On the Genesis of Species

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 of the flying-dragon, for the latter is formed of certain ribs, and not of limb bones.

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 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."

"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 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.

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), 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.

Now, these various appendages are distinct and different enough as we 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.

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.

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 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 the right and left sides respectively.

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 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.

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 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.

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 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, 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 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.

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 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.

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 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 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.

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 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 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 of correlation presents numerous and remarkable exceptions.

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 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.

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 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 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.

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 quelques metacarpiens et metatarsiens, enfin de quelques os du carpe et du tarse."

Professor Burt G. Wilder, in his paper on extra digits,^[188] has 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 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 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 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 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 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 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.

CHAPTER IX.

EVOLUTION AND ETHICS.

The origin of morals an inquiry not foreign to the subject of this book.—Modern utilitarian view as to that origin.—Mr. Darwin's speculation as to the origin of the abhorrence of incest.—Cause assigned by him insufficient.—Care of the aged and infirm opposed by "Natural Selection;" also self-abnegation and asceticism.—Distinctness of the ideas "right" and "useful."—Mr. John Stuart Mill.—Insufficiency of "Natural Selection" to account for the origin of the distinction between duty and profit.—-Distinction of moral acts into "material" and "formal."—No ground for believing that formal morality exists in brutes.—Evidence that it does exist in savages.—Facility with which savages may be misunderstood.—Objections as to diversity of customs.—Mr. Hutton's review of Mr. Herbert Spencer.—Anticipatory character of morals.—Sir John Lubbock's explanation.—Summary and conclusion.

Any inquiry into the origin of the notion of "morality"—the conception of "right"—may, perhaps, be considered as somewhat remote from the question of the Genesis of Species; the more so, since Mr. Darwin, at one time, disclaimed any pretension to explain the origin of the higher psychical phenomena of man. His disciples, however, were never equally reticent, and indeed he himself is now not only about to produce a work on man (in which this question must be considered), but he has distinctly announced the extension of the application of his theory to the very phenomena in question. He says:^[202] "In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Light will be thrown on the origin of man and his history." It may not be amiss then to glance slightly at the question, so much disputed, concerning the origin of ethical conceptions and its bearing on the theory of "Natural Selection."

The followers of Mr. John Stuart Mill, of Mr. Herbert Spencer, and apparently, also, of Mr. Darwin, assert that in spite of the great present difference between the ideas "useful" and "right," yet that they are, nevertheless, one in origin, and that that origin consisted ultimately of pleasurable and painful sensations.

They say that "Natural Selection" has evolved moral conceptions from perceptions of what was useful, i.e. pleasurable, by having through long ages preserved a predominating number of those individuals who have had a natural and spontaneous liking for practices and habits of mind useful to the race, and that the same power has destroyed a predominating number of those individuals who possessed a marked tendency to contrary practices. The descendants of individuals so preserved have, they say, come to inherit such a liking and such useful habits of mind, and that at last (finding this inherited tendency thus existing in themselves, distinct from their tendency to conscious self-gratification) they have become apt to regard it as fundamentally distinct, innate, and independent of all experience. In fact, according to this school, the idea of "right" is only the result of the gradual accretion of useful predilections which, from time to time, arose in a series of ancestors naturally selected. In this way, "morality" is, as it were, the congealed past experience of the race, and "virtue" becomes no more than a sort of "retrieving," which the thus improved human animal practises by a perfected and inherited habit, regardless of self-gratification, just as the brute animal has acquired the habit of seeking prey and bringing it to his master, instead of devouring it himself.

Though Mr. Darwin has not as yet expressly advocated this view, yet some remarks made by him appear to show his disposition to sympathise with it. Thus, in his work on "Animals and Plants under Domestication,"^[203] he asserts that "the savages of Australia and South America hold the crime of incest in abhorrence;" but he considers that this abhorrence has probably arisen by "Natural Selection," the ill effects of close interbreeding causing the less numerous and less healthy offspring of incestuous unions to disappear by degrees, in favour of the descendants (greater both in number and strength) of individuals who naturally, from some cause or other, as he suggests, preferred to mate with strangers rather than with close blood-relations; this preference being transmitted and becoming thus instinctive, or habitual, in remote descendants.

