PROOFS FROM HOMOLOGIES OF THE VERTEBRATE SKELETON.
The proposition to be established here is, that all vertebrates
have not only a common general plan of structure, but an essential
identity even in detail, although this identity is obscured by adaptive
modifications. We will try to show first a common general plan, and
then, taking parts most familiar to the general reader, will show
essential identity even in detail.
=Common General Plan.=--1. All vertebrate animals, and none other, have
an _internal_ jointed skeleton worked by muscles on the _outside_.
As we shall see hereafter, the relation of skeleton and muscle in
arthropods is exactly the reverse.
2. In all vertebrates, and in none other, the axis of this skeleton
is a jointed backbone (vertebral column) inclosing and protecting the
nervous centers (cerebro-spinal axis). These, therefore, may well be
called back-boned animals.
3. All vertebrates, and none other, have a number of their anterior
vertebral joints enlarged and consolidated into a box to form the
skull,[22] in order to inclose and protect a similar enlargement of
the nervous center, viz., the brain; and also usually, but not always,
a number of posterior joints, enlarged and consolidated to form the
pelvis, to serve as a firm support to the hind-limbs.
4. All vertebrates, and none other, have two cavities, inclosed
and protected by the skeleton, viz., the neural cavity above, and
the visceral or body cavity below, the vertebral column; so that a
cross-section of the body is diagrammatically represented by Fig. 4.
[Illustration:
FIG. 4.--Diagram cross-section through the body of a vertebrate,
showing the relation of skeleton to the cavities. _n_, neutral
cavity; _v_, visceral cavity; _c_, centrum of vertebra.
]
5. All vertebrates, with few exceptions, and no other animals, have
two and only two pair of limbs. The exceptions are of two kinds,
viz.: _a_, some lowest fishes, amphioxus and lampreys, which probably
represent the vertebrate condition before limbs were acquired; and _b_,
degenerate forms like snakes and some lizards, which have lost their
limbs by disuse.
So much concerns the general plan of skeletal structures, and is
strongly suggestive of--in fact, is inexplicable without--common
origin. But much more remains which is not only suggestive, but
demonstrative of such origin. By extensive comparison in the taxonomic
and ontogenic series, the whole vertebrate structure in all its
details in different animals may be shown to be modifications one
of another. Sometimes a piece is enlarged, sometimes diminished, or
even becomes obsolete; sometimes several pieces are consolidated into
one; but, in spite of all these obscurations, corresponding parts may
usually be made out. This is the main subject of this chapter.
=Special Homology of Vertebrate Limbs.=--It would lead us much too far
into unfamiliar technicalities to take up the whole skeleton. We select
the limbs, both because their general structure is more familiar, and
because in them the two fundamental ideas of essential identity and of
adaptive modification are both admirably illustrated. The reason of
this is, that it is by the limbs that the organism chiefly reacts on
the environment, and is modified by it.
=Fore-limbs.=--In the accompanying figures (Figs. 5-18) we have
represented, side by side, the fore-limbs of many vertebrates, taken
from all the classes--mammals, birds, reptiles, and fishes. For
convenience of comparison, the corresponding parts are similarly
lettered in all. Also, in order to identify easily certain important
corresponding segments, we have drawn through them a continuous
dotted line. In man, nearly all the parts are present, and his limbs,
therefore, may be taken as a term of comparison; for man’s structure,
except his brain, is far less modified than that of many animals.
Note, then, the following points: 1. The collar-bone (clavicle) is
associated with wide separation of the shoulders, and the free use of
the fore-limb for prehension or for flight, but is gradually lost in
proportion as the fore-limb is brought nearer together and used for
support, because it is no longer wanted. I say _gradually_, for all
the steps of the passing away may be found. The useless rudimentary
condition is not uncommon.
[Illustration:
FIGS. 5-9.--5. Fore-limb of man. 6. Dog 7. Hog. 8. Sheep. 9. Horse.
_sc_, scapula; _c_, coracoid; _a_, _b_, two bones of fore-arm.
(Taken from various sources and grouped.)
]
[Illustration:
FIGS. 10-13.--10. Fore-limb of bat. 11. Bird. 12. Archæopteryx.
13. Pterodactyl. (Lettered as in previous figures;
grouped from various sources.)
]
2. The coracoid (_c_), it is seen, is a small, beak-like process
of the blade-bone (scapula) in man and mammals; but in birds (Fig.
