On the Genesis of Species Part 10

(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 Acanthometrae 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 a.s.sertion, 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 h.o.m.ology 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_ planariae in this fashion.[169] But even if this were as a.s.serted, {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 ma.s.ses 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.

[Ill.u.s.tration: 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 larvae within it, which develop and burst the body of the primary larva. The secondary larvae give rise, similarly, to another set within them, and these again to another[171] set.

Again, the fact that in _Taenia 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 h.o.m.ology to the mollusca, but it is difficult to see why the sh.e.l.l segments of chiton are not such h.o.m.ologues because the segmentation is superficial. Similarly the external processes of eolis, doris, &c., are good examples of serial h.o.m.ology, as also are plainly the successive chambers of the orthoceratidae. 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 palaeozoic strata; and annelids, probably formed mainly like those of the present day, abounded during the deposition of the oldest fossiliferous rocks.

[Ill.u.s.tration: TRILOBITE.]

Thirdly, and lastly, as regards such serial h.o.m.ology 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, const.i.tute 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. Gunther informs me that the sluggishness of the common tope (_Galeus vulgaris_) is much like that of the sturgeon, and yet the bodies of its vertebrae are distinct and well-ossified. Moreover, the great salamander of j.a.pan 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 Muller 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 trabeculae 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 cla.s.s, 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 h.o.m.ology. 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 a.s.sisted 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 h.o.m.ological 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 h.o.m.ological internal relations, but that they are so strong and energetic as to re-a.s.sert 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 h.o.m.ological 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.

[Ill.u.s.tration: THE AARD-VARK (ORYCTEROPUS).]

[Ill.u.s.tration: 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 a.s.sertion 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 dent.i.tion 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 cla.s.sed {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.

[Ill.u.s.tration: DUGONG.]

To return, however, to the subject of h.o.m.ological relations: it is surely inconceivable that indefinite variation with survival of the fittest can ever have built up these serial, bilateral, and vertical h.o.m.ologies, 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 a.n.a.logous 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 h.o.m.ological 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 h.o.m.ologous 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.

[Ill.u.s.tration: SKELETON OF AN ICHTHYOSAURUS.]

[Ill.u.s.tration: 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 h.o.m.ology--a repet.i.tion 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} ident.i.ty. 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.

[Ill.u.s.tration: 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 h.o.m.ologous 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 versa_.[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 repete d'un membre thoracique au membre abdominal du meme cote." And he afterwards quotes from Weitbrecht,[187] who had "observe dans un cas l'absence simultanee aux deux mains et aux deux pieds, de quelques doigts, de {180} quelques metacarpiens et metatarsiens, enfin de quelques os du carpe et du ta.r.s.e."

[Ill.u.s.tration: LONG FLEXOR MUSCLES AND TENDONS OF THE HAND.

_P.t._ p.r.o.nator 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 h.o.m.ologous 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 'b.a.s.t.a.r.d 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 h.o.m.ologous 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 ill.u.s.trate this h.o.m.ological tendency. My friend and colleague Mr.

George G. Gascoyen, a.s.sistant 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 pota.s.sium 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 tibiae 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 h.o.m.ologous. 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 G.o.dfrey), 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 ill.u.s.trated in Plate III., "The a.n.a.logy 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 h.o.m.ologies, 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 h.o.m.ologies.

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 ma.s.s, 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 apt.i.tude to aggregate into the form of that species; just as{185} in the atoms of a salt, there dwells the intrinsic apt.i.tude 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 a.n.a.logous force displayed by organic limits."