatest have the skin always thickest; and that in each person special
points exposed to special pressures become specially dense--often as
dense as horn. Further, we have the converse fact that the skin of
little-used hands becomes abnormally thin--even losing, in places, that
ribbed structure which distinguishes skin subject to rough usage. Of
increased density directly following increased tension, the skeletons,
whether of men or animals, furnish abundant evidence. Anatomists easily
discriminate between the bones of a strong man and those of a weak
man, by the greater development of those ridges and crests to which
the muscles are attached; and naturalists, on comparing the remains of
domesticated animals with those of wild animals of the same species,
find kindred differences. The first of these facts shows unmistakably
the immediate effect of function on structure, and by obvious alliance
with it the second may be held to do the same: both implying that the
deposit of dense substance capable of great resistance, constantly
takes place at points where the tension is excessive.
Taking into account, then, this adaptive process, continually aided by
the survival of individuals in which it has taken place most rapidly,
we may expect, on tracing up the evolution of the vertebrate axis, to
find that as the muscular power becomes greater there arise larger and
harder masses of tissue, serving the muscles as _points d’appui_; and
that these arise first in those places where the strains are greatest.
Now this is just what we _do_ find. The _myocommata_ are so placed
that their actions are likely to affect first that upper coat of the
notochord, where there are found “quadrate masses of somewhat denser
tissue,” which “seem faintly to represent neural spines,” even in the
_Amphioxus_. It is by the development of the neural spines, and after
them of the hæmal spines, that the segments of the vertebral column are
first marked out; and under the increasing strains of more-developed
_myocommata_, it is just these peripheral appendages of the vertebral
segments that must be most subject to the forces which cause the
formation of denser tissue. It follows from the mechanical hypothesis
that as the muscular segmentation must begin externally and progress
inwards, so, too, must the vertebral segmentation. Besides thus
finding reason for the fact that in fishes with wholly cartilaginous
skeletons, the vertebral segments are indicated by these processes,
while yet the notochord is unsegmented; we find a like reason for the
fact that the transition from the less-dense cartilaginous skeleton
to the more-dense osseous skeleton, pursues a parallel course. In the
existing _Lepidosiren_, which by uniting certain piscine and amphibian
characters betrays its close alliance with primitive types, the axial
part of the vertebral column is unossified, while there is ossification
of the peripheral parts. Similarly with numerous genera of fishes
classed as palæozoic. The fossil remains of them show that while the
neural and hæmal spines consisted of bone, the central parts of the
vertebræ were not bony. It may in some cases be noted, too, both in
extant and in fossil forms, that while the ossification is complete at
the outer extremities of the spines it is incomplete at their inner
extremities--thus similarly implying centripetal development.
§ 257. After these explanations the process of eventual segmentation
in the spinal axis itself, will be readily understood. The original
cartilaginous rod has to maintain longitudinal rigidity while
permitting lateral flexion. As fast as it becomes definitely marked
out, it will begin to concentrate within itself a great part of those
pressures and tensions caused by transverse strains. As already said,
it must be acted upon much in the same manner as a bow, though it is
bent by forces acting in a more indirect way; and like a bow, it must,
at each bend, have the substance of its convex side extended and the
substance of its concave side compressed. So long as the vertebrate
animal is small or inert, such a cartilaginous rod may have sufficient
strength to withstand the muscular strains; but, other things equal,
the evolution of an animal that is large, or active, or both, implies
muscular strains which must tend to cause modification in such a
cartilaginous rod. The results of greater bulk and of greater vivacity
may be best dealt with separately. As the animal increases in size,
the rod will grow both longer and thicker. On looking back at the
diagrams of forces caused by transverse strains, it will be seen that
as the rod grows thicker, its outer parts must be exposed to more
severe tensions and pressures if the degree of bend is the same. It is
doubtless true that when the fish, advancing by lateral undulations,
becomes longer, the curvature assumed by the body at each movement
becomes less; and that from this cause the outer parts of the notochord
are, other things equal, less strained--the two changes thus partially
neutralizing one another. But other things are _not_ equal. For while,
supposing the shape of the body to remain constant, the force exerted
in moving the body increases as the cubes of its dimensions, the
sectional area of the notochord, on which fall the reactions of this
exerted force, increases only as the squares of the dimensions: whence
results a greater stress upon its substance. This, however, will not
be very decided where there is no considerable activity. It is clear
that augmenting bulk, taken alone, involves but a moderate residuary
increase of strain on each portion of the notochord; and this is
probably the reason why it is possible for a large _sluggish_ fish like
the Sturgeon, to retain the notochordal structure. But now, passing to
the effects of greater activity, a like dynamical inquiry at once shows
us how rapidly the violence of the actions and reactions rises as the
movements become more vivacious. In the first place, the resistance
of a medium such as water increases as the square of the velocity of
the body moving through it; so that to _maintain_ double the speed, a
fish has to expend four times the energy. But the fish has to do more
than this--it has to _initiate_ this speed, or to impress on its mass
the force implied by this speed. Now the _vis viva_ of a moving body
varies as the square of the velocity; whence it follows that the energy
required to generate that _vis viva_ is measured by the square of the
velocity it produces. Consequently, did the fish put itself in motion
_instantaneously_, the expenditure of energy in generating its own
_vis viva_ and simultaneously overcoming the resistance of the water,
would vary as the fourth power of the velocity. But the fish cannot
put itself in motion instantaneously--it must do it by increments; and
thus it results that the amounts of the forces expended to give itself
different velocities must be represented by some series of numbers
falling between the squares and the fourth powers of those velocities.
