Bones of the Leg. (Pls. III., IX., X. & XI.)
Of the five femora in the above defined series of remains of the Dodo, two measure 6 inches 3 lines in length; one (Pl. IX.) is 6 inches 4½ lines; the shortest is a little under 6 inches, with proportionate differences in the diameter of the shaft. All of them show a small pneumatic foramen (Pl. IX. figs. 1 & 2, p) on the inner side of the anterior ridge of the great trochanter (ib. c), and on the same transverse line with the head of the bone. This part shows an oblong depression (ib. figs. 2 & 3, a) for the “ligamentum teres” at the upper and back part. The articular surface on the same aspect of the neck (ib. fig. 3, b), adapted to the trochanterian prominence of the pelvis (Pl. VII. t), is well-defined. The trochanter (Pl. IX. fig. 1, c) rises, ridge-like, above the level of the head, and is continued from behind the middle of the articular surface on the neck, forward, with a convex outline upon the fore and outer part of the shaft, where it gradually subsides; a narrow intermuscular ridge (ib. fig. 1, r), inclining to the middle of the fore part of the shaft, is continued from the trochanterian one. The small trochanter (ib. fig. 3, d) is a small subcircular tuberosity, in some specimens a ridge, 3 to 4 lines in length, on the inner side of the shaft, about an inch below the head. The muscular impressions on the fore part of the bone are well defined. A minute medullary canal (ib. fig. 3, m) perforates the middle of the back part of the shaft; the popliteal fossa (ib. fig. 3, o) shows a few small pneumatic orifices; a triangular rough flat surface divides the fossa from the outer condyle. Above the fibular depression (ib. fig. 3, g) there is a well-defined, slightly raised, rough surface (ib. k) for the head of the ectogastrocnemius muscle. The ridge (ib. n) extending to the back part of the inner condyle is not sharp; the rotular groove (ib. fig. 1, p) is deep and moderately wide, with the inner boundary, formed by the narrow anterior part of the inner condyle (ib. fig. 5, e′), most produced. The breadth of this end of the longer femora is 1 inch 9 lines; the character of the distal articular surface is shown in Pl. IX. fig. 5.
The head, neck, and great trochanter (Pl. XI. fig. 6) are occupied by a pneumatic cancellous structure, with a thin compact wall on the upper part and sides: this begins to gain thickness at the under part of the neck and at the lower and back part of the trochanter, the compact wall acquiring a thickness of a line at the beginning of the shaft, where the cancellous structure is confined to the outer side of the pneumatic cavity; this structure gives way to a few delicate filaments of bone crossing the cavity of the major part of the shaft, and is not resumed until the bone expands to form the distal condyles (ib. fig. 7).
The five tibiæ of Didus in the same collection range in length from 8 inches 8 lines to 9 inches. The procnemial ridge (Pl. X. figs. 1, 2, 4, p) is a triangular plate, with the base longest and the apex rounded off: it inclines outwardly, and does not extend much more than half an inch from the level of the proximal end of the bone: the length of its base rather exceeds an inch: on its inner side a triangular muscular surface is well defined by an irregular inferior line or ridge (ib. fig. 2, n). The ectocnemial process (ib. figs. 1, 3, 4, e) is thicker, shorter, and terminates roughly and obtusely. There is a low, narrow ridge (ib. fig. 2, g), about half an inch in length, on the inner side of the proximal end of the shaft, beginning about 9 lines below the articular surface at that end. The fibular ridge (ib. figs. 1 & 3, h), beginning 1 inch 8 lines from the proximal end, extends about 2 inches down the outer side of the shaft. The epicnemial ridge (ib. figs. 1 & 4, k) is obtuse, and but little produced above the upper articular surfaces or condyles (t d) of the tibia: the breadth of that end of the bone, in the longest specimen, is 2 inches 3 lines. The tendinal canal at the fore part of the distal end is bridged by bone (ib. fig. 1, l), and is situated on the inner half of that aspect of the shaft; the lower opening is subcircular and close to the anterior end of the inner lower condyle (ib. a), which is more produced forward than the outer one (ib. b). Their hind ends project very little beyond the level of that aspect of the shaft of the tibia. An intermuscular ridge (ib. fig. 1, r) strengthens into a tuberosity (r′) at the inner side of the tendinal groove.
The cancellous structure in the tibia is limited to an extent of about half an inch below the proximal articular surfaces (Pl. XI. fig. 8), and to about an inch and a half from the distal end of the line (ib. fig. 9): the shaft is occupied by a large air-cavity, with a compact wall of half a line in thickness at the upper third, gradually increasing to about a line at the lower fourth, until the cancellous structure is reestablished; the transverse direction of a plate of this structure indicates the extent of the original distal epiphysis of the tibia (fig. 8).
The fibula (Pl. X. figs. 6–8) presents the usual ornithic characters of the bone: it varies from 4 inches 4 lines to 4 inches 6 lines in length, with a greatest proximal breadth of 8 lines. No adequate gain would result from a detailed description or comparison of this bone; and the rest of the bones of the foot have received every requisite attention in this way in the excellent work on the Dodo and its kindred, already quoted. A longitudinal section of the metatarsus, taken in the direction from side to side (Pl. XI. fig. 10), shows the loose cancellous texture of the common epiphysis of the three long metatarsals, and the remnant of their contiguous coalesced walls reduced to a thin lamella of bone. As the moiety of the bone figured is the posterior one (of the left metatarsus), the usual oblique position of the middle metatarsal (iii), with its proximal end nearer the back part and its distal end nearer the fore part of the coalesced series, produces a corresponding direction of the section, with narrowing and termination of the exposed part of the medullary canal about one-third from the distal end of that metatarsal. The medullary canal of the outer metatarsal (iv) is wider and descends lower before the breaking up of the inner surface into decussating lamellæ or filaments, than that of the inner metatarsal (ii): the peripheral compact wall of the inner is twice the thickness of that of the outer metatarsal. I may remark that the more posterior position of the middle metatarsal at its proximal end, from which and the corresponding part of the common epiphysis the calcaneal process is developed, is related to the greater share taken by the middle toe in the act of walking and scratching. I will only remark that of the four metatarsals of as many Dodos in the present series, one exceeds by a line the length of that figured in plate xi. op. cit., and one falls short thereof to the same trifling amount.
