HOMOLOGIES.
Animals made-up of parts mutually related in various ways.--What
homology is.--Its various kinds.--Serial homology.--Lateral
homology.--Vertical homology.--Mr. Herbert Spencer's explanations.--An
internal power necessary, as shown by facts of comparative anatomy.--Of
teratology.--M. St. Hilaire.--Professor Burt
Wilder.--Foot-wings.--Facts of pathology.--Mr. James Paget.--Dr.
William Budd.--The existence of such an internal power of individual
development diminishes the improbability of an analogous law of
specific origination.
That concrete whole which is spoken of as "an individual" (such, _e.g._, as
a bird or a lobster) is formed of a more or less complex aggregation of
parts which are actually (from whatever cause or causes) grouped, together
in a harmonious interdependency, and which have a multitude of complex
relations amongst themselves.
The mind detects a certain number of these relations as it contemplates the
various component parts of an individual in one or other direction--as it
follows up different lines of thought. These perceived relations, though
subjective, _as relations_, have nevertheless an objective foundation as
real parts, or conditions of parts, of real wholes; they are, therefore,
true relations, such, _e.g._, as those between the right and left hand,
between the hand and the foot, &c.
The component parts of each concrete whole have also a relation of
resemblance to the parts of other concrete wholes, whether of the same{156}
or of different kinds, as the resemblance between the hands of two men, or
that between the hand of a man and the fore-paw of a cat.
Now, it is here contended that the relationships borne one to another by
various component parts, imply the existence of some innate, internal
condition, conveniently spoken of as a power or tendency, which is quite as
mysterious as is any innate condition, power, or tendency, resulting in the
orderly evolution of successive specific manifestations. These
relationships, as also this developmental power, will doubtless, in a
certain sense, be somewhat further explained as science advances. But the
result will be merely a shifting of the inexplicability a point backwards,
by the intercalation of another step between the action of the internal
condition or power and its external result. In the meantime, even if by
"Natural Selection" we could eliminate the puzzles of the "origin of
species," yet other phenomena, not less remarkable (namely, those noticed
in this chapter), would still remain unexplained and as yet inexplicable.
It is not improbable that, could we arrive at the causes conditioning all
the complex inter-relations between the several parts of one animal, we
should at the same time obtain the key to unlock the secrets of specific
origination.
It is desirable, then, to see what facts there are in animal organization
which point to innate conditions (powers and tendencies), as yet
unexplained, and upon which the theory of "Natural Selection" is unable to
throw any explanatory light.
The facts to be considered are the phenomena of "homology," and especially
of serial, bilateral, and vertical homology.
The word "homology" indicates such a relation between two parts that they
may be said in some sense to be "the same," or at least "of similar
nature." This similarity, however, does not relate to the _use_ to which
parts are put, but only to their relative position with regard to other
parts, or to their mode of origin. There are many kinds of {157}
homology,[161] but it is only necessary to consider the three kinds above
enumerated.
[Illustration: WINGBONES OF PTERODACTYLE, BAT, AND BIRD.]
The term "homologous" may be applied to parts in two individual animals of
different kinds, or to different parts of the same individual. Thus "the
right and left hands," or "joints of the backbone," or "the teeth of the
two jaws," are homologous parts of the same individual. But the arm of a
man, the fore-leg of the horse, the paddle of the whale, and the wing of
the bat and the bird are all also homologous parts, yet of another kind,
_i.e._ they are the same parts existing in animals of different species.
On the other hand, the wing of the humming-bird and the wing of the
humming-bird moth are not homologous at all, or in any sense; for the
resemblance between them consists solely in the use to which they are put,
and is therefore only a relation of _analogy_. There is no relation of
_homology_ between them, because they have no common resemblance as to
their relations to surrounding parts, or as to their mode of origin.
Similarly, there is no homology between the wing of the bat and that {158}
of the flying-dragon, for the latter is formed of certain ribs, and not of
limb bones.
[Illustration: SKELETON OF THE FLYING DRAGON.
(Showing the elongated ribs which support the flitting organ.)]
Homology may be further distinguished into (1) a relationship which, on
evolutionary principles, would be due to descent from a common ancestor, as
the homological relation between the arm-bone of the horse and that of the
ox, or between the singular ankle bones of the two lemurine genera,
cheirogaleus and galago, and which relation has been termed by Mr. Ray
Lankester "homogeny;"[162] and (2) a relationship induced, not
derived--such as exists between parts closely similar in relative position,
but with no genetic affinity, or only a remote one, as the homological
relation between the chambers of the heart of a bat and those of a {159}
bird, or the similar teeth of the thylacine and the dog before spoken of.
For this relationship Mr. Bay Lankester has proposed the term "homoplasy."
[Illustration: TARSAL BONES OF DIFFERENT LEMUROIDS.