Were the increments slowly accumulated, the ratios of increasing effort
would but little exceed the ratios of the squares; but whoever observes
the sudden, convulsive action with which an alarmed fish darts out
of a shallow into deep water, will see that the velocity is rapidly
generated, and that therefore the ratios of increasing effort probably
exceed the ratios of the squares very considerably. At any rate it
will be clear that the efforts made by fishes in rushing upon prey or
escaping enemies (and it is these extreme efforts which here concern
us) must, as fishes become more active, rapidly exalt the strains to
be borne by their motor organs; and that of these strains, those which
fall upon the notochord must be exalted in proportion to the rest. Thus
the development of locomotive power, which survival of the fittest must
tend in most cases to favour, involves such increase of stress on the
primitive cartilaginous rod as will tend, other things equal, to cause
its modification.
[Illustration: Figs. 291–293.]
What must its modification be? Considering the complication of the
influences at work, conspiring, as above indicated, in various ways and
degrees, we cannot expect to do more than form an idea of its average
character. The nature of the changes which the notochord is likely
to undergo, where greater bulk is accompanied by higher activity, is
rudely indicated by Figs. 291, 292, and 293. The successively thicker
lines represent the successively greater strains to which the outer
layers of tissue are exposed; and the widening interspaces represent
the greater extensions which they have to bear when they become convex,
or else the greater gaps that must be formed in them. Had these outer
layers to undergo extension only, as on the convex side, continued
natural selection might result in the formation of a tissue elastic
enough to admit of the requisite stretching. But at each alternate
bend these outer layers, becoming concave, are subject to increased
compression--a compression which they cannot withstand if they have
become simply more extensible. To withstand this greater compression
they must become harder as well as more extensible. How are these two
requirements to be reconciled? If, as facts warrant us in supposing, a
formation of denser substance occurs at those parts of the notochord
where the strain is greatest; it is clear that this formation cannot
so go on as to produce a continuous mass: the perpetual flexions must
prevent this. If matter that will not yield at each bend, is deposited
while the bendings are continually taking place, the bendings will
maintain certain places of discontinuity in the deposit--places at
which the whole of the stretching consequent on each bend will be
concentrated. And thus the tendency will be to form segments of hard
tissue capable of great resistance to compression, with intervals
filled by elastic tissue capable of great resistance to extension--a
vertebral column.
And now observe how the progress of ossification is just such as
conforms to this view. That centripetal development of segments which
holds of the vertebrate animal as a whole, as, if caused by transverse
strains, it ought to do, and which holds of the vertebral column as
a whole, as it ought to do, holds also of the central axis. On the
mechanical hypothesis, the outer surface of the notochord should be
the first part to undergo induration, and that division into segments
which must accompany induration. And accordingly, in a vertebral column
of which the axis is beginning to ossify, the centrums consist of bony
rings inclosing a still-continuous rod of cartilage.
§ 258. Sundry other general facts disclosed by the comparative
morphology of the _Vertebrata_, supply further confirmation. Let
us take first the structure of the skull.