Skull. (Plates III. & XI. fig. 1.)
Of the skull of the Dodo, the series of bones transmitted to me include the cranial part with the detached upper mandibular bone (more or less mutilated) of two mature birds, and the lower mandible of three individuals. In the latter the dentary elements (Pl. XI. fig. 1, 32), confluent at the “gonys,” are distinct from the hinder halves of the rami formed by the confluent, or perhaps connate, articular, surangular and angular elements (ib. 31): if the “splenial” were ever distinct, it has coalesced with the dentary, where its upper boundary is indicated by a linear groove or series of small foramina.
In size, shape, and all other characters of these important evidences of the specific nature of the remains from the Mahébourg morass[29], they agree with those of Didus ineptus detailed in the ‘Proceedings of the Zoological Society’ for January 11th, 1848 (part xvi. pp. 2–8), and in the work entitled “The Dodo and its Kindred,” pp. 76–96.
The occipital condyle (ib. 1) presents the same hemispheroid or reniform shape, with the median vertical notch or depression above. The upper margin of the foramen magnum is broad, as it were excised, with the sides slightly prominent. The superoccipital foramen is present in both specimens, as in the one originally described (Proc. Zool. Soc. part xvi. p. 2). This foramen also exists in Owls and Parrots, but not in all Pigeons; the Didunculus (Pl. III. fig. 2) shows no trace of it; I have also failed to find it in the skull of a Crown-pigeon (Goura coronata). The superoccipital ridge is defined by the subsidence of the surface beneath it being continued directly from the upper, almost flat, smooth surface of the cranium: the middle part of the ridge is more produced than the angles. In the great breadth of the occipital surface compared with its depth, in its flatness from side to side, and its aspect backward and a little upward, Didus most resembles Dinornis. The basioccipital curves downward, and unites with the basisphenoid in developing the pair of larger tuberosities (Pl. XI. fig. 1, 5), which terminate about ½ an inch below the occipital condyle. There is nothing of this structure in the Columbine cranium. In one of my Dodo’s skulls there is a pair of small tubercles between the larger basioccipital ones; these are not developed in the other cranium. The basisphenoid is subquadrate, and flattish below, impressed by a shallow median longitudinal channel.
The hypoglossal nerve escapes by two small foramina on each side of the base of the condyle; external to these is the vagal foramen; still more external is the depression (ib. a) perforated below by the entocarotid, glossopharyngeal, and sympathetic, above by the tympanic vein. The entocarotid canal opens into the hind part of the sella or pituitary fossa: the vagal canal begins within the skull, above the hypoglossal foramina. The paroccipital carries the posterior surface of the skull downward and outward to a much greater degree than in any Dove, but to a less degree than in Dinornis. The Eustachian tubes impress the outer and fore part of the basisphenoid.
The temporal fossæ (Pl. III.), in the present specimens, show the same contraction in proportion to their depth by which the original skull of the Dodo, compared with that of the Dinornis, ‘Proc. Zool. Soc.’ (1848, p. 3), differed from the larger extinct wingless bird. In the approximation of the postorbital process to the mastoid, Didunculus shows a closer resemblance to Didus than does Goura, in which the temporal fossa, besides being narrow, is shallow. The temporal muscle appears to spread its origin above the fossa upon the sides of the cranium, forward half an inch in advance of the postfrontal process, and backward to the outer angle of the superoccipital ridge.
The parietal region is broad, flat, and short, as in Dinornis, not convex as in Doves; it is also impressed at its middle part by a shallow transverse groove, continued outward and forward of less depth and definition, so as to mark off the convex interorbital part of the swollen frontals.
The outer side of the mastoid is convex, smooth, overhanging the tympanic cavity, and sending off a short process, the base of which is defined in one cranium by a transverse ridge in front of the anterior articular cup for the tympanic bone. A similar process is developed in Didunculus, not in Goura, where it is barely indicated.
The presphenoid is compressed, but thickened and rounded below, where the palatines and pterygoids at their junction with each other abut against it: the pterygoid sends off a short process from the middle of its hinder border; but this is not met by a corresponding “pterygoid process” of the basisphenoid as in Didunculus.
The frontals are broad and convex, rising abruptly (as in Didunculus) above the coalesced cranial ends of the nasals and premaxillary (Pl. III.); in Didus the breadth greatly exceeds the length of the interorbital frontal convexity, as compared with Didunculus, and the convexity reigns in the transverse as well as the antero-posterior direction; in Didunculus, however, it is less concave transversely than in Goura. In the breadth or thickness of the interorbital septum Didus resembles Apteryx and Palapteryx and shows the same pneumatic cancellous structure. The posterior olfactory chambers are partially divided, as in Dinornis, by an upper median septum; each compartment, which is 7 lines across and an inch in length, is perforated posteriorly by an olfactory foramen more than a line in diameter, from which grooved impressions of ramifications of the nerve diverge upon the hind and upper wall of the chamber: external to the olfactory foramen is a longer one for the passage of a vein into the fore and inner part of the orbit.
The cranial ends of the nasals and nasal process of the premaxillary (Pl. XI. fig. 1, 22) are flat, depressed, thin plates; the latter at its junction with the frontal is 6 lines broad, partially divided by a median groove above and a ridge below, and by short linear fissures from the nasals: the forward extension of these bones is feebly indicated by linear grooves terminating at the outer margins of the nasal branch of the premaxillary, about 4 inches from its vertical end. The proportion of the base of the upper mandible attached to the frontal contributed by the nasals is the same as that indicated in the ‘Proc. Zool. Soc.’ l. c. The nasal branch of the premaxillary presents a full elliptical transverse section where it quits the maxillary processes, losing both depth and breadth as it recedes to join the nasals; here it retains its breadth, viz. 6 lines, but continues to be thinned off vertically to the plate above named joining the frontal. The under surface of the narrower part of the stem is angular, the upper one being gently convex.