(Right tarsus of Galago; left tarsus of Cheirogaleus.)]
[Illustration: A CENTIPEDE.]
"Serial homology" is a relation of resemblance existing between two or more
parts placed in series one behind the other in the same individual.
Examples of such homologues are the ribs, or joints of the backbone of{160}
a horse, or the limbs of a centipede. The latter animal is a striking
example of serial homology. The body (except at its two ends) consists of a
longitudinal series of similar segments. Each segment supports a pair of
limbs, and the appendages of all the segments (except as before) are
completely alike.
[Illustration: SQUILLA.]
A less complete case of serial homology is presented by Crustacea (animals
of the crab class), notably by the squilla and by the common lobster. In
the latter animal we have a six-jointed abdomen (the so-called tail), {161}
in front of which is a large solid mass (the cephalo-thorax), terminated
anteriorly by a jointed process (the rostrum). On the under surface of the
body we find a quantity of moveable appendages. Such are, _e.g._, feelers
(Fig. 9), jaws (Figs. 6, 7, and 8), foot-jaws (Fig. 5), claws and legs
(Figs. 3 and 4), beneath the cephalo-thorax; and flat processes (Fig. 2),
called "swimmerets," beneath the so-called tail or abdomen.
[Illustration: PART OF THE SKELETON OF THE LOBSTER.]
Now, these various appendages are distinct and different enough as we {162}
see them in the adult, but they all appear in the embryo as buds of similar
form and size, and the thoracic limbs at first consist each of two members,
as the swimmerets always do.
This shows what great differences may exist in size, in form, and in
function, between parts which are developmentally the same, for all these
appendages are modifications of one common kind of structure, which becomes
differently modified in different situations; in other words, they are
serial homologues.
The segments of the body, as they follow one behind the other, are also
serially alike, as is plainly seen in the abdomen or tail. In the
cephalo-thorax of the lobster, however, this is disguised. It is therefore
very interesting to find that in the other crustacean before mentioned, the
squilla, the segmentation of the body is more completely preserved, and
even the first three segments, which go to compose the head, remain
permanently distinct.
[Illustration: SPINE OF GALAGO ALLENII.]
Such an obvious and unmistakeable serial repetition of parts does not
obtain in the highest, or backboned animals, the Vertebrata. Thus in man
and other mammals, nothing of the kind is _externally_ visible, and we have
to penetrate to his skeleton to find such a series of homologous parts.
There, indeed, we discover a number of pairs of bones, each pair so
obviously resembling the others, that they all receive a common name--the
ribs. There also (_i.e._ in the skeleton) we find a still more remarkable
series of similar parts, the joints of the spine or backbone (vertebræ),
which are admitted by all to possess a certain community of structure.{163}
It is in their limbs, however, that the Vertebrata present the most obvious
and striking serial homology--almost the only serial homology noticeable
externally.
The facts of serial homology seem hardly to have excited the amount of
interest they certainly merit.
Very many writers, indeed, have occupied themselves with investigations and
speculations as to what portions of the leg and foot answer to what parts
of the arm and hand, a question which has only recently received a more or
less satisfactory solution through the successive concordant efforts of
Professor Humphry,[163] Professor Huxley,[164] the Author of this
work,[165] and Professor Flower.[166] Very few writers, however, have
devoted much time or thought to the question of serial homology in general.
Mr. Herbert Spencer, indeed, in his very interesting "First Principles of
Biology," has given forth ideas on this subject, which are well worthy
careful perusal and consideration, and some of which apply also to the
other kinds of homology mentioned above. He would explain the serial
homologies of such creatures as the lobster and centipede thus: Animals of
a very low grade propagate themselves by spontaneous fission. If certain
creatures found benefit from this process of division remaining incomplete,
such creatures (on the theory of "Natural Selection") would transmit their
selected tendency to such incomplete division to their posterity. In this
way, it is conceivable, that animals might arise in the form of long chains
of similar segments, each of which chains would consist of a number of
imperfectly separated individuals, and be equivalent to a series of
separate individuals belonging to kinds in which the fission was complete.
In other words, Mr. Spencer would explain it as the coalescence of {164}
organisms of a lower degree of aggregation in one longitudinal series,
through survival of the fittest aggregations. This may be so. It is
certainly an ingenious speculation, but facts have not yet been brought
forward which demonstrate it. Had they been so, this kind of serial
homology might be termed "homogenetic."
The other kind of serial repetitions, namely, those of the vertebral
column, are explained by Mr. Spencer as the results of alternate strains
and compressions acting on a primitively homogeneous cylinder. The serial
homology of the fore and hind limbs is explained by the same writer as the
result of a similarity in the influences and conditions to which they are
exposed. Serial homologues so formed might be called, as Mr. Ray Lankester
has proposed, "homoplastic." But there are, it is here contended, abundant
reasons for thinking that the predominant agent in the production of the
homologies of the limbs is an _internal_ force or tendency. And if such a
power can be shown to be necessary in this instance, it may also be
legitimately used to explain such serial homologies as those of the
centipede's segments and of the joints of the backbone. At the same time it
is not, of course, pretended that external conditions do not contribute
their own effects in addition. The presence of this internal power will be
rendered more probable if valid arguments can be brought forward against
the explanations which Mr. Herbert Spencer has offered.
