The study of Notornis suggests certain questions of interest as to flightless birds in general, and especially as to that group of preeminently flightles birds, the Ratitæ, and their relations on the one hand to the Carinatæ, and on the other to the reptilian ancestors of the class.
The Ratiæ; are usually regarded, from certain undoubted reptilian characteristics, as being nearer the reptilian stock from which birds sprang, than the Carinatæ, and there seems to be a general opinion that large ratite birds of some sort formed the actual connecting link between reptiles of a dinosaurian type and the Carinatæ.
But it appears to me that there are serious difficulties in the way of this view. The Ratitæ, in many of the most distinctive avian characteristics, approach no nearer to reptiles than do the Carinatæ. For instance, in the characters of the vertebræ, the femur, the tibio-tarsus, the tarso-metatarsus, the pelvis and sacrum, and, in the case of Dinornis and Apteryx, of the small, backwardly-directed hallux.
The chief skeletal characters, except those of the skull, in which the Ratitæ differ from the Carinatæ, are:—
The absence of a keel to the sternum:
The great width, and, except in Rhea, the extreme flatness of the sternum, i.e. the openness of the transverse sternal angle.
The comparatively small size of the scapula and coracoid.
The coraco-scapular angle is equal to two right-angles.
The axis of the coracoid is vertical or inclined backwards.
The furcula is rudimentary or absent.
If the Ratitæ are to be looked upon as in any way an ancestral group, these characters must be considered of primary importance, that is, as having a true phylogenetic significance. But in all these points, the Ratitæ merely exaggerate what we find in the flightless members of the Carinate order. There is no more keel to the sternum in Stringops or Cnemiornis than in Struthio, and the transverse sternal angle of Rhea is very considerably less than in the flightless Carinatæ. In these, also, there is a progressive diminution in size of the coracoid and scapula in passing from good fliers to flightless members of the same class, and at the same time a gradual rotation backwards of the dorsal end of the coracoid, and increase of the coraco-scapular angle. In fact, with the exception of the foramen in the coracoid of the ostrich, I know of no character in the shoulder-girdle of Ratitæ which can be pointed out as distinctively reptilian. One important distinction between the shoulder-girdle of reptiles and that of birds, is the position of the bones. In reptiles the coracoid passes from its sternal articulation outwards and slightly upwards, and the scapula (including the supra-scapula), from its
coracoid articulation upwards and somewhat inwards, so that the two pairs of bones form, with the adjacent portions of the sternum and vertebral column, a transverse arch or true shoulder “girdle”: in Carinatæ, on the other hand, the coracoid passes from the sternum forwards, upwards, and outwards, and the scapula, from its coracoid end, backwards and upwards. In the Ratitæ the bones mate no closer an approach to reptiles in this than in other characters, the coracoid is still directed upwards and slightly outwards, the chief alteration in its position being that it has a slight backward inclination, this being, however, only an extreme development of what occurs in Notornis and Ocydromus: the scapula also passes upwards and backwards, and not inwards. Finally, a diminution of the furcula occurs in all birds with functionless wings.
In a suggestive paper on the phylogeny of Mammals,* Professor Huxley has brought out the fact that it gives a wholly erroneous notion of the pedigree of that class to suppose that either the Marsupials or the Monotrermes lie in the direct line of descent of the Monodelphia. He points out that the ancestors of the Monodelphia—the Metatheria—were probably didelphous but not marsupial, and that the Marsupialia are to be looked upon as an offshoot of the Metatheria, which, while retaining the lower characters of brain and urinogenitals, and the large præ-pubes, have undergone great specialization in other directions. In the same way Professor Huxley supposes the Monotremata to be a specially modified offshoot of the Prototheria, the forerunners of the Metatheria.
It appears to me that a far juster view of the affinities of the Ratitæ than that alluded to above, is to be had by considering them as the greatly specialized but degenerate (using that word in the sense in which I have applied it to Notornis and other flightless birds) descendants of Carinate birds. Professor Huxley remarks†. that “in all probability the existing Ratitæ are but the waifs and strays of what was once a very large and important group.” What I wish to insist upon is that this hypothetical group, like the mammalian Metatheria gave, rise to two races of descendants: one continuing the direct line of descent-the Carinatæ-the other arising by a gradual modification of structure correlated with disuse of the wings-the Ratitæ. Just as the Metatheria gave rise to marsupial descendants which exist now only in the Austro-Columbian and Australian regions, so we may suppose that a widely distributed group of primitive typical birds— Proto-Carinatæ-gave rise to Ratite descendants, now confined to the Austro-Columbian, African, and Australian regions. The fact that such
[Footnote] * “Nature,” vol. xxiii., p. 227.
[Footnote] † Proc. Zool. Soc. 1867, p. 419
a group must have been intermediate between Odontornithes* and Carinatæ is quite sufficient to account for all the reptilian affinities of the Ratitæ: the assumption that they had acquired the distinctive characteristics of shoulder-girdle and sternum, pelvis, vertebral column, and fore and hind limbs, and had lost all trace of teeth, will account for the fact that in these points the Ratitæ make no approach whatever to reptiles. The same hypothesis also explains the fact that while the Ratitæ agree in certain common characters, some because they are of ancestral importance, others because they have been acquired by the same law of degeneration, they differ in the most remarkable way in other points. For instance, while Dinornis retains such ancestral structures as the hallux, large external xiphoid processes, and free ischium and pubis, it has undergone the greatest amount of degeneration of the shoulder-girdle and forelimb. Struthio, in the same way, shows the extreme of modification in the foot, Struthio and Rhea in the pelvis, and so on.
Probably the great size of the Ratitæ is also connected with their cursorial life, and is, like so many points in the skeleton, merely an exaggeration of what is found in Notornis, Cnemiornis and Didus. The Proto-Carinatæ being, by the hypothesis, good fliers, were presumably not of gigantic size; moreover they probably possessed feathers with connected barbs, so that the special characters of the Ratite plumage should be looked upon as a secondary or degenerate, not as an ancestral, character.
Finally, if we look upon the typical Odontornithes and Archæopteryx as approximately linear types in the ancestry of birds, we must assume that the latter arose from ornithoscelidan reptiles of comparatively small size, the gigantic Dinosauria being a special development of the same type.
The following diagram expresses the results to which a consideration of the above facts seems to lead:—
[Footnote] * I mean such Odontornithes as Ichthyornis: Hesperornis is another instance of degeneration induced by disuse of the wings.