“Where the nasal and maxillary processes diverge, there is a deep groove externally, terminating in a canal directed forwards into the rostral part or body of the premaxillary”[30]. This part is subdecurved, pointed, roughened by irregular vascular perforations and grooves, with a sharp alveolar border, which describes a sigmoid curve lengthwise, and with a deeper concavity of the palatal surface than in Dinornis or Didunculus. Moreover the concavity is partially divided lengthwise by a median ridge. The palatal surfaces of the maxillary processes and maxillaries are narrow and very convex transversely, intercepting a long narrow palato-nasal fissure. The outer side of the maxillary process is deep vertically and slightly concave lengthwise—a structure not known in Didunculus or any Dove, and related, like most other deviations from the Columbine cranial characteristics, to the provision of unwonted strength of beak in the Dodo. The maxillary branches of the premaxillary have completely coalesced with the maxillaries, as these have with the palatines; and the halves of the upper mandible here swell out laterally and more so vertically, the maxillaries rising to combine with the outer divisions of the nasals, and sending back a short process from their lower and lateral part to join the malar. The inner surface of the maxillary process (Pl. XI. fig. 1, 22*) is smooth and slightly convex vertically; both upper and lower borders are obtuse and thick.
The palatines arch outward from their posterior attachments, are broad and smooth mesially; the margin here is angular, with a slightly produced obtuse apex, divided by a channel on the under surface of the palatine from the outer convex border; the upper and outer ridge extends forward to the maxillary; the inner one subsides before reaching that bone. “The palatines form the posterior boundaries of the naso-palatine aperture, and approximate each other at both ends, but more closely posteriorly, yet here without meeting; whilst in Didunculus they coalesce before receiving the abutment of the pterygoids.
“The tympanic bone is subquadrate, with the four angles produced, and the upper and hinder are bifurcate, forming the double condyle for the mastoid articulation”[31]. There is a larger pneumatic foramen, communicating with the tympanic cavity, between the articulating cavities for these condyles.
The brain is singularly small in the present species of Didus: and if it be viewed as an index of intelligence of the bird, the latter may well be termed ineptus. The length of the cranial cavity (Pl. XI. fig. 1, v c) is 1 inch 8 lines, its extreme breadth 1 inch 6 lines, its greatest height 1 inch (and this is at the cerebellar fossa). The most remarkable feature in the cranial structure of Didus is the disproportionate size of the brain-case to the important part of the neural axis it contained and protected: some approximation to this condition is made by Dinornis[32], the Owls, and a few large Cockatoos, e. g. Microglossum aterrimum; but it is fully paralleled only by the Elephant among air-breathing vertebrates, as may be seen by comparing the section Pl. XI. fig. 1 with the figures of a similar section quoted below[33].
Not only was the brain of very small proportional size in the present large extinct bird, but the division of the cranial cavity appropriate to the cerebrum proper is less in proportion to that for the cerebellum and optic lobes, at least in vertical and longitudinal diameters, than in any other known bird.
In the Elephant the thickness of the pneumatic diploë between the fore part of the cerebral cavity and that of the outer cranial wall equals the longitudinal diameter of the cavity containing the cerebral hemispheres: in Didus it exceeds that diameter. The thickness of the pneumatic diploë above the cerebral cavity equals the vertical diameter of that cavity in Didus: the diploë gradually decreases in thickness as it approaches the foramen magnum. The disposition of the osseous lamellæ forming the cells or cavities of the diploë is very different in the Elephant and Dodo: they extend for the most part vertically between the outer and inner tables of the skull in the proboscidian mammal, leaving long and narrow interspaces; in the heavy ground-bird they form a congeries of small subequal and subspherical air-cells, and this structure obtains in the basal and lateral walls as well as in the superior or “roofing” wall of the cranial cavity. The extent of this cancellous structure at the sides of the cranial cavity may be known by the ratio of the breadth of that cavity to the breadth of the cranium, which is 3 inches and 8 lines at the broadest part of the brain, viz. the prosencephalon. It would seem, at first sight, as if the poorly developed brain of the Dodo had needed, on some account, unusual protection; but the true explanation rests on the size, weight, and power of the bill, and the concomitant necessity for adequate extent of attachment of the facial to the cranial part of the skull, and of the muscles from the trunk destined to sustain and wield the long and heavy-beaked head. The cerebrum of the Dodo does not greatly, and by no means proportionally, exceed the size of that part of the brain in the Crown-pigeons (Goura). If the great Ground-dove of the Mauritius gradually gained bulk in the long course of successive generations in that uninhabited thickly-wooded island, and, exempt from the attacks of any enemy, with food enough scattered over the ground, ceased to exert the wings to raise the heavy trunk, then, on Lamarck’s principle, the disused members would atrophy, while the hind limbs, through the increased exercise by habitual motion on land, with increasing weight to support, would hypertrophy.
In the long course of generations subject to this slow rate of change, there would be nothing in the contemporaneous condition of the Mauritian fauna to alarm or in any way to put the Dodo to its wits; being, like other Pigeons, monogamous, the excitement, even, of a seasonal or prenuptial combat, might, as in them, be wanting: we may well suppose the bird to go on feeding and breeding in a lazy, stupid fashion, without call or stimulus to any growth of cerebrum proportionate to the gradually accruing increment of the bulk of the body. Whatever part of the brain was concerned in regulating or controlling muscular actions, might, indeed, be expected to show some concurrent rate of increase with the growing mass of the voluntary contractile fibres; and the size of the cerebellar division (Pl. XI. fig. 1, n o) of the cranial cavity accords with the generally accepted physiology of the superincumbent mass of the epencephalon. The lateral depression at the fore and under part of the side of the postcerebral division of the cranial cavity indicates that the optic lobes, like the eyes, remained almost stationary during the progressive acquisition of the bulk that distinguishes the Dodo from the largest existing Doves.
The proportions of Didus, Pezophaps, Casuarius, Rhea, Dromaius, Struthio, Aptornis, Cnemiornis, Palapteryx, Æpyornis, Dinornis, &c. among terrestrial birds, of Notornis among the lake-haunting Coots, and of Aptenodytes and Alca impennis among seabirds, point to the disuse of wings in flight as the main condition of increase of size in species of birds—the next condition being absence of lethal enemies during the years requisite for such course and rate of growth.
Let foes arise from whom a power of flight is the main condition of escape, and the wingless giants of the feathered class soon succumb. Among the genera above-cited, Aptornis, Cnemiornis, Æpyornis, Palapteryx, Dinornis, Didus, and Pezophaps, with the largest of the Auks, have thus passed away, while Notornis and Apteryx are on the verge of extinction through the rapid increase of population in the small island to which they are restricted. In sparsely peopled continents, such as Africa, South America, and Australia, brevipennate giants may still range the deserts, pampas, and unfrequented wilds. The ascertained recent advent of Man in New Zealand, New Britain, Ceram, Banda, Salwattie, Mauritius, Rodriguez, significantly points to the conditions under which have come to pass, in lapse of time, so strange an anomaly as a bird with the specially modified instruments of flight reduced below the power of exerting that mode of locomotion, yet, as a bird, retaining the conditions of the respiratory and tegumentary systems of the volant class, of which it has become a degenerate member. With the cessation of the chief of those conditions, viz. the absence of enemies, such birds necessarily perish.
Refraining, however, from further indulgence in an easy and seductive vein of speculation, I would recall attention to the notable protuberance in the cranial cavity of the Dodo (Pl. XI. fig. 1, o) developed towards the upper part of the vertical tentorium, contracting at its lower part into the ridge dividing the prosencephalic from the mesencephalic chamber. In the latter are the orifices for the issue of the trigeminal nerve, the larger and posterior (ib. tr) giving passage to the third and second divisions, and answering to the combined foramen ovale and rotundum of mammals, and the smaller and anterior foramen dismissing the first or orbital division of the fifth nerve. At the upper part of the mesencephalic fossa the narrow groove for the lateral venous sinus impresses and defines the back part of the tentorial protuberance, above which it bifurcates, the lower branch bounding or defining the wall of the superior semicircular canal and the upper part of the primitive acoustic capsule. Below this arch is an oblong cerebellar fossa (ib. n) which appears to have received veins from the cranial diploë. Beneath this fossa, and just behind the mesencephalic chamber, is the multiperforate internal auditory depression. Next behind this is the outlet for the vagal nerve and entojugular vein. Below this are the small precondyloid foramina. There is a falcial ridge, low and thick, indicating the division of the prosencephalic chamber into lateral compartments for hemispheres; and this ridge shows a narrow groove as for a small longitudinal sinus. A transverse linear groove abruptly defines the fore part of the ridge.
The vertically expanded anterior part of the premaxillary (ib. fig. 1, 22) has a large pneumatic cavity communicating by a reticulate wall with the cells of a cancellous structure, larger than those of the cranial diploë. The maxillary branch of the premaxillary (ib. 22*) consists of a light open-work air-diploë, with a very thin outer case of bone. The short symphysis mandibulæ shows a small cavity, surrounded by more minutely cancellous structure and thicker compact walls, especially at the upper and hinder parts.
Although some characters have been too much insisted on (e. g. the “superoccipital foramen”) as exemplifying the affinity of the Dodo, the more essential characters of the skull relate to its true Columbine character, while the deviations from that part of the skeleton of volant Doves are explicable in the adaptive developments needed for the wielding of long, powerful, massive mandibles, serving most probably to enable the bird to subsist on some proportion of animal diet, in addition to such vegetable food as it might gain from the ground. Such indiscriminate feeding doubtless rendered its flesh less palatable than that of the winged Pigeons of the Mauritius to the Dutch navigators of the sixteenth and seventeenth centuries.
But the affinities of Didus will be more fully and decisively brought out in the comparison of the, in this respect, more instructive and light-giving parts of the skeleton.
§ 3. Comparison of the Skeleton.
The dorsal region of the vertebral column shows, in some birds, a confluence of certain vertebræ: I have observed four to be so welded together by both centrums and neural spines in Phœnicopterus, viz. the second to the fifth dorsal inclusive, leaving the sixth free, which articulates with the first costigerous sacral vertebra. In Platalea three dorsals coalesce in advance of the antepenultimate free vertebra. In the smaller diurnal birds of prey five dorsal vertebræ are usually confluent, leaving one free vertebra for the lateral movements of the trunk between such dorsal “sacrum” and the pelvic one. In Vultures, Plovers, Bustards, Cranes, Psophia, Cariama, Palamedea, Auks, Penguins, and in all flightless land-birds save the Dodo, no such anchylosis takes place. The Columbidæ are the species in which the dorsal vertebræ, homologous and the same in number with those of Didus, undergo the process of confluence into one mass of bone: they are the three which immediately precede the last (moveable) dorsal vertebra; and of these the two anterior develope, in Goura and Didunculus, hypapophyses closely corresponding in shape and proportion with those in the Dodo.
The chief difference which Didus offers in the present region of the vertebral column from that of Columbidæ is in the greater number of the vertebræ or segments which are typically completed by bony hæmapophyses articulating with pleurapophyses and directly with their mass of coalesced and expanded hæmal spines constituting the sternum. Of these typical thoracic segments there were five in Didus (Pl. III.); Didunculus (ib.) shows four; Goura three. In both existing genera these segments are succeeded by a single one, anchylosed to the fore part of the sacrum, but with the pleurapophysis long and moveable, with its hæmapophysis terminating in a point before reaching the sternum, and extensively connected with the antecedent hæmapophysis or sternal rib: in both genera two dorsal vertebræ in advance of the typically complete one have moveable pleurapophyses terminating freely in a point, with no hæmapophyses other than the costal processes of the sternum may represent. In Goura, which has six pairs of moveable or thoracic ribs, the second pair belong to the first of the three anchylosed dorsal vertebræ: in Didunculus, which has seven pairs of thoracic ribs, the second pair belongs to the free dorsal immediately in advance of the anchylosed mass. Supposing Didus to have had one pair of ribs behind, and two pairs in front of those that directly articulate with the sternum, as the vertebra Pl. V. fig. 7 indicates, it would have had eight pairs of thoracic ribs; and I think this excess of one pair beyond the formula in Didunculus to be very probable in the large-bodied, small-winged, extinct Ground-dove.
As far as the series of Dodo’s neck-vertebræ under my observation exhibit such characters, the proportion of those with neural spines, or with hypapophyses, or both, is the same as in the Columbidæ. In this family, as in most birds, the greater part of the series want both processes. The cervical parapophyses, descending to form the sides of the carotid canal, do not meet, coalesce, and circumscribe it in any cervical vertebra of Goura or Didunculus; and not any of the vertebræ of Didus, which I have yet received, shows such circumscription of the hæmal canal. The majority of the cervicals in Didus (those, viz., that lack both neural spines and hypapophyses) are broader and more massive in proportion to their length than in the winged Doves. The third cervical in Didus has both the above processes, as in Columbidæ: the characters of the axis vertebra in the same family are closely repeated in that of the Dodo. In the Raptores the axis vertebra is shorter in proportion to its length, and a greater proportion of the cervical vertebræ at both ends of the series have both neural spines and hypapophyses.
The ribs of the Dodo are as broad, in proportion to their length, as in Doves, but are relatively longer in proportion to the dorsal region, encompassing a more capacious thoracic-abdominal cavity. The ribs of the Vulture are more expanded than in Didus, especially where they afford the extensive attachment to the epipleurals. But I shall not dwell further on the comparative characters of this part of the skeleton, as more decisive ones of the affinity of Didus are afforded by other parts.
In comparing the sternum of the Dodo with that of Doves of flight, the first well-marked difference is in the adaptive development of the keel in the last (Pl. III. fig. 2, Didunculus), and in the provision for the concomitantly broader coracoids, the grooves for which meet and run into each other across the fore part of the bone in existing Columbidæ (Pl. XII. fig. 2, b); consequently the inner or upper wall of the confluent grooves forms a median prominence (ib. e) at the front margin of the sternum, contrasting with the wide notch at that part of the bone in the Dodo (Pl. IV. fig. 4). The next difference, as compared with Goura and most Pigeons, is the absence of the entolateral processes (Pl. XII. fig. 3, i) in the Dodo’s sternum: but Didunculus singularly exemplifies its nearer affinity to Didus by a like absence of those processes; only the sternal margins behind the ectolateral processes (ib. fig. 1, h), instead of converging with a slight convexity to an obtuse apex, as in Pl. VI., describe a concavity, through an expansion of the posterior truncate end of the breast-bone. The sternum of Didunculus may be said to show one pair of posterior notches (Pl. XII. fig. 1, f), that of other Pigeons two pairs (ib. fig. 3, f f′); but the sternum of Didus, which is relatively broader, shows no other trace of the anterior notch (Pl. VI. f) than is afforded by the rounded angle at which the ectolateral process (h) rises from the bone. Although the costal margin is relatively shorter in Doves of flight than in the Dodo, again an intermediate condition is manifested by Didunculus as compared with Goura, in which latter Dove there are articular surfaces for three sternal ribs (Pl. XII. fig. 3, o 1, 2, 3), whilst in Didunculus there are four (ib. fig. 1, c). Didunculus also exhibits, more strongly than Goura, the obtuse ridges (ib. fig. 2, r) converging like buttresses from the outer wall of the coracoid groove to the fore part of the keel, where they subside. In Didunculus there is a pneumatic foramen exterior to the coracoid groove, corresponding with p, fig. 4, Pl. IV., which I do not find in the sternum of Goura; but in the Crown-pigeons the pneumatic foramina along the middle line of the upper surface of the sternum are conspicuous; they are confined to the fore part of that surface in Didunculus (Pl. XII. fig. 1).
In the direction of the ectolateral processes Goura (ib. fig. 3, h) is intermediate between Didunculus and Didus. The pectoral ridge on the outer surface of the sternum, continued backward from the outer end of the coracoid groove, is adaptively better marked in Pigeons of flight than in the Dodo; and the pair of ridges are more nearly parallel in their backward course, not so convergent as in Didus. In Goura the subcostal ridge is better marked than in Didunculus. In no Dove of flight is the body of the sternum so broad and hollow as in Didus (Pl. XI. fig. 4); in this respect the Vulture more nearly resembles the Dodo, as it does also in the more convex anterior contour of the keel: but the vulturine sternum does not lose breadth as it extends backward; it is a square-shaped shield in birds of prey, shorter in proportion to its breadth, with a greater extent of costal process and margin, and with the ectolateral processes, when they exist, extending backward as far as the hinder border of the bone. In the thorough quest of resemblances to the Dodo’s sternum which I have made through the class of Birds, I came upon an unexpected superficial likeness to it in the sternum of a Night-jar (Podargus humeralis). The ectolateral processes (Pl. XII. fig. 4, h) rise behind the moderately extended costal borders, c; and beyond them the body of the sternum converges to an obtuse end, with a contour similar to that in Didus. Moreover the coracoid grooves are divided from each other by a free concave border, less deep and extensive, indeed, than in Didus, but as free from any trace of episternal projection. The ectolateral processes, however, are extended backward to beyond the sternal body; and this part usually shows a pair of small entolateral notches, f′, of which one was present on one side in the specimen figured.