_Lateral homology_ (or bilateral symmetry) is the resemblance between the
right and left sides of an animal, or of part of an animal; as, _e.g._,
between our right hand and our left. It exists more or less at one or other
time of life in all animals, except some very lowly organized creatures. In
the highest animals this symmetry is laid down at the very dawn of life,
the first trace of the future creature being a longitudinal streak--the
embryonic "primitive groove." This kind of homology is explained by Mr.
Spencer as the result of the similar way in which conditions affect {165}
the right and left sides respectively.
[Illustration: VERTEBRÆ OF AXOLOTL.]
_Vertical homology_ (or vertical symmetry) is the resemblance existing
between parts which are placed one above the other beneath. It is much less
general and marked than serial, or lateral homology. Nevertheless, it is
plainly to be seen in the tail region of most fishes, and in the
far-extending dorsal (back) and ventral (belly) fins of such kinds as the
sole and the flounder.
It is also strikingly shown in the bones of the tail of certain efts, as in
_Chioglossa_, where the complexity of the upper (neural) arch is closely
repeated by the inferior one. Again, in _Spelerpes rubra_, where almost
vertically ascending articular processes above are repeated by almost
vertically descending articular processes below. Also in the axolotl, where
there are douple pits, placed side by side, not only superiorly but at the
same time inferiorly.[167]
This kind of homology is also explained by Mr. Spencer as the result of the
similarity of conditions affecting the two parts. Thus he explains the very
general absence of symmetry between the dorsal and ventral surfaces of
animals by the different conditions to which these two surfaces are
respectively exposed, and in the same way he explains the asymmetry of the
flat-fishes (_Pleuronectidæ_), of snails, &c.
Now, first, as regards Mr. Spencer's explanation of animal forms by means
of the influence of external conditions, the following observations may be
made. Abundant instances are brought forward by him of admirable adaptation
of structure to circumstances, but as to the immense majority of these it
is very difficult, if not impossible, to see _how_ external conditions{166}
can have produced, or even tended to have produced them. For example, we
may take the migration of one eye of the sole to the other side of its
head. What is there here either in the darkness, or the friction, or in any
other conceivable external cause, to have produced the first beginning of
such an unprecedented displacement of the eye? Mr. Spencer has beautifully
illustrated that correlation which all must admit to exist between the
forms of organisms and their surrounding external conditions, but by no
means proved that the latter are _the cause_ of the former.
[Illustration: PLEURONECTIDÆ, WITH THE PECULIARLY PLACED EYE IN DIFFERENT
POSITIONS.]
Some internal conditions (or in ordinary language some internal power and
force) must be conceded to living organisms, otherwise incident forces must
act upon them and upon non-living aggregations of matter in the same way
and with similar effects.
If the mere presence of these incident forces produces so ready a response
in animals and plants, it must be that there are, in their case, conditions
disposing and enabling them so to respond, according to the old maxim,
_Quicquid recipitur, recipitur ad modum recipientis_, as the same rays of
light which bleach a piece of silk, blacken nitrate of silver. If,
therefore, we attribute the forms of organisms to the action of {167}
external conditions, _i.e._ of incident forces on their modifiable
structure, we give but a partial account of the matter, removing a step
back, as it were, the action of the internal condition, power, or force
which must be conceived as occasioning such ready modifiability. But indeed
it is not at all easy to see how the influence of the surface of the ground
or any conceivable condition or force can produce the difference which
exists between the ventral and dorsal shields of the carapace of a
tortoise, or by what differences of merely external causes the ovaries of
the two sides of the body can be made equal in a bat and unequal in a bird.
[Illustration: AN ECHINUS, OR SEA-URCHIN.
(The spines removed from one-half.)]
There is, on the other hand, an _a priori_ reason why we should expect to
find that the symmetrical forms of all animals are due to internal causes.
This reason is the fact that the symmetrical forms of minerals are
undoubtedly due to such causes. It is unnecessary here to do more than
allude to the beautiful and complex forms presented by inorganic
structures. With regard to organisms, however, the wonderful Acanthometræ
and the Polycystina may be mentioned as presenting complexities of form
which can hardly be thought to be due to other than _internal_ causes. The
same may be said of the great group of Echinoderms, with their amazing{168}
variety of component parts. If then internal forces can so build up the
most varied structures, they are surely capable of producing the serial,
lateral, and vertical symmetries which higher animal forms exhibit. Mr.