On considering the arrangement of the muscular flakes, or _myocommata_,
in any ordinary fish which comes to table--an arrangement already
sketched out in the _Amphioxus_--it is not difficult to see that that
portion of the body out of which the head of the vertebrate animal
becomes developed, is a portion which cannot subject itself to bendings
in the same degree as the rest of the body. The muscles developed
there must be comparatively short, and much interfered with by the
pre-existing orifices. Hence the cephalic part will not partake in any
considerable degree of the lateral undulations; and there will not tend
to arise in it any such distinct segmentation as arises elsewhere. We
have here, then, an explanation of the fact, that from the beginning
the development of the head follows a course unlike that of the spinal
column; and of the fact that the segmentation, so far as it can be
traced in the head, is most readily to be traced in the occipital
region and becomes lost in the region of the face. For if, as we have
seen, the segmentation consequent on mechanical actions and reactions
must progress from without inwards, affecting last of all the axis; and
if, as we have seen, the region of the head is so circumstanced that
the causes of segmentation act but feebly even on its periphery; then
that terminal portion of the primitive notochord which is included in
the head, having to undergo no lateral bendings, may ossify without
division into segments.
Of other incidental evidences supplied by comparative morphology, let
me next refer to the supernumerary bones, which the theory of Goethe
and Oken as elaborated by Prof. Owen, has to get rid of by gratuitous
suppositions. In many fishes, for example, there are what have been
called interneural spines and interhæmal spines. These cannot by any
ingenuity be affiliated upon the archetypal vertebra, and they are
therefore arbitrarily rejected as bones belonging to the exo-skeleton;
though in shape and texture they are similar to the spines between
which they are placed. On the hypothesis of evolution, however, these
additional bones are accounted for as arising under actions like those
that gave origin to the bones adjacent to them. And similarly with such
bones as those called sesamoid; together with others too numerous to
name.
§ 259. Of course the foregoing synthesis is to be taken simply as an
adumbration of the process by which the vertebrate structure may have
arisen through the continued actions of known agencies. The motive
for attempting it has been two-fold. Having, as before said, given
reasons for concluding that the segments of a vertebrate animal are not
homologous in the same sense as are those of an annulose animal, it
seemed needful to do something towards showing how they are otherwise
to be accounted for; and having here, for our general subject, the
likenesses and differences among the parts of organisms, as determined
by incident forces, it seemed out of the question to pass by the
problem presented by the vertebrate skeleton.
Leaving out all that is hypothetical, the general argument may be
briefly presented thus:--The evolution from the simplest known
vertebrate animal of a powerful and active vertebrate animal, implies
the development of a stronger internal fulcrum. The internal fulcrum
cannot be made stronger without becoming more dense. And it cannot
become more dense while retaining its lateral flexibility, without
becoming divided into segments. Further, in conformity with the general
principles thus far traced, these segments must be alike in proportion
as the forces to which they are exposed are alike, and unlike in
proportion as these forces are unlike; and so there necessarily results
that unity in variety by which the vertebral column is from the
beginning characterized. Once more, we see that the explanation extends
to those innumerable and more marked divergences from homogeneity,
which vertebræ undergo in the various higher animals. Thus, the
production of vertebræ, the production of likenesses among vertebræ,
the production of unlikenesses among vertebræ, and the production of
unlikenesses among vertebral columns, are interpretable as parts of one
general process, and as harmonizing with one general principle.
Whether sufficient or insufficient, the explanation here given assigns
causes of known kinds producing effects such as they are known to
produce. It does not, as a solution of one mystery, offer another
mystery of which no solution is to be asked. It does not allege a
Platonic ἰδέα, or fictitious entity, which explains the vertebrate
skeleton by absorbing into itself all the inexplicability. On the
contrary, it assumes nothing beyond agencies by which structures in
general are moulded--agencies by which these particular structures
are, indeed, notoriously modifiable. An ascertained cause of certain
traits in vertebræ and other bones, it extends to all other traits of
vertebræ; and at the same time assimilates the morphological phenomena
they present to much wider classes of morphological phenomena.
* * * * *
[NOTE.--The theory set forth in the foregoing chapter, is an
elaboration of one suggested at the close of a criticism of Prof.
