9. The surface of the posterior moveable part of the body is slightly concave both before and after transformation; the anterior part is, however, less concave in Amblystoma than in Siredon (Duméril).
I have not yet been able to verify Duméril’s 7th and 9th statements, as I did not want to kill any of my living Amblystomas,[240] simply in order to confirm the observations of a naturalist in whom one may certainly place complete confidence. Neither have I as yet observed the transformation of the branchial arches, but all the other statements of Kölliker and Duméril I can entirely corroborate.
The structural differences between Axolotl and Amblystoma are considerably greater and of more importance than those between allied genera, or indeed than between the families of the Urodela. The genus Siredon undoubtedly belongs to a different sub-order to the genus Amblystoma into which it occasionally becomes transformed. Strauch, the most recent systematic worker at this group, distinguishes the sub-order Salamandrida from that of the Ichthyodea by the possession of eyelids, and by the situation of the palatine teeth in single rows on the posterior edge of the palatal bone: in Ichthyodea the eyelids are wanting and the palatine teeth are either “situated on the anterior edge of the palatal bone,” or “cover the whole surface of the palatal plates in brush-like tufts.”
How is it possible to regard such widely divergent anatomical characters as changes suddenly produced by the action (but once exerted) of deviating conditions of life? Hand in hand with the shedding of the old and the appearance of new palatine teeth, there occurs a change in the anatomical structure of the vertebral column, and also—as we may fairly conclude from Kölliker’s correct observation of the cessation of the slimy secretion—in the histological structure of the skin. Who would undertake to explain all these profound modifications as the direct and sudden action of certain external influences? And if any one were inclined to explain such changes as a consequence of the disappearance of the gills, i.e. as correlative changes, what else is such a correlation than the phyletic vital force under another name?
If from one change arising from the direct action of external agencies, the whole body can in two days become transformed in all its parts, in the precise manner which appears best adapted for the new conditions of life under which it is henceforward to exist, then the word “correlation” is only a phrase which explains nothing, but which prevents any attempt at a better explanation, and it would be preferable to profess simply the belief in a phyletic vital force.
Moreover, it is hardly permissible to seek such an explanation, since Urodela are known which have no gills in the adult state, and which nevertheless possess all the other characters of the Ichthyodea, viz. want of eyelids, characteristic palatine teeth, and the tongue bone. This is the case with the genera Amphiuma (Linn.), Menopoma (Harl.), and Cryptobranchus (v. d. Hoev.). The two first genera, as is known, still possess gill-clefts, but Cryptobranchus has even lost these clefts, which, as in Amblystoma, are overgrown by skin; nevertheless Cryptobranchus is, according to the concurrent testimony of all systematists, a true salamander in habits, tongue bone, palatine teeth,[241] &c. It must further be added that the Axolotl itself can lose the gills without thereby becoming transformed into an Amblystoma. I have previously mentioned that in Axolotls which were kept in shallow water the gills frequently became diminutive, and it also sometimes happens that they completely shrivel up. I possess an Axolotl preserved in alcohol in which the gills have shrivelled up into small irregular bunches, and the dorsal crest is also so completely absent that its place is occupied by a long furrow, and even on the tail the crest has entirely disappeared from the lower edge and about half from the upper edge. Notwithstanding this, the creature is widely removed from Amblystoma in structure; it possesses the arched branchial apparatus, the palatine teeth, the skin, &c., of the Axolotl.
These facts prove, therefore, that the shedding of the gills by no means always entails all the other modifications which we observe in the metamorphosis of Axolotl, so that these modifications are thus not by any means the necessary and immediate consequence of such gill shedding.
Whether these modifications will occur after a long series of generations—whether the successors of Cryptobranchus will also one day acquire the salamandriform structure is another question, and one which I could not exactly answer in the negative. But this question does not here come into consideration, as we are now only concerned with the immediate result of the shedding of the gills.
The problem appears therefore to be as follows:—Either the hitherto received interpretation of the transformational history of the Axolotl as a further development of the species is incorrect, or else the case of Axolotl incontestably proves the existence of a phyletic vital force.