11) and reptiles (Figs. 14, 18) it is a separate bone as large as
the blade-bone itself, jointed with the latter at the shoulder and
with the breast-bone (sternum) in front, thus making together a
strong shoulder-girdle for the attachment of the fore-limb. This was
undoubtedly the condition in the original or earliest walking animal,
viz., reptiles. It was inherited and retained by birds, because
necessary for powerful action of the wings in flight. In mammals
it gradually dwindled and became united with the blade-bone as a
process. In one mammal, the lowest and most reptilian living--the
ornithorhynchus--the coracoid is much like that of reptiles--a large,
flat bone, separated from the blade-bone and articulated with the
breast-bone. It is a significant fact that, in the mammalian embryo,
it is first developed as a separate bone and afterward united with the
scapula.
[Illustration: FIGS. 14-17.--14. Fore-limb of turtle. 15. Mole. 16.
Whale. 17. Fish.]
3. In man, monkeys, bears, and some other mammals, the limb is fairly
free from the body and the elbow half-way down the limb; while in
herbivores (Figs. 8, 9), such as the horse, ox, and deer, etc., the
elbow is high on the side of the body, and the limb is free only
from the elbow downward. Perhaps in these cases most observers do
not recognize it as an elbow at all. All gradations between these
extremes are easily traced. The free condition of the limb is evidently
the original one, the condition in herbivores being an extreme
modification associated with another modification mentioned under 5.
4. In man and in many mammals, and in all reptiles and birds, there are
two bones in the forearm (radius and ulna). In the more specialized
forms of hoofed animals (ungulates), such as horse and ruminants (Figs.
8, 9), there is apparently but one. Two is the normal and original
number; but one of them, the ulna, has gradually become smaller and
smaller, and finally is reduced to a short splint, and consolidated
with the radius as a process extending backward to form the point
of the elbow. In the horse family every step of this reduction and
consolidation may be traced in the course of its geological history.
[Illustration: FIG. 18.--Mosasaur.]
5. The _wrist_ of many mammals and all birds differs in structure from
that of man, chiefly in containing a smaller number of bones. The
normal number, as in man, seems to be eight. The decrease takes place
mainly by consolidation of two or more into one. In such cases usually
the embryo will show the bones still separate, thus revealing the
ancestral condition. Again, the _position_ of the wrist is noteworthy.
In man, monkeys, the bear family, and several other mammalian families,
and in all reptiles, the hand bends forward at the wrist, so that
the tread is on the whole palm (palmigrade). But, in all the most
specialized mammals, the wrist can not bend in this direction, and
therefore this joint can not be brought to the ground. The tread is
therefore on the toes (digitigrade), and the wrist is high up above
the ground. In the horse (Fig. 9), the ox, and many other mammals, for
example, the wrist is so high that it is not usually recognized as a
wrist, and is often called the _fore-knee_. Now, homologous parts ought
to have the same _scientific_ name; but to use the word “_hand_” in
the case of lower animals might produce confusion and misconception.
Therefore it has been agreed among comparative anatomists to use
instead the Latin word “_manus_” for all that corresponds, in any
animal, to the hand of man--i. e., all from the wrist downward.
The manus of a horse is about fifteen inches long. The manus of a
pterodactyl, such as that found by Marsh in the cretaceous strata of
the West, with an expanse of wings of twenty-five feet, was probably
not less than seven or eight feet long.
6. The number of palm-bones (metapodal) and toes deserves special
notice. In fishes, and in some extinct swimming reptiles, these are
or were very numerous, but in the earliest land-animals they became
five. This is the number now in nearly all reptiles, and in all the
more generalized mammals. It may be called the normal number for a
walking animal. In very many mammals, such, for example, as the dog
family, they are reduced to four, though the fifth often remains as a
useless, rudimentary splint and dew-claw (Fig. 6), thus showing the
process of dwindling in the ancestry. In hoofed animals the process of
gradual diminution is shown even in existing forms, and still better in
extinct forms. Confining ourselves, now, only to existing forms, in the
elephant there are five palm-bones and toes, and in the hippopotamus
there are four, all functional. In the hog (Fig. 7) there are still
four, but two are behind the others and much smaller, and do not touch
the ground--are not functional unless in soft ground. In the cow, deer,
etc., the palm-bones are reduced to two, and these are consolidated
into _one_ (canon-bone), and the toes are reduced to two efficient and
two useless rudiments. In the sheep and the goat (Fig. 8) these useless
rudiments are dropped, and there are two only. Finally, in the horse
(Fig. 9), the _toes_ are reduced to one, although the palm-bones are
still three, two of them, however, being reduced to rudimentary splints.
How is it with birds? Have these also palm-bones and fingers? Yes,
in birds (Fig. 11) there are three palm-bones and three fingers (the
fourth and fifth being wanting); one of them--the thumb--is free, and
sometimes carries a claw. In the earliest known and most reptilian
bird, the archæopteryx (Fig. 12), all the three fingers are free, have
the full number of joints, and all of them carry claws. In the embryo
of living birds the fingers are all free, as in the archæopteryx.
[Illustration: FIG. 19.--Restoration of Rhamphorhynchus phyllurus
(after Marsh). One-seventh natural size.]
7. Observe, finally, as an admirable illustration of different
adaptative modifications for the same purpose--flight--the structure
of the manus of flying animals. In the bat (Fig. 10), the flat
flying-plane is made by enormous elongation of the palm-bones and
finger-bones, their wide separation and the stretching of a thin
membrane between them. In the pterosaurs, or extinct flying reptiles
(Fig. 13), one finger only is greatly enlarged and elongated, and
the flying-membrane is stretched between it and the hind-leg (Fig.
19), while the other three fingers are free and provided with claws.
If it be asked which finger is it that is so greatly enlarged in
this animal, we answer, it is the _little finger_. In birds, on
the contrary, the manus is consolidated to the last degree, to
form a strong basis for attachments for the quills which form the
flying-plane, and which are themselves extreme modifications of the
scales of reptiles. But throughout all these extreme modifications the
same essential structure is detectable.
It is perhaps unnecessary to dwell upon the still greater modifications
of limbs for swimming, as in the whale (Fig. 16), the ichthyosaur,
mosasaur (Fig. 18), and the fish (Fig. 17). A careful inspection of the
figures, after what we have said, will be sufficient to explain them.
In the fish alone the upper segments of the limb, viz., shoulder-girdle
and humerus, are wanting, not being yet introduced, and the manus is
not yet differentiated into palm-bones and fingers, and the fingers
are indefinitely multiplied. All these characters are indications of
low position in the scale of evolution. The earliest vertebrates were
fishes. Limbs were not yet completely formed. In embryos of higher
animals, also, the outer segments are first formed.
=Hind-Limbs.=--Figs. 20 to 24 represent, in a similar way, the
hind-limbs of several animals--in this case all mammals. As before,
corresponding parts are similarly lettered, and a dotted line is
carried through certain prominent parts, especially the knee, heel,
instep, and toes. By careful inspection the figures explain themselves.
Nevertheless, it will be well to draw special attention to several of
the more important points:
[Illustration: FIGS. 20-24.--20. Hind-limb of man. 21. Monkey. 22. Dog.
23. Sheep. 24. Horse.]
1. See, then, the position of the knee. The thigh-bone in man, monkeys,
bears, and several other families of mammals, and all reptiles, is free
from the body, and the knee is far removed and half-way down the limb
(Figs. 20, 21). This is undoubtedly the original and normal condition
of land-animals. But in all the more highly specialized and swifter
animals the knee is brought nearer and nearer to the body, until, in
the swiftest of all, such as the ruminants and the horse (Figs. 23,
24), it is high up on the side of the body, in the middle of what is
usually called the thigh but which really includes the thigh and the
upper part of the lower leg or shank.
2. See, again, the position of the heel. In man, monkey, bear, and many
other mammals, and all _living_ reptiles, the heel is on the ground,
the tread is on the whole foot, plantigrade; while in all the more
specialized and agile animals, and especially in the swiftest of all,
such as the horse, the deer, etc., the heel is high in the air, and the
tread is digitigrade.
3. Observe, again: there are two degrees of digitigradeness. The one
we find in carnivorous or clawed digitigrades, the other in herbivores
or hoofed digitigrades. In the one the tread is on the whole length of
the toes to the balls, as in man when he _tip-toes_; in the other the
_tread is on the tip of the last joint alone_. All that in any animal
corresponds to the foot of a man--i. e., from the hamstring and heel
downward--is called, in comparative anatomy, the “_pes_.” The pes,
or foot of a horse, is eighteen inches long. It is easy to see what
spring and activity this mode of treading gives to an animal. Think how
helpless a horse would be if he trod on the whole foot, heel down!