Owen’s _Archetype and Homologies of the Vertebrate Skeleton_,
already referred to in § 210 as having been published in the
_Medico-Chirurgical Review_ for October, 1858. It is now reproduced in
Appendix B. Since the issue of this elaborated exposition, in No. 15 of
my serial in December, 1865, verifications of it have from time to time
been published. In his work _The Primary Factors of Organic Evolution_,
Prof. Cope of Philadelphia writes:--
“Mr. Herbert Spencer has endeavoured to account for the origin of the
segmentation of muscles into myotomes, and the division of the sheath
of the notochord into vertebræ, by supposing it to be due to the
lateral swimming movements of the fishes, which first exhibit these
structures. With this view various later authors have agreed, and I
have offered some additional evidence of the soundness of this position
with respect to the vertebral axis of Batrachia, and the origin of
limb articulations. It is true that the origin of segmentation in the
vertebral column of the true fishes and the Batrachia turns out to have
been less simple in its process than was suggested by Mr. Spencer, but
his general principle holds good, now that paleontology has cleared up
the subject” (pp. 367–8).
An allusion in the foregoing extract is made by Prof. Cope to certain
observations set forth in his work entitled _The Origin of the
Fittest_. On pp. 305–6 of it will be found the following sentences:--
“Now, all the Permian land-animals, reptiles and batrachians, retain
this notochord with the elements of osseous vertebræ, in a greater or
less degree of completeness. There are some in South Africa, I believe,
in which the ossification has come clear through the notochord; but
they are few.... There is something to be said as to the condition
of the column from a mechanical standpoint, and it is this: that the
chorda exists, with its osseous elements disposed about it; and in the
Permian batrachians, equally related to salamanders and frogs, these
osseous elements are arranged in the sheath or skin of the chorda;
and they are in the form of regular concave segments, very much like
such segments as you can take from the skin of an orange--but parts
of a cylinder, and having greater or less dimensions according to the
group or species. Now, the point of divergence of these segments is
on the side of the column. The contacts are placed on the side of the
column where the segments separate--the upper segments rising and the
lower segments coming downward. To the upper segments are attached the
arches and their articulations, and the lower segments are like the
segments of a cylinder. If you take a flexible cylinder, and cover it
with a more or less inflexible skin or sheath, and bend that cylinder
sidewise, you of course will find that the wrinkles or fractures of
that part of the surface will take place along the line of the shortest
curve, which is on the side; and, as a matter of fact, you have breaks
of very much the character of the segments of the Permian Batrachia....
In the cylinder bending both ways, of course the shortest line of
curve is right at the centre of the side of that cylinder, and the
longest curve is of course at the summit and base, and the shortest
curve will be the point of fracture. And that is exactly what I presume
has happened in the case of the construction of the segments of the
sheath of the vertebral column, by the lateral motion of the animal in
swimming, and which has been the actual cause of the disposition of the
osseous material in its form.... That is the state of the vertebral
column of many of the Vertebrata of the Permian period.”
In his essay on “The Mechanical Causes of the Development of the
Hard Parts of the Mammalia,” published in the American _Journal of
Morphology_ (Vol. III), Prof. Cope has carried the interpretation
further, by showing that in kindred ways the genesis of articulations
and limb-bones may be explained. On p. 163 he enunciates the general
principle of his interpretation as follows:--
“It cannot have been otherwise than that, since the motions of animals
continued during the evolution of their hard parts, these hard parts
grew in exact adaptation to these movements. Thus at the points of
greatest flexure joints would be formed, and between these joints the
deposit would be continuous.”
Evidently if osseous structures are produced by deposits of calcareous
matters in pre-existing cartilaginous structures, or other structures
of flexible materials, the deposits must be so carried on that while
dense resistant masses are produced these must admit of such free
movements as the creature’s life necessitates, and must so form adapted
joints.
Let it be understood, however, that the hypothesis set forth in
the foregoing chapter and extended by Prof. Cope, which serves to
interpret a large part of the phenomena of osseous structures in the
_Vertebrata_, does not serve to interpret them all. While the formation
of hard parts has been in large measure initiated and regulated by
tensions and pressures, there are hard parts the formation of which
cannot be thus explained. The bones of the skull are the most obvious
instances. These are apparently referable to no other cause than the
survival of the fittest--the survival of individual animals in which
greater density of the brain-covering yielded better protection against
external injuries. Without enumerating other instances which might be
given, it will suffice to recognize the truth that natural selection
of favourable variations and the inheritance of functionally-produced
changes have all along co-operated: each of them in some cases acting
alone, but in other cases both acting together.]