Part 6

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.”