Spencer is the more bound to admit this, inasmuch as in his doctrine of
"physiological units" he maintains that these organic atoms of his have an
innate power of building up and evolving the whole and perfect animal from
which they were in each case derived. To build up and evolve the various
symmetries here spoken of is not one whit more mysterious. Directly to
refute Mr. Spencer's assertion, however, would require the bringing forward
of examples of organisms which are ill-adapted to their positions, and out
of harmony with their surroundings--a difficult task indeed.[168]
Secondly, as regards the last-mentioned author's explanation of such serial
homology as exists in the centipede and its allies, the very groundwork is
open to objection. Multiplication by spontaneous fission seems from some
recent researches to be much less frequent than has been supposed, and more
evidence is required as to the fact of the habitual propagation of _any_
planariæ in this fashion.[169] But even if this were as asserted, {169}
nevertheless it fails to explain the peculiar condition presented by
_Syllis_ and some other annelids, where a new head is formed at intervals
in certain segments of the body. Here there is evidently an innate tendency
to the development at intervals of a complex whole. It is not the budding
out or spontaneous fission of certain segments, but the transformation in a
definite and very peculiar manner of parts which already exist into other
and more complex parts. Again, the processes of development presented by
some of these creatures do not by any means point to an origin through{170}
the linear coalescence of primitively distinct animals by means of
imperfect segmentation. Thus in certain Diptera (two winged flies) the
legs, wings, eyes, &c., are derived from masses of formative tissue (termed
imaginal disks), which by their mutual approximation together build up
parts of the head and body,[170] recalling to mind the development of
Echinoderms.
[Illustration: AN ANNELID DIVIDING SPONTANEOUSLY.
(A new head having been formed towards the hinder end of the body of the
parent.)]
Again, Nicholas Wagner found in certain other Diptera, the Hessian flies,
that the larva gives rise to secondary larvæ within it, which develop and
burst the body of the primary larva. The secondary larvæ give rise,
similarly, to another set within them, and these again to another[171] set.
Again, the fact that in _Tænia echinococcus_ one egg produces numerous
individuals, tends to invalidate the argument that the increase of segments
during development is a relic of specific genesis.
Mr. H. Spencer seems to deny serial homology to the mollusca, but it is
difficult to see why the shell segments of chiton are not such homologues
because the segmentation is superficial. Similarly the external processes
of eolis, doris, &c., are good examples of serial homology, as also are
plainly the successive chambers of the orthoceratidæ. Nor are parts of a
series less serial, because arranged spirally, as in most gasteropods. Mr.
Spencer observes of the molluscous as of the vertebrate animal, "You cannot
cut it into transverse slices, each of which contains a digestive organ, a
respiratory organ, a reproductive organ, &c."[172] But the same may be said
of every single arthropod and annelid if it be meant that all these organs
are not contained in every possible slice. While if it be meant that parts
of all such organs are contained in certain slices, then some of the
mollusca may also be included.
Another objection to Mr. Spencer's speculation is derived from
considerations which have already been stated, as to past time. For if{171}
the annulose animals have been formed by aggregation, we ought to find this
process much less perfect in the oldest form. But a complete development,
such as already obtains in the lobster, &c., was reached by the Eurypterida
and Trilobites of the palæozoic strata; and annelids, probably formed
mainly like those of the present day, abounded during the deposition of the
oldest fossiliferous rocks.
[Illustration: TRILOBITE.]
Thirdly, and lastly, as regards such serial homology as is exemplified by
the backbone of man, there are also several objections to Mr. Spencer's
mechanical explanation.
On the theory of evolution most in favour, the first Vertebrata were
aquatic. Now, as natation is generally effected by repeated and vigorous
lateral flexions of the body, we ought to find the segmentation much more
complete laterally than on the dorsal and ventral aspects of the spinal
column. Nevertheless, in those species which, taken together, constitute a
series of more and more distinctly segmented forms, the segmentation
gradually increases _all round_ the central part of the spinal column.
Mr. Spencer[173] thinks it probable that the sturgeon has retained the
notochordal (that is, the primitive, unsegmented) structure because it{172}
is sluggish. But Dr. Günther informs me that the sluggishness of the common
tope (_Galeus vulgaris_) is much like that of the sturgeon, and yet the
bodies of its vertebræ are distinct and well-ossified. Moreover, the great
salamander of Japan is much more inert and sluggish than either, and yet it
has a well-developed, bony spine.
I can learn nothing of the habits of the sharks _Hexanchus_, _Heptanchus_,
and _Echinorhinus_, but Müller describes them as possessing a persistent
_chorda dorsalis_.[174] It may be they have the habits of the tope, but
other sharks are amongst the very swiftest and most active of fishes.
In the bony pike (_lepidosteus_), the rigidity of the bony scales by which
it is completely enclosed must prevent any excessive flexion of the body,
and yet its vertebral column presents a degree of ossification and
vertebral completeness greater than that found in any other fish whatever.
Mr. Spencer supports his argument by the non-segmentation of the anterior
end of the skeletal axis, _i.e._ by the non-segmentation of the skull. But
in fact the skull _is_ segmented, and, according to the quasi-vertebral
theory of the skull put forward by Professor Huxley,[175] is probably
formed of a number of coalesced segments, of some of which the trabeculæ
cranii and the mandibular and hyoidean arches are indications. What is,
perhaps, most remarkable however is, that the segmentation of the
skull--its separation into the three occipital, parietal, and frontal
elements--is most complete and distinct in the highest class, and this can
have nothing, however remotely, to do with the cause suggested by Mr.
Spencer.
Thus, then, there is something to be said in opposition to both the
aggregational and the mechanical explanations of serial homology. The
explanations suggested are very ingenious, yet repose upon a very {173}
small basis of fact. Not but that the process of vertebral segmentation may
have been sometimes assisted by the mechanical action suggested.
It remains now to consider what are the evidences in support of the
existence of an internal power, by the action of which these homological
manifestations are evolved. It is here contended that there _is_ good
evidence of the existence of some such special internal power, and that not
only from facts of comparative anatomy, but also from those of
teratology[176] and pathology. These facts appear to show, not only that
there are homological internal relations, but that they are so strong and
energetic as to re-assert and re-exhibit themselves in creatures which, on
the Darwinian theory, are the descendants of others in which they were much
less marked. They are, in fact, sometimes even more plain and distinct in
animals of the highest types than in inferior forms, and, moreover, this
deep-seated tendency acts even in diseased and abnormal conditions.
Mr. Darwin recognizes[177] these homological relations, and does "not doubt
that they may be mastered more or less completely by Natural Selection." He
does not, however, give any explanation of these phenomena other than the
imposition on them of the name "laws of correlation;" and indeed he says,
"The nature of the bond of correlation is frequently quite obscure." Now,
it is surely more desirable to make use, if possible, of one conception
than to imagine a number of, to all appearance, separate and independent
"laws of correlation" between different parts of each animal.
[Illustration: THE AARD-VARK (ORYCTEROPUS).]
[Illustration: THE PANGOLIN (MANIS).]
But even some of these alleged laws hardly appear well founded. Thus Mr.
Darwin, in support of such a law of concomitant variation as regards hair
and teeth, brings forward the case of Julia Pastrana,[178] and a man {174}
of the Burmese Court, and adds,[179] "These cases and those of the hairless
dogs forcibly call to mind the fact that the two orders of mammals, namely,
the Edentata and Cetacea, which are the most abnormal in their dermal
covering, are likewise the most abnormal either by deficiency or redundancy
of teeth." The assertion with regard to these orders is certainly true, but
it should be borne in mind at the same time that the armadillos, which are
much more abnormal than are the American anteaters as regards their dermal
covering, in their dentition are less so. The Cape ant-eater, on the other
hand, the Aard-vark (Orycteropus), has teeth formed on a type quite
different from that existing in any other mammal; yet its hairy coat is not
known to exhibit any such strange peculiarity. Again, those remarkable
scaly ant-eaters of the Old World--the pangolins (Manis)--stand alone
amongst mammals as regards their dermal covering; having been classed {175}
with lizards by early naturalists on account of their clothing of scales,
yet their mouth is like that of the hairy ant-eaters of the New World. On
the other hand, the duck-billed platypus of Australia (Ornithorhynchus) is
the only mammal which has teeth formed of horn, yet its furry coat is
normal and ordinary. Again, the Dugong and Manatee are dermally alike, yet
extremely different as regards the structure and number of their teeth. The
porcupine also, in spite of its enormous armature of quills, is furnished
with as good a supply of teeth as are the hairy members of the same family,
but not with a better one; and in spite of the deficiency of teeth in the
hairless dogs, no converse redundancy of teeth has, it is believed, been
remarked in Angora cats and rabbits. To say the least, then, this law {176}
of correlation presents numerous and remarkable exceptions.
[Illustration: DUGONG.]
To return, however, to the subject of homological relations: it is surely
inconceivable that indefinite variation with survival of the fittest can
ever have built up these serial, bilateral, and vertical homologies,
without the action of some special innate power or tendency so to build up,
possessed by the organism itself in each case. By "special tendency" is
meant one the laws and conditions of which are as yet unknown, but which is
analogous to the innate power and tendency possessed by crystals similarly,
to build up certain peculiar and very definite forms.