4. Observe, again, the number of toes. In the process of specialization
there is a tendency for these to become fewer and stronger.[23] The
normal number, as already seen, is five. All the earliest mammals,
and many orders of mammals still living, have five; but in the most
specialized orders, such as the ungulates or hoofed animals, they
were steadily reduced in number in the course of evolution. In the
elephant there are still five, in the hippopotamus there are four, in
the rhinoceros three, in the goat two, in the horse one. Still more
the order of the dropping is regular. If an animal have but four toes,
it is usually the first, or great toe, or thumb, that is wanting, or
may be rudimentary. If, as in the rhinoceros, there are only three,
then No. 5, or little toe, is also wanting, and the existing toes are
Nos. 2, 3, and 4. If an animal has only two toes, as the goat, these
are Nos. 3 and 4; and if only one, as the horse, it is the third or
middle toe. Or, to put it more definitely: hoofed animals are divided
into two groups, even-toed (artiodactyl) and odd-toed (perissodactyl).
The even-toed may have four, as in the hippopotamus; or two, as in the
goat. The odd-toed may have three, as in the rhinoceros; or but one, as
in the horse. Now, both of these orders came by differentiation, far
back in the Eocene Tertiary, from a five-toed plantigrade ancestor.
After dropping No. 1 (thumb or great toe) it is not yet decided, so far
as number of toes is concerned, whether the resulting four-toed animal
shall become artiodactyl or perissodactyl. If the former, then the
two side-toes (Nos. 2 and 5) become shortened up, as in the hog; then
rudimentary, as in the ox and deer; and finally pass away entirely, as
in the goat. If, on the other hand, the four-toed animal is on the line
of perissodactyl evolution, it becomes first a three-toed animal by
dropping No. 5. Now, the two side-toes (Nos. 2 and 4) shorten up more
and more, and the middle toe increases in size, until finally, in the
modern horse, only the greatly enlarged middle toe (No. 3) remains.
We look with wonder and admiration at the _danseuse_ pirouetting on
the point of one toe. The horse is performing this feat all the time.
Yes, the one toe of a horse has all the three joints like ours. The
coffin-bone is the last joint, and the hoof is the nail.
[Illustration: _a b c d e f g_
Equus: Quaternary and Recent.
Pliohippus: Pliocene.
Protohippus: Lower Pliocene.
Miohippus: Miocene.
Mesohippus: Lower Miocene.
Orohippus: Eocene.
FIG. 25.--Diagram illustrating gradual changes in the horse family.
Throughout _a_ is fore-foot; _b_, hind-foot; _c_, fore-arm; _d_,
shank; _e_, molar on side-view; _f_ and _g_, grinding surface of
upper and lower molars (after Marsh).
]
=Genesis of the Horse.=--Every step of this process on the
perissodactyl line may be traced in the history of the genesis of
the horse. The beautiful form and structure of this animal were not
made at once, but by a slow process of integration of small changes
from generation to generation, and from epoch to epoch of the earth’s
history. The horse (as in fact did all ungulates) came from a five-toed
_plantigrade_ ancestor, but we are not able to trace the direct line
of genesis quite so far. The earliest stage that we can trace with
certainty, in this line of descent, is found in the eohippus of Marsh.
This was a small animal, no bigger than a fox, with three toes behind
and four serviceable toes in front, with an additional fifth palm-bone
(splint), and perhaps a rudimentary fifth toe like a dew-claw. This was
in early Eocene times. Then, in later Eocene, came the orohippus, which
differs from the last chiefly in the disappearance of the rudimentary
fifth toe and splint. (See Fig. 25.) Next, in the Miocene, came the
mesohippus and miohippus. These were larger animals (about the size of
a sheep), and had three serviceable toes all around; but in the former
the rudiment of a fourth splint in the fore-limb yet remained. Then,
in the Miocene, came the protohippus and pliohippus. These were still
larger animals, being about the size of an ass. In the former the two
side-toes were shortening up and the middle toe becoming larger. In
the latter the two side-toes have become splints. Lastly, only in
the Quaternary comes the genus _Equus_, or true horse. The size of the
animal is become greater, the middle toe stronger, the side-splints
smaller; but in the side-splints of the modern horse we have still
remaining the evidence of its three-toed ancestor.