But on Mr. Darwin's own ground, it maybe objected that this notion fails to account for "abhorrence," and "moral reprobation;" for, as no stream can rise higher than its source, the original "slight feeling" which was useful would have been perpetuated, but would never have been augmented beyond the degree requisite to ensure this beneficial preference, and therefore would not certainly have become magnified into "abhorrence." It will not do to assume that the union of males and females, each possessing the required "slight feeling," must give rise to offspring with an intensified feeling of the same kind; for, apart from reversion, Mr. Darwin has called attention to the unexpected modifications which sometimes result from the union of similarly constituted parents. Thus, for example, he tells us:^[204] "If two top-knotted canaries are matched, the young, instead of having very fine top-knots, are generally bald." From examples of this kind, it is fair, on Darwinian principles, to infer that the union of parents who possessed a similar inherited aversion might result in phenomena quite other than the augmentation of such aversion, even if the two aversions should be altogether similar; while, very probably, they might be so different in their nature as to tend to neutralize each other. Besides, the union of parents so similarly emotional would be rare indeed amongst savages, where marriages would be owing to almost anything rather than to congeniality of mind between the spouses. Mr. Wallace tells us,^[205] that they choose their wives for "rude health and physical beauty," and this is just what might be naturally supposed. Again, we must bear in mind the necessity there is that many individuals should be similarly and simultaneously affected with this aversion from consanguineous unions; as we have seen in the second chapter, how infallibly variations presented by only a few individuals, tend to be eliminated by mere force of numbers. Mr. Darwin indeed would throw back this aversion, if possible, to a pre-human period; since he speculates as to whether the gorillas or orang-utans, in effecting their matrimonial relations, show any tendency to respect the prohibited degrees of affinity.^[206] No tittle of evidence, however, has yet been adduced pointing in any such direction, though surely if it were of such importance and efficiency as to result (through the aid of "Natural Selection" alone) in that "abhorrence" before spoken of, we might expect to be able to detect unmistakeable evidence of its incipient stages. On the contrary, as regards the ordinary apes (for with regard to the highest there is no evidence of the kind) as we see them in confinement, it would be difficult to name any animals less restricted, by even a generic bar, in the gratification of the sexual instinct. And although the conditions under which they have been observed are abnormal, yet these are hardly the animals to present us in a state of nature, with an extraordinary and exceptional sensitiveness in such matters.

To take an altogether different case. Care of, and tenderness towards, the aged and infirm are actions on all hands admitted to be "right;" but it is difficult to see how such actions could ever have been so useful to a community as to have been seized on and developed by the exclusive action of the law of the "survival of the fittest." On the contrary, it seems probable that on strict utilitarian principles the rigid political economy of Tierra del Fuego would have been eminently favoured and diffused by the impartial action of "Natural Selection" alone. By the rigid political economy referred to, is meant that destruction and utilization of "useless mouths" which Mr. Darwin himself describes in his highly interesting "Journal of Researches."^[207] He says: "It is certainly true, that when pressed in winter by hunger, they kill and devour their old women before they kill their dogs. The boy being asked why they did this, answered, 'Doggies catch otters, old women no.' They often run away into the mountains, but they are pursued by the men and brought back to the slaughter-house at their own firesides." Mr. Edward Bartlett, who has recently returned from the Amazons, reports that at one Indian village where the cholera made its appearance, the whole population immediately dispersed into the woods, leaving the sick to perish uncared for and alone. Now, had the Indians remained, undoubtedly far more would have died; as doubtless, in Tierra del Fuego, the destruction of the comparatively useless old women has often been the means of preserving the healthy and reproductive young. Such acts surely must be greatly favoured by the stern and unrelenting action of exclusive "Natural Selection."

