Thus, if the Axolotls transformed into Amblystomas are regarded as individuals which, impelled by external influences, have anticipated the phyletic development of the others, then this advance can only be ascribed to a phyletic vital force, since the transformation is sudden, and leaves no time for gradual adaptation in the course of generations. The indirect influence of the external conditions of life, i.e. natural selection, is thus excluded from the beginning. But the direct action of the changed conditions of life by no means furