Kerr's Theory of Modified External Gills.—"It is because I feel that in the present state of our knowledge neither of the two views I have mentioned has a claim to any higher rank than that of extremely suggestive speculations that I venture to say a few words for the third view, which is avowedly a mere speculation.
"Before proceeding with it I should say that I assume the serial homology of fore and hind limbs to be beyond dispute. The great and deep-seated resemblances between them are such as to my mind seem not to be adequately explicable except on this assumption.
"In the Urodela (salamanders) the external gills are well-known structures—serially arranged projections from the body-wall near the upper ends of certain of the branchial arches. When one considers the ontogenetic development of these organs, from knob-like outgrowth from the outer face of the branchial arch, covered with ectoderm and possessing a mesoblastic core, and which frequently if not always appear before the branchial clefts are open, one cannot but conclude that they are morphologically projections of the outer skin and that they have nothing whatever to do with the gill-pouches of the gut-wall. Amongst the Urodela one such gill projects from each of the first three branchial arches. In Lepidosiren there is one on each of the branchial arches I-IV. In Polypterus and Calamoichthys (Erpetoichthys) there is one on the hyoid arch. Finally, in many Urodelan larvæ we have present at the same time as the external gills a pair of curious structures called balancers. At an early stage of my work on Lepidosiren, while looking over other vertebrate embryos and larvæ for purposes of comparison, my attention was arrested by these structures, and further examinations, by section or otherwise, convinced me that there were serial homologues of the external gills, situated on the mandibular arch. On then looking up the literature, I found that I was by no means first in this view. Rusconi had long ago noticed the resemblance, and in more recent times both Orr and Maurer had been led to the same conclusion as I had been. Three different observers having been independently led to exactly the same conclusions, we may, I think, fairly enough regard the view I have mentioned of the morphological nature of the balancers as probably a correct one.
"Here, then, we have a series of homologous structures projecting from each of the series of visceral arches. They crop up on the Crossopterygii, the Dipnoi, and the Urodela, i.e., in three of the most archaic of the groups of Gnathostomata. But we may put it in another way. The groups in which they do not occur are those whose young possess a very large yolk-sac (or which are admittedly derived from such forms). Now wherever we have a large yolk-sac we have developed on its surface a rich network of blood-vessels for purposes of nutrition. But such a network must necessarily act as an extraordinarily efficient organ of respiration, and did we not know the facts we might venture to prophesy that in forms possessing it any other small skin-organ of respiration would tend to disappear.
"No doubt these external gills are absent also in a few of the admittedly primitive forms such as, e.g., (Neo-) Ceratodus. But I would ask that in this connection one should bear in mind one of the marked characteristics of external gills—their great regenerative power. This involves their being extremely liable to injury and consequently a source of danger to their possessor. Their absence, therefore, in certain cases may well have been due to natural selection. On the other hand, the presence in so many lowly forms of these organs, the general close similarity in structure that runs through them in different forms, and the exact correspondence in their position and relations to the body can, it seems to me, only be adequately explained by looking on them as being homologous structures inherited from a common ancestor and consequently of great antiquity in the vertebrate stem."
As to the third theory, Professor Kerr suggests tentatively that the external gill may be the structure modified to form the paired limbs. Of the homology of fore and hind limbs and consequently of their like origin there can be no doubt.
The general gill-structures have, according to Kerr, "the primary function of respiration. They are also, however, provided with an elaborate muscular apparatus comprising elevators, depressors, and adductors, and larvæ possessing them may be seen every now and then to give them a sharp backward twitch. They are thus potentially motor organs. In such a Urodele as Amblystoma their homologues on the mandibular arch are used as supporting structures against a solid substratum exactly as are the limbs of the young Lepidosiren.
Fig. 62.—Polypterus congicus, a Crossopterygian fish from the Congo River. Young, with external gills. (After Boulenger.)
"I have, therefore, to suggest that the more ancient Gnathostomata possessed a series of potentially motor, potentially supporting structures projecting from their visceral arches; it was inherently extremely probable that these should be made use of when actual supporting, and motor appendages had to be developed in connection with clambering about a solid substratum. If this had been so, we should look upon the limb as a modified external gill; the limb-girdle, with Gegenbaur, as a modified branchial arch.