In the same way that admiration which all feel for acts of self-denial done for the good of others, and tending even towards the destruction of the actor, could hardly be accounted for on Darwinian principles alone; for self-immolators must but rarely leave direct descendants, while the community they benefit must by their destruction tend, so far, to morally deteriorate. But devotion to others of the same community is by no means all that has to be accounted for. Devotion to the whole human race, and devotion to God—in the form of asceticism—have been and are very generally recognized as "good;" and the Author contends that it is simply impossible to conceive that such ideas and sanctions should have been developed by "Natural Selection" alone, from only that degree of unselfishness necessary for the preservation of brutally barbarous communities in the struggle for life. That degree of unselfishness once attained, further improvement would be checked by the mutual opposition of diverging moral tendencies and spontaneous variations in all directions. Added to which, we have the principle of reversion and atavism, tending powerfully to restore and reproduce that more degraded anterior condition whence the later and better state painfully emerged.

Very few, however, dispute the complete distinctness, here and now, of the ideas of "duty" and "interest" whatever may have been the origin of those ideas. No one pretends that ingratitude may, in any past abyss of time, have been a virtue, or that it may be such now in Arcturus or the Pleiades. Indeed, a certain eminent writer of the utilitarian school of ethics has amusingly and very instructively shown how radically distinct even in his own mind are the two ideas which he nevertheless endeavours to identify. Mr. John Stuart Mill, in his examination of "Sir William Hamilton's Philosophy," says,^[208] if "I am informed that the world is ruled by a being whose attributes are infinite, but what they are we cannot learn, nor what the principles of his government, except that 'the highest human morality which we are capable of conceiving' does not sanction them; convince me of it, and I will bear my fate as I may. But when I am told that I must believe this, and at the same time call this being by the names which express and affirm the highest human morality, I say in plain terms that I will not. Whatever power such a being may have over me, there is one thing which he shall not do: he shall not compel me to worship him. I will call no being good, who is not what I mean when I apply that epithet to my fellow-creatures; and if such a being can sentence me to hell for not so calling him, to hell I will go."

This is unquestionably an admirable sentiment on the part of Mr. Mill (with which every absolute moralist will agree), but it contains a complete refutation of his own position, and is a capital instance^[209] of the vigorous life of moral intuition in one who professes to have eliminated any fundamental distinction between the "right" and the "expedient." For if an action is morally good, and to be done, merely in proportion to the amount of pleasure it secures, and morally bad and to be avoided as tending to misery, and if it could be proved that by calling God good—whether He is so or not, in our sense of the term,—we could secure a maximum of pleasure, and by refusing to do so we should incur endless torment, clearly, on utilitarian principles, the flattery would be good.

Mr. Mill, of course, must also mean that, in the matter in question, all men would do well to act with him. Therefore, he must mean that it would be well for all to accept (on the hypothesis above given) infinite and final misery for all as the result of the pursuit of happiness as the only end.

It must be recollected that in consenting to worship this unholy God, Mr. Mill is not asked to do harm to his neighbour, so that his refusal reposes simply on his perception of the immorality of the requisition. It is also noteworthy that an omnipotent Deity is supposed incapable of altering Mr. Mill's mind and moral perceptions.

Mr. Mill's decision is right, but it is difficult indeed to see how, without the recognition of an "absolute morality," he can justify so utter and final an abandonment of all utility in favour of a clear and distinct moral perception.

