This last statement, however, gives no certainty to the matter. Of much more importance is the above-mentioned fact, that the true Axolotl of the lakes about the city of Mexico does not, as a rule, become transformed into an Amblystoma in that locality, although this species in certain cases undergoes metamorphosis when in confinement. From this circumstance and from the fact that the Paris Axolotl likewise experienced but a very small percentage of metamorphosis in captivity, we may venture to conclude that this species also, in its native habitat, either does not become transformed at all or does so only exceptionally.
But there is yet another body of facts which come prominently into view on considering the history of the transformations. I refer to the existence of numerous species of Amblystoma in a natural state. In the “Revision of the genera of Salamandridæ,” published some years ago by Strauch,[234] this author, following Cope,[235] gives twenty species of Amblystoma as inhabiting North America. Although some of these species are based on single examples, and consequently, as Strauch justly remarks, “may well have to be reduced in the course of time,” there must nevertheless always remain a large number of species which live and propagate as true Amblystomas, and of which the habitat extends from the latitude of New York to that of New Mexico. There are therefore true species of Siredon which regularly assume the Amblystoma form under their natural conditions of life, and which propagate in this form, while, on the other hand, there are at least two species which, under their existing natural conditions of life, always propagate as Siredon. It is but another mode of expression for the same facts if we say that the Mexican Axolotl and the Paris Siredon—whether this is Lichenoides or some other species—stand at a lower grade of phyletic development than those species of Amblystoma which propagate in the salamander form. No one can raise any objection to this statement, while the alternative view maintained by all authors contains a theory either expressed or implied which is, as I believe, incorrect, viz. that the Mexican Axolotl has remained at an inferior stage of phyletic development.
All zoologists[236] who have expressed an opinion upon the transformation of the Axolotl, and who are not, like the first observer of this fact, embarrassed by Cuvier’s views as to the immutability of species, regard the phenomenon as though a species, which owing to some special conditions had hitherto remained at a low stage of development, had now through some other influences been compelled to advance to a higher stage.
I believed for a long time that the phenomenon could not otherwise be comprehended, so little was I then in a position to bring all the facts into harmony with this view. Thus in the year 1872 I expressed myself as follows[237]:—“Why should not a sudden change in all the conditions of life (transference from Mexico to Paris) have a direct action on the organization of the Axolotl, causing it suddenly to reach a higher stage of development, such as many of its allies have already attained, and which obviously lies in the nature of its organization—a stage which it would perhaps itself have reached, although later, in its native country? Or is it inconceivable that the sudden removal from 8000 feet above the sea (Mexican plateau) to the altitude of Paris, may have given the respiratory organs an impetus in the direction of the transformation imminent? In all probability we have here to do with the direct action of changed conditions of life.”
That the substance of this last statement must still hold good is obvious from the experiments previously described, which show that by the application of definite external influences, we have it to a certain extent in our power to produce the transformation. It is precisely in this last point that there lies the new feature furnished by these experiments.
But are we also compelled to interpret the phenomenon in the above manner? i.e. as a sudden advance in the phyletic development of the species occurring, so to speak, at one stroke? I believe not.
What first made this view appear to me erroneous, was the appearance of the living Amblystomas bred from my Axolotl larvæ. These creatures by no means differed from the Axolotls merely in single characters, but they were distinct from the latter in their entire aspect; they differed in some measure in all their parts, in some but slightly and in other parts strongly—in brief, they had become quite different animals. In accordance with this, their mode of life had become completely modified; they no longer lived in the water, but remained concealed by day among the damp moss of the vivarium, coming forth only by night in search of food in dry places.
I had been able to perceive the great difference between the two stages of development from the anatomical data with which I had long been familiar, and which Duméril had made known with respect to the structure of his Amblystomas. But the collecting of numerous details gives no very vivid picture of the metamorphosis; it was the appearance of the living animal that first made me conscious how deep-seated was the transformation which we have here before us, and that this change not merely affected those parts which would be directly influenced by the change in the conditions of life, such as the gills, but that most if not all the parts of the animal underwent a transformation, which could in part be well explained as morphological adaptation to new conditions of life, and partly as a consequence of this adaptation (correlative changes), but could not possibly be regarded as the sudden action of these changed conditions.
Such at least is my view of the case, according to which a per saltum development of the species of such a kind as must here have taken place, is quite inconceivable.
I may venture to assume that most observers of the metamorphosis of Axolotl have, like myself, not been hitherto aware of the extent of the transformation, and it may thus be explained why the theoretical bearings of the case have on all sides been incorrectly conceived. We have here obviously a quite extraordinary case of the first order of importance. I believe that it can easily be shown that the explanation of the history of the metamorphosis of the Paris Axolotl which has hitherto been pretty generally accepted, necessarily comprises a very far-reaching principle. If this interpretation is correct, then in my opinion must also hold good the ideas of those who, like Kölliker, Askenasy, Nägeli, and, among the philosophers, Hartmann and Hübner, would refer the transformation of species in the first instance to a power innate in the organism, to an active, i.e. a self-urging “law of development”—a phyletic vital force.