Similar gradual changes may be traced in the two leg-bones, which
have gradually consolidated into one; in the teeth, which have become
progressively longer and more complex in structure, and therefore a
better grinder; in the position of the heel and wrist, which have
become higher above-ground; in the general form, which has become
more graceful and agile; and, lastly, in the brain, which has become
progressively larger and more complex in its convolutions--to give
greater battery-power, to make a more powerful dynamo--to work the
improved skeletal machine. See, then, how long it has taken Nature to
produce that beautiful finished article we call the horse!
* * * * *
We have taken only limbs as examples of what is true of the whole
skeleton. To the superficial observer the bodies of animals of
different classes seem to differ fundamentally in plan--to be entirely
different machines, made each for its own purposes, at once, out of
hand. Extensive comparison, on the contrary, shows them to be the same,
although the essential identity is obscured by adaptive modifications.
The simplest, in fact the only scientific, explanation of the phenomena
of vertebrate structure is the idea of a primal vertebrate, modified
more and more through successive generations by the necessities of
different modes of life.
See, then, in conclusion, the difference between man’s mode of working
and Nature’s. A man having made a steam-engine, and desiring to use it
for a different purpose from that for which it was first designed and
used, will nearly always be compelled to add new parts not contemplated
in the original machine. Nature rarely makes new parts--never, if
she can avoid it--but, on the contrary, adapts an old part to the
new function. It is as if Nature were not free to use any and every
device to accomplish her end, but were conditioned by her own plans
of structure; as, indeed, she must be according to the derivation
theory. For example: In early Devonian times fishes were the only
representatives of the vertebrate type of structure. The vertebrate
machine was then a _swimming-machine_. In the course of time, when all
was ready and conditions were favorable, reptiles were introduced.
Here, then, is a new function--that of locomotion on land. We want a
_walking-machine_. Shall we have a new organ for this new function? No:
the old swimming-organ is modified so as to adapt it for walking. Time
went on, until the middle Jurassic, and birds were introduced. Here
is a new and wonderful function, that of flying in the air. We want a
_flying-machine_. We know how man would have done this; for we have
the result of his imagination in angels of Christian art and griffins
of Greek mythology. He would have added wings to already existing
parts, and this would have necessitated the alteration of the whole
plan of structure, both skeletal and muscular. Nature only modifies
the fore-limbs for this new purpose. If we must have wings, we must
sacrifice fore-legs. We can not have both without violating the laws of
morphology. Finally, ages again passed, and, when time was fully ripe,
man was introduced. Now we want some part to perform a new and still
more wonderful function. We want a _hand_, the willing and efficient
servant of a rational mind. We know, again, how man would have done
this, for we have the result in the centaurs of Greek mythology, in
which man’s chest, and arms, and head are added to the body of a
quadruped. But natural laws must not be violated, even for man. If we
want hands, we must sacrifice feet. Again, therefore, the fore-limbs
are modified for this new and exquisite function. Thus, in the fin of
a fish, the fore-paw of a reptile or a mammal, the wing of a bird, and
the arm and hand of a man, we have the same part, variously modified
for many purposes.
Many other illustrations might be taken from the skeleton and from
other systems, especially the muscular and nervous. But in the
muscular system the modifications have been so extreme that homology
is much more difficult to trace, and therefore requires more extensive
knowledge than we yet possess, and more extended comparison than has
yet been attempted. It has been traced with some success through
mammals, and probably will be through air-breathing vertebrates--i. e.,
also through birds, reptiles, and amphibians; but to trace it through
fishes seems almost hopeless. In the case of the nervous system, and
especially of the brain, it is again distinct; but this had better be
taken up under another head, viz., proofs from ontogeny, Chapter VI.
In the visceral organs homology is very plain, in fact too plain.
There is not modification enough in most cases even to obscure it,
because function is the same in all animals. These organs do not,
therefore, furnish good illustrations of that essential identity in the
midst of adaptive modification which constitutes the argument for the
derivative origin of structure. It is the organs of _animal life_ that
show this most perfectly, because it is these that take hold on the
environment and are modified by it. There are, however, a few striking
illustrations to be found among the visceral organs, especially the
blood-system. This, however, had better also be deferred to the chapter
on ontogeny.