First, with regard to comparative anatomy. The correspondence between the
thoracic and pelvic limbs is notorious. Professor Gegenbaur has lately
endeavoured[180] to explain this resemblance by the derivation of each limb
from a primitive form of fin. This fin is supposed to have had a marginal
external (radial) series of cartilages, each of which supported a series of
secondary cartilages, starting from the inner (ulnar) side of the distal
part of the supporting marginal piece. The root marginal piece would become
the humerus or femur, as the case might be: the second marginal piece, with
the piece attached to the inner side of the distal end of the root marginal
piece, would together form either the radius and ulna or the tibia and
fibula, and so on.
Now there is little doubt (from _a priori_ considerations) but that the
special differentiation of the limb bones of the higher Vertebrates has
been evolved from anterior conditions existing in some fish-like form or
other. But the particular view advocated by the learned Professor is open
to criticism. Thus, it may be objected against this view, first, that it
takes no account of the radial ossicle which becomes so enormous in the
mole; secondly, that it does not explain the extra series of ossicles {177}
which are formed on the _outer_ (radial or marginal) side of the paddle in
the Ichthyosaurus; and thirdly, and most importantly, that even if this had
been the way in which the limbs had been differentiated, it would not be at
all inconsistent with the possession of an innate power of producing, and
an innate tendency to produce similar and symmetrical homological
resemblances. It would not be so because resemblances of the kind are found
to exist, which, on the Darwinian theory, must be subsequent and secondary,
not primitive and ancestral. Thus we find in animals of the eft kind
(certain amphibians), in which the tarsus is cartilaginous, that the carpus
is cartilaginous likewise. And we shall see in cases of disease and of
malformation what a tendency there is to a similar affection of homologous
parts. In efts, as Professor Gegenbaur himself has pointed out,[181] there
is a striking correspondence between the bones or cartilages supporting the
arm, wrist, and fingers, and those sustaining the leg, ankle, and toes,
with the exception that the toes exceed the fingers in number by one.
[Illustration: SKELETON OF AN ICHTHYOSAURUS.]
[Illustration: A. SKELETON OF ANTERIOR EXTREMITY OF AN EFT.
B. SKELETON OF POSTERIOR EXTREMITY OF THE SAME.]
Yet these animals are far from being the root-forms from which all the
Vertebrata have diverged, as is evidenced from the degree of specialization
which their structure presents. If they have descended from such {178}
primitive forms as Professor Gegenbaur imagines, then they have built up a
secondary serial homology--a repetition of similar modifications--fully as
remarkable as if it were primary. The Plesiosauria--those extinct marine
reptiles of the Secondary period, with long necks, small heads, and
paddle-like limbs--are of yet higher organization than are the efts and
other Amphibia. Nevertheless they present us with a similarity of structure
between the fore and hind limb, which is so great as almost to be {179}
identity. But the Amphibia and Plesiosauria, though not themselves
primitive vertebrate types, may be thought by some to have derived their
limb-structure by direct descent from such. Tortoises, however, must be
admitted to be not only highly differentiated organisms, but to be far
indeed removed from primeval vertebrate structure. Yet certain
tortoises[182] (notably _Chelydra Temminckii_) exhibit such a remarkable
uniformity in fore and hind limb structure (extending even up to the
proximal ends of the humerus and femur) that it is impossible to doubt its
independent development in these forms.
[Illustration: SKELETON OF A PLESIOSAURUS.]
Again in the Potto (Perodicticus) there is an extra bone in the foot,
situated in the transverse ligament enclosing the flexor tendons. It is
noteworthy that in the _hand_ of the same animal a serially homologous
structure should also be developed.[183] In the allied form called the slow
lemur (Nycticebus) we have certain arrangements of the muscles and tendons
of the hand which reproduce in great measure those of the foot and _vice
versâ_.[184] And in the Hyrax another myological resemblance appears.[185]
It is, however, needless to multiply instances which can easily be produced
in large numbers if required.
Secondly, with regard to teratology, it is notorious that similar
abnormalities are often found to co-exist in both the pelvic and thoracic
limbs.
M. Isidore Geoffroy St. Hilaire remarks,[186] "L'anomalie se répète d'un
membre thoracique au membre abdominal du même côté." And he afterwards
quotes from Weitbrecht,[187] who had "observé dans un cas l'absence
simultanée aux deux mains et aux deux pieds, de quelques doigts, de {180}
quelques metacarpiens et metatarsiens, enfin de quelques os du carpe et du
tarse."
[Illustration: LONG FLEXOR MUSCLES AND TENDONS OF THE HAND.
_P.t._ Pronator teres. _F.s._ Flexor sublimis digitorum. _F.p._ Flexor
profundus digitorum. _F.l.p._ Flexor longus pollicis.]
Professor Burt G. Wilder, in his paper on extra digits,[188] has {181}
recorded no less than twenty-four cases where such excess coexisted in both
little fingers; also one case in which the right little finger and little
toe were so affected; six in which it was both the little fingers and both
the little toes; and twenty-two other cases more or less the same, but in
which the details were not accurately to be obtained.