These two ideas, the "right" and the "useful," being so distinct here and now, a greater difficulty meets us with regard to their origin from some common source, than met us before when considering the first beginnings of certain bodily structures. For the distinction between the "right" and the "useful" is so fundamental and essential that not only does the idea of benefit not enter into the idea of duty, but we see that the very fact of an act not being beneficial to us makes it the more praiseworthy, while gain tends to diminish the merit of an action. Yet this idea, "right," thus excluding, as it does, all reference to utility or pleasure, has nevertheless to be constructed and evolved from utility and pleasure, and ultimately from pleasurable sensations, if we are to accept pure Darwinianism: if we are to accept, that is, the evolution of man's psychical nature and highest powers, by the exclusive action of "Natural Selection," from such faculties as are possessed by brutes; in other words, if we are to believe that the conceptions of the highest human morality arose through minute and fortuitous variations of brutal desires and appetites in all conceivable directions.

It is here contended, on the other hand, that no conservation of any such variations could ever have given rise to the faintest beginning of any such moral perceptions; that by "Natural Selection" alone the maxim fiat justitia, ruat cœlum could never have been excogitated, still less have found a widespread acceptance; that it is impotent to suggest even an approach towards an explanation of the first beginning of the idea of "right." It need hardly be remarked that acts may be distinguished not only as pleasurable, useful, or beautiful, but also as good in two different senses: (1) materially moral acts, and (2) acts which are formally moral. The first are acts good in themselves, as acts, apart from any intention of the agent which may or may not have been directed towards "right." The second are acts which are good not only in themselves, as acts, but also in the deliberate intention of the agent who recognizes his actions as being "right." Thus acts may be materially moral or immoral, in a very high degree, without being in the least formally so. For example, a person may tend and minister to a sick man with scrupulous care and exactness, having in view all the time nothing but the future reception of a good legacy. Another may, in the dark, shoot his own father, taking him to be an assassin, and so commit what is materially an act of parricide, though formally it is only an act of self-defence of more or less culpable rashness. A woman may innocently, because ignorantly, marry a married man, and so commit a material act of adultery. She may discover the facts, and persist, and so make her act formal also.

Actions of brutes, such as those of the bee, the ant, or the beaver, however materially good as regards their relation to the community to which such animals belong, are absolutely destitute of the most incipient degree of real, i.e. formal "goodness," because unaccompanied by mental acts of conscious will directed towards the fulfilment of duty. Apology is due for thus stating so elementary a distinction, but the statement is not superfluous, for confusion of thought, resulting from confounding together these very distinct things, is unfortunately far from uncommon.

Thus some Darwinians assert that the germs of morality exist in brutes, and we have seen that Mr. Darwin himself speculates on the subject as regards the highest apes. It may safely be affirmed, however, that there is no trace in brutes of any actions simulating morality which are not explicable by the fear of punishment, by the hope of pleasure, or by personal affection. No sign of moral reprobation is given by any brute, and yet had such existed in germ through Darwinian abysses of past time, some evidence of its existence must surely have been rendered perceptible through "survival of the fittest" in other forms besides man, if that "survival" has alone and exclusively produced it in him.

Abundant examples may, indeed, be brought forward of useful acts which simulate morality, such as parental care of the young, &c. But did the most undeviating habits guide all brutes in such matters, were even aged and infirm members of a community of insects or birds carefully tended by young which benefited by their experience, such acts would not indicate even the faintest rudiment of real, i.e. formal, morality. "Natural Selection" would, of course, often lead to the prevalence of acts beneficial to a community, and to acts materially good; but unless they can be shown to be formally so, they are not in the least to the point, they do not offer any explanation of the origin of an altogether new and fundamentally different motive and conception.

It is interesting, on the other hand, to note Mr. Darwin's statement as to the existence of a distinct moral feeling, even in, perhaps, the very lowest and most degraded of all the human races known to us. Thus in the same "Journal of Researches"^[210] before quoted, bearing witness to the existence of moral reprobation on the part of the Fuegians, he says: "The nearest approach to religious feeling which I heard of was shown by York Minster (a Fuegian so named), who, when Mr. Bynoe shot some very young ducklings as specimens, declared in the most solemn manner, 'Oh, Mr. Bynoe, much rain, snow, blow much.' This was evidently a retributive punishment for wasting human food."