Through the reduction of the coracoids in all flightless birds, there is an interval between their sternal articulations: this is long and concave in the Dodo, but is longest and most deeply concave in Apteryx; it is long but almost straight in Rhea; in Casuarius and Dromaius it is narrow but deeply notched; in Struthio it developes a short episternal process. In no Grallatorial sternum with both ecto- and ento-lateral processes (as e.g. Otis, Œdicnemus, Charadrius) do the former project, as in Didus and the Rasores, immediately behind the costal margin, but they are continued, parallel with the keel, from the outer and posterior angle of the sternum, distant from the costal margin. In old Plovers the entolateral process joins the contiguous angle of the sternal body, and converts the inner notch into a foramen.
In the breast-bone of the Dodo we plainly discern the Columbine modification of the Gallinaceous type, simplified in the minor development of those parts relating adaptively to the power of flight, and expanded and excavated for the support of the larger gizzard with its heavier grindstones[34].
In comparing the pelvis of Didunculus and Goura (Pl. XII. fig. 5) with that of Didus (Pl. VII. fig. 1), the correspondences are:—in the general shape, proportions and disposition of the ilia; in the articulation therewith of the last pair of moveable ribs, and of the short straight confluent pleurapophyses of the three succeeding sacral vertebræ; then follow, as in Didus, three vertebræ without pleurapophyses, these reappearing in the next two with their extremities converging to abut against a prominence of the inner surface of the ilium in the same relative position. The difference here is in the two equal and more slender rib-buttresses, in place of the single stronger one, which is the more common structure in Didus; but in Goura I have noted an instance in which it agreed with the Didunculus on the left side, and with Didus on the right, in the last-specified character. In the Crown-pigeons, also, there is an indication of the transverse ridge marking off the under part of the centrum of the first sacral from the rest, and those that follow are less expanded than in the Dodlets; moreover in Didunculus they show a median canal instead of a ridge, while the ridge is feebly indicated here and there and there is no canal in Goura. In neither Didunculus nor Goura do the sacral centrums behind the last rib-abutments diminish in breadth so suddenly as in Didus: in both the winged Pigeons the hinder part of the pelvic cavity is relatively deeper and narrower than in Didus; in both, also, the upper and anterior concave tracks of the ilia are deeper; and in Didunculus the mesial borders do not attain the neural crest, but leave a pair of open longitudinal canals at that part of the pelvis; in Goura those margins reach the neural crest, but do not overtop it at any part. In Goura the acetabula are more in advance of a median position than in Didunculus, Columba magnifica, or Didus. Although the ischiadic foramina are completed by terminal confluence of the ilium and ischium in Dromaius and Casuarius, yet the length of those foramina (which are unclosed) in Struthio and Apteryx, concomitant with the greater relative length of the pelvis, shows the difference of Didus from the cursorial Brevipennates in this part of the skeleton. The ischia of the winged Pigeons resemble those of the Dodo; but the inner longitudinal ridge is more strongly marked in Didunculus: in the Goura it is less developed than in Didus; the bone is longer also in proportion to its breadth, and the ischiadic foramen is longer and narrower: the proportions of that in Didunculus are more like those in Didus. In Didunculus the pubis coalesces with the ischium behind the small obturator foramen, but leaves a second or posterior elongate ischio-pubic vacuity. The greatest amount of resemblances with the pelvis of the Dodo is found in that of different members of the Dove-tribe.
In comparing the pelvis of the Dodo with that of the Vulture (Pl. XII. fig. 6), we find in the latter that the first two confluent sacral vertebræ; supporting moveable ribs are succeeded by several with short abutting ribs, the extent of this part of the sacrum being nearly one-half of the whole, instead of one-fourth as in Didus and the Doves. The reappearance of rib-abutments after four ribless sacrals is in the posterior third of the sacrum, and they are continued to the end of that bone from the last four vertebræ of the series, constituting a very marked difference, both as to number and the character of the vertebræ in the sacral part of the pelvis.
With regard to the iliac bones, the anterior concave track occupies two-thirds of the extent of the bone in Vultur, not one-half as in Didus and most Doves; the breadth of the posterior parts of the ilia with the intervening sacrum in the Vulture is relatively less than in the winged Doves, and differs in a greater degree from that characteristic part in the sacrum of Didus. In Ciconia the antacetabular part of the pelvis is relatively longer, and the iliac bones are more expanded anteriorly. In Platalea the proportions are more nearly those in Didus. In Otis the ilia touch the fore part of the sacro-spinal ridge, but leave both posterior and anterior apertures of the ilio-neural canals widely open. In Œdicnemus and Charadrius they are grooves, the ilia not reaching the sacral spines. The external concavity of the ilium is longer, narrower, and deeper, in most waders, than in Didus. In Eudyptes and Aptenodytes the ilia are more expanded anteriorly, but the whole pelvis is narrower and longer than in Didus. The Gar-fowl (Alca impennis)[35], Uria, Podiceps, and Colymbus, all show still longer and narrower proportions of the pelvis.
In the Doves of flight the proportions and relative position of the three compartments of the cranial cavity differ from those in the Dodo. Both the pros- and mes-encephalic ones are proportionally larger than the epencephalic; and the mesencephalic compartment lies more directly below the prosencephalic one. A very thin stratum of finely cellular diploë divides the two tables of the skull along the medial line of the upper surface: it is thicker between the orbits. The falcial ridge at the inner surface of the prosencephalic roof resembles that in Didus. The tentorial ridge bifurcates halfway down, the front portion dividing, almost horizontally, the pros- from the mesencephalic compartment, the hinder and more obtuse ridge dividing, almost vertically, the mes- from the epencephalic compartment. The angle of bifurcation is slightly produced and obtuse, but represents very feebly the tentorial tuberosity (Pl. XI. fig. 1, o) in the Dodo: from it, in Goura, is continued backward the arch of bone formed by the superior semicircular canal, above which is the groove for the venous sinus, as in Didus. The internal auditory fossa is less deep than in Didus: above it is a similarly vertically oblong cerebellar pit. The nerve-foramina correspond with those in Didus: the entocarotid canal opens into a rather deeper sella in Columba palumbus.