Mr. Darwin cites[189] a remarkable instance of what he is inclined to
regard as the development in the foot of birds of a sort of representation
of the wing-feathers of the hand. He says: "In several distinct breeds of
the pigeon and fowl the legs and the two outer toes are heavily feathered,
so that, in the trumpeter pigeon, they appear like little wings. In the
feather-legged bantam, the 'boots,' or feathers, which grow from the
outside of the leg, and generally from the two outer toes, have, according
to the excellent authority of Mr. Hewitt, been seen to exceed the
wing-feathers in length, and in one case were actually nine and a half
inches in length! As Mr. Blyth has remarked to me, these leg-feathers
resemble the primary wing-feathers, and are totally unlike the fine down
which naturally grows on the legs of some birds, such as grouse and owls.
Hence it may be suspected that excess of food has first given redundancy to
the plumage, and then that the law of homologous variation has led to the
development of feathers on the legs, in a position corresponding with those
on the wing, namely, on the outside of the tarsi and toes. I am
strengthened in this belief by the following curious case of correlation,
which for a long time seemed to me utterly inexplicable,--namely, that in
pigeons of any breed, if the legs are feathered, the two outer toes are
partially connected by skin. These two outer toes correspond with our third
and fourth toes. Now, in the wing of the pigeon, or any other bird, the
first and fifth digits are wholly aborted; the second is rudimentary, and
carries the so-called 'bastard wing;' whilst the third and fourth {182}
digits are completely united and enclosed by skin, together forming the
extremity of the wing. So that in feather-footed pigeons not only does the
exterior surface support a row of long feathers like wing-feathers, but the
very same digits which in the wing are completely united by skin become
partially united by skin in the feet; and thus, by the law of the
correlated variation of homologous parts, we can understand the curious
connexion of feathered legs and membrane between the outer toes."
Irregularities in the circulating system are far from uncommon, and
sometimes illustrate this homological tendency. My friend and colleague Mr.
George G. Gascoyen, assistant surgeon at St. Mary's Hospital, has supplied
me with two instances of symmetrical affections which have come under his
observation.
In the first of these the brachial artery bifurcated almost at its origin,
the two halves re-uniting at the elbow-joint, and then dividing into the
radial and ulnar arteries in the usual manner. In the second case an
aberrant artery was given off from the radial side of the brachial artery,
again almost at its origin. This aberrant artery anastomosed below the
elbow-joint with the radial side of the radial artery. In each of these
cases the right and left sides varied in precisely the same manner.
Thirdly, as to pathology. Mr. James Paget,[190] speaking of symmetrical
diseases, says: "A certain morbid change of structure on one side of the
body is repeated in the exactly corresponding part of the other side." He
then quotes and figures a diseased lion's pelvis from the College of
Surgeons Museum, and says of it: "Multiform as the pattern is in which the
new bone, the product of some disease comparable with a human rheumatism,
is deposited--a pattern more complex and irregular than the spots upon a
map--there is not one spot or line on one side which is not represented, as
exactly as it would be in a mirror, on the other. The likeness has more
than daguerreotype exactness." He goes on to observe: "I need not {183}
describe many examples of such diseases. Any out-patients' room will
furnish abundant instances of exact symmetry in the eruptions of eczema,
lepra, and psoriasis; in the deformities of chronic rheumatism, the
paralyses from lead; in the eruptions excited by iodide of potassium or
copaiba. And any large museum will contain examples of equal symmetry in
syphilitic ulcerations of the skull; in rheumatic and syphilitic deposits
on the tibiæ and other bones; in all the effects of chronic rheumatic
arthritis, whether in the bones, the ligaments, or the cartilages; in the
fatty and earthy deposits in the coats of arteries."[191]
He also considered it to be proved that, "Next to the parts which are
symmetrically placed, none are so nearly identical in composition as those
which are homologous. For example, the backs of the hands and of the feet,
or the palms and soles, are often not only symmetrically, but similarly,
affected with psoriasis. So are the elbows and the knees; and similar
portions of the thighs and the arms may be found affected with ichthyosis.
Sometimes also specimens of fatty and earthy deposits in the arteries
occur, in which exact similarity is shown in the plan, though not in the
degree, with which the disease affects severally the humeral and femoral,
the radial and peroneal, the ulnar and posterior tibial arteries."
Dr. William Budd[192] gives numerous instances of symmetry in disease, both
lateral and serial. Thus, amongst others, we have one case (William
Godfrey), in which the hands and feet were distorted. "The distortion of
the right hand is greater than that of the left, of the right foot greater
than that of the left foot." In another (Elizabeth Alford) lepra affected
the extensor surfaces of the thoracic and pelvic limbs. Again, in the case
of skin disease illustrated in Plate III., "The analogy between the {184}
elbows and knees is clearly expressed in the fact that these were the only
parts affected with the disease."[193]
Professor Burt Wilder,[194] in his paper on "Pathological Polarities,"
strongly supports the philosophical importance of these peculiar relations,
adding arguments in favour of antero-posterior homologies, which it is here
unnecessary to discuss, enough having been said, it is believed, to
thoroughly demonstrate the existence of these deep internal relations which
are named lateral and serial homologies.
What explanation can be offered of these phenomena? To say that they
exhibit a "nutritional relation" brought about by a "balancing of forces"
is merely to give a new denomination to the unexplained fact. The changes
are, _of course_, brought about by a "nutritional" process, and the
symmetry is undoubtedly the result of a "balance of forces," but to say so
is a truism. The question is, what is the cause of this "nutritional
balancing"? It is here contended that it must be due to an internal cause
which at present science is utterly incompetent to explain. It is an
internal property possessed by each living organic whole as well as by each
non-living crystalline mass, and that there is such internal power or
tendency, which may be spoken of as a "polarity," seems to be demonstrated
by the instances above given, which can easily be multiplied indefinitely.
Mr. Herbert Spencer[195] (speaking of the reproduction, by budding, of a
Begonia-leaf) recognizes a power of the kind. He says, "We have, therefore,
no alternative but to say that the living particles composing one of these
fragments have an innate tendency to arrange themselves into the shape of
the organism to which they belong. We must infer that a plant or animal of
any species is made up of special units, in all of which there dwells the
intrinsic aptitude to aggregate into the form of that species; just as{185}
in the atoms of a salt, there dwells the intrinsic aptitude to crystallize
in a particular way. It seems difficult to conceive that this can be so;
but we see that it _is_ so." ... "For this property there is no fit term.
If we accept the word polarity as a name for the force by which inorganic
units are aggregated into a form peculiar to them, we may apply this word
to the analogous force displayed by organic limits."
Dr. Jeffries Wyman,[196] in his paper on the "Symmetry and Homology of
Limbs," has a distinct chapter on the "Analogy between Symmetry and
Polarity," illustrating it by the effects of magnets on "particles in a
polar condition."
Mr. J. J. Murphy, after noticing[197] the power which crystals have to
repair injuries inflicted on them and the modifications they undergo
through the influence of the medium in which they may be formed, goes on to
say:[198] "It needs no proof that in the case of spheres and crystals the
forms and the structures are the effect, and not the cause, of the
formative principles. Attraction, whether gravitative or capillary,
produces the spherical form; the spherical form does not produce
attraction. And crystalline polarities produce crystalline structure and
form; crystalline structure and form do not produce crystalline polarities.
The same is not quite so evident of organic forms, but it is equally true
of them also." ... "It is not conceivable that the microscope should reveal
peculiarities of structure corresponding to peculiarities of habitual
tendency in the embryo, which at its first formation has no structure
whatever;"[199] and he adds that "there is something quite inscrutable and
mysterious" in the formation of a new individual from the germinal {186}
matter of the embryo. In another place[200] he says: "We know that in
crystals, notwithstanding the variability of form within the limits of the
same species, there are definite and very peculiar formative laws, which
cannot possibly depend on anything like organic functions, because crystals
have no such functions; and it ought not to surprise us if there are
similar formative or morphological laws among organisms, which, like the
formative laws of crystallization, cannot be referred to any relation of
form or structure to function. Especially, I think, is this true of the
lowest organisms, many of which show great beauty of form, of a kind that
appears to be altogether due to symmetry of growth; as the beautiful
star-like rayed forms of the _acanthometræ_, which are low animal organisms
not very different from the Foraminifera." Their "definiteness of form does
not appear to be accompanied by any corresponding differentiation of
function between different parts; and, so far as I can see, the beautiful
regularity and symmetry of their radiated forms are altogether due to
unknown laws of symmetry of growth, just like the equally beautiful and
somewhat similar forms of the compound six-rayed, star-shaped crystals of
snow."
Altogether, then, it appears that each organism has an innate tendency to
develop in a symmetrical manner, and that this tendency is controlled and
subordinated by the action of external conditions, and not that this
symmetry is superinduced only _ab externo_. In fact, that each organism has
its own internal and special laws of growth and development.
If, then, it is still necessary to conceive an internal law or "substantial
form," moulding each organic being,[201] and directing its development{187}
as a crystal is built up, only in an indefinitely more complex manner, it
is congruous to imagine the existence of some internal law accounting at
the same time for specific divergence as well as for specific identity.
A principle regulating the successive evolution of different organic forms
is not one whit more mysterious than is the mysterious power by which a
particle of structureless sarcode develops successively into an egg, a
grub, a chrysalis, a butterfly, when all the conditions, cosmical,
physical, chemical, and vital, are supplied, which are the requisite
accompaniments to determine such evolution. [Page 188]
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