On comparing the cranial cavity, as exposed by a vertical longitudinal section in the Dodo (Pl. XI. fig. 1), with that of a Dinornis similarly exposed[36], the first difference is the smaller proportional depth of the diploë in the larger wingless bird, which is not greater over the prosencephalic than over the epencephalic compartment; next may be noticed the larger relative size of the former compartment, indicating the larger cerebrum of the Dinornis, then the absence of the tentorial tuberosity, the sharper and more produced superior part of the tentorial ridge arching transversely between the cerebrum and cerebellum, the smaller internal auditory fossa, and the deeper sella: the mesencephalic compartment, or cavity for the optic lobe, is less in proportion to the prosencephalic compartment than in Didus; it holds, however, a similar relative position: finally, the cerebellar pit, above the internal auditory fossa, is wanting in the Dinornis.
The Dodo agrees with the Doves in possessing a slender furculum, forming an acute angle: it resembles Columba galeata, more especially, in the halves of that bone being united by ligament below, and forming separate styles or “clavicles.”
The humerus of the Goura closely repeats most of the characters described in that of the Dodo; but its length is proportionally greater, being 3 inches 9 lines, nearly equal to that of the sternum or pelvis, whereas the humerus of the Dodo is little more than half the length of either sternum or pelvis. The processes for the attachment of the muscles are, nevertheless, fully as strongly developed in Didus (Pl. VIII. figs. 12 & 14) as in the volant Doves (Pl. XII. figs. 8 & 9, Goura); that, indeed, which is a ridge (r) on the back part of the shaft in Didus, is a mere rough surface in Goura, and does not show in Didunculus. The pneumatic fossa, which varies in depth in the two humeri of the Dodo, is in both relatively larger and shallower than in Goura. The pectoral process is thinner, but relatively rather more produced, in Didunculus. The humerus in Œdicnemus, Otis, and Charadrius has a more longitudinally extended, thinner, and more produced pectoral ridge than in Didus and the Columbidæ; there is a more marked ectocondyloid tuberosity, which in Charadrius becomes a pointed process.
There is nothing to be gained by giving the details of the more striking differences which the humerus presents in Penguins, Auks, and birds of prey, as compared with that bone in the Dodo; but a few words may be recorded of the comparison of the humerus of the Dodo with that of the flightless bird of New Zealand so nearly approaching to it in size, which bird is described in the 5th volume of the ‘Transactions’ of the Society under the name of Cnemiornis (p. 395, pl. 66. figs. 7–10). In that extinct species, although the humerus is 5½ inches in length, the parts indicative of the forces by which it was worked are comparatively feebly developed. The ulnar tuberosity is narrower, thicker, more obtuse, and its base has neither the upper nor lower excavation; it rises above the articular head, which is less prominent and narrower than in Didus; the pectoral ridge is shorter and situated lower down upon the shaft, not on the same level with the radial tuberosity as it is in Didus; the distal articulation is of the same size as in Didus, but neither the radial nor the ulnar convexity is so prominent or well-defined.
The ulna of the Dodo is shorter absolutely, and much more so proportionally, than in the Goura and most other volant Doves. In these it exceeds the humerus by about one-fourth its own length; in Didunculus (Pl. III.) it is a little longer than the humerus; in the Dodo (ib.) it is shorter than the humerus. The length of the ulna in Goura coronata is 4 inches 6 lines; it is more bent than in the Dodo; the quill-tubercles, seven or eight in number, are more prominent; nevertheless the rough depression for the insertion of the chief flexor is less deep and less defined. The plumed winglet of the Dodo would seem, therefore, to have been frequently and forcibly moved.
In comparing the femur of the Dodo with that of the largest Dove, the bone appears gigantic. The length of the femur in Goura coronata (Pl. XII. fig. 11) is but 3 inches 3 lines, and it is more slender in proportion to its length than in the Dodo; it, however, repeats the few characteristics, if they may be so termed, of the Dodo’s femur. It has the pneumatic foramen in the same position, perhaps proportionally larger; it has the same large oblong surface for the ligament at the head of the bone; the great trochanter has the same form and disposition, but is not quite so much produced anteriorly; there is a slight depression instead of a ridge for the trochanter minor; the fore part of the inner condyle is relatively thicker and less produced. The femur in Otis and Œdicnemus has a thicker and shorter trochanter major, & more narrow and shallow rotular channel; it is shorter in comparison with the tibia, and more especially with the metatarsus, than in Didus and the Doves.
The femur of Aptornis otidiformis[37] is of the same size as that of the Dodo; but it has no pneumatic foramen, the head is more hemispheroid and inclined forward, the ligamentous pit is deeper and more circular, the supracervical articular surface is not defined from that of the head, there is a wider and deeper depression at the fore part of the proximal end of the femur, and a more prominent tuberosity on the back part; the ridge continued from the back part of the shaft to that of the inner condyle is more produced and sharper in Aptornis, the fore part of the same condyle is less produced.
The femur in Cnemiornis[38] and Dinornis[39] is much thicker, in proportion to its length, than in either Aptornis or Didus. In Pezophaps the great trochanterian ridge rises higher above the neck, and the shaft has a more uniform thickness, with the inner contour less concave, than in Didus.
The characters which have been noted at the proximal and distal ends of the tibia of Didus are repeated in those of the tibia of the Goura. The difference in size is more marked than in the femur; the length of the tibia of Goura coronata is 4 inches 7 lines, and its shaft is more slender, in proportion to its length (Pl. XII. fig. 13), than in Didus (Pl. X.). The tendency to a trihedral form of the shaft is less marked in Goura; the anterior prominences of the distal condyles are thicker in proportion to the intervening fossa.
In the Vulture the fibular ridge is more parallel with the long axis of the shaft than in Didus; the tendinal canal is less cylindrical, has an oblique course from the middle of the anterior surface towards the inner condyle; the fore parts of both distal condyles are less produced and less convex; the distal end is narrower from before backwards in proportion to its breadth; both extremities of the bone are less expanded in proportion to the shaft than in the Dodo.
In the great Plover (Œdicnemus crepitans) the tibia, as in other Grallæ, is longer in proportion to its thickness than in Didus; the epicnemial process rises higher above and projects further in front of the condylar surfaces before it divides into the pro- and ectocnemial plates; and these are relatively more produced. The fibular ridge is shorter in proportion to the length of the tibia, is more prominent, and more parallel with the axis of the shaft. The distal condyles project further backward than in Didus. The tibia in Charadrius, Otis, Tantalus, Grus, Ciconia, Mycteria, Porphyrio, opposes similar or equivalent differences to those in Œdicnemus, against the affinity of Didus to any of those Grallæ.
In the comparison of the tibia of this extinct flightless bird with that of the Cnemiornis, the wonderful development of the plates and processes at the proximal end of the bones in the New Zealand bird is strikingly manifested. In Cnemiornis the fibular ridge runs in a line with the shaft, and does not incline from above obliquely forward as in Didus and the Doves; the ridge on the outer side of the distal fourth of the bone is stronger and sharper in Cnemiornis; the tendinal canal is transversely elliptical, medial in position, with a slight inward inclination; the intercondyloid fossa is much wider in Cnemiornis. The differences, indeed, in all the characters of the tibia, as compared with Didus, in the Vultures, Plovers, Penguins, and terrestrial flightless birds tend to render more instructive and convincing the resemblances which Pigeons present in the same characters to the extinct Mauritian bird.
§ 4. Conclusion.
The affinities or place in nature of the Dodo being thus determined by the characters of its skeleton, but few words remain to be said on the bearings of present knowledge of this species upon other zoological generalizations.
The researches and observations of naturalists have been carried out to such an extent as to support the conclusion that the Didus ineptus does not now live in any part of the world, and that it never existed save in that part of which the island of Mauritius may be a remnant. Consequently the species there originated; and the most intelligible conception of its mode of origin is that to which I have alluded in the description of the brain-case (p. 39).
The Dodo exemplifies Buffon’s idea[40] of the origin of species through departure from a more perfect original type by degeneration; and the known consequences of the disuse of one locomotive organ and extra use of another indicate the nature of the secondary causes that may have operated in the creation of this species of bird, agreeably with Lamarck’s philosophical conception of the influence of such physiological conditions of atrophy and hypertrophy[41]. The young of all Doves are hatched with wings as small as in the Dodo: that species retained the immature character. The main condition making possible the production and continuance of such a species in the island of Mauritius was the absence of any animal that could kill a great bird incapable of flight. The introduction of such a destroyer became fatal to the species which had lost such means of escape[42]. The Mauritian Doves (Columba nitidissima and C. meyeri) that retained their powers of flight continue to exist there.
As I have no reason to offer why one kind of Pigeon should have retained and another lost its powers of flight, nor am able to adduce a particle of evidence of the hypothetical degrees of diminution of the wing-bones to their stunted proportions in Didus, any more than in Dinornis, I feel that in the foregoing remarks I lay myself open to the rebuke of fellow-labourers who may think with the able authors who last treated of the present subject.
They warn their readers to “beware of attributing anything like imperfection to these anomalous organisms, however deficient they may be in those complicated structures which we so much admire in other creatures. Each animal and plant has received its peculiar organization for the purpose, not of exciting the admiration of other beings, but of sustaining its own existence. Its perfection, therefore, consists, not in the number or complication of its organs, but in the adaptation of its whole structure to the external circumstances in which it is destined to live. And, in this point of view, we shall find that every department of the organic creation is equally perfect, the humblest animalcule or the simplest conferva being as completely organized with reference to its appropriate habitat and its destined functions as Man himself, who claims to be lord of all. Such a view of the creation is surely more philosophical than the crude and profane ideas entertained by Buffon and his disciples”[43].
Nevertheless the truth, as we have or feel it, should be told. In the end it may prove to be the more acceptable service. The Didus ineptus, L., through its degenerate or imperfect structure, howsoever acquired, has perished. What have the stigmatizers of Buffon to offer in lieu of his theory as applied to the origin of this species of bird? They begin by asking, “Why does the whale possess the germs of teeth which are never used for mastication? and why was the Dodo endowed with wings at all, when those wings were useless for locomotion? This question,” they own, “is too wide and too deep to plunge into at present.” They nevertheless proceed to remark, “These apparently anomalous facts are really the indications of laws which the Creator has been pleased to follow in the construction of organized beings; they are inscriptions in an unknown hieroglyphic, which we are quite sure mean something, but of which we have scarcely begun to master the alphabet. There appear, however, reasonable grounds for believing that the Creator has assigned to each class of animals a definite type or structure, from which He has never departed, even in the most exceptional or eccentric modifications of form. Thus, if we suppose, for instance, that the abstract idea of a Mammal implied the presence of teeth, and the idea of a Bird the presence of wings, we may then comprehend why in the Whale and the Dodo these organs are merely suppressed, not wholly annihilated”[44].
This notion of type-forms or centres, unfortunately, has not merely relation to abstract biological speculations or theories, but to practical questions on which the true progress of Natural History vitally depends. If such types do exist, the National Museum, it is argued, may be restricted to their exhibition: and so our legislators and the public were assured by the Professor of Natural History in the Government School of Mines[45], when the question was before the “House” four years ago. I have let slip no suitable occasion[46] to combat and expose what has seemed to me to be both an erroneous and mischievous view, most obstructive to the best interests of the science; and, standing alone as I seemed to do on this point in the array of evidence before the “Parliamentary Committee on the British Museum, 1860,” I was glad to find my views on type-forms adopted and paraphrased by the President of the British Association in his Inaugural Address at the Meeting at Nottingham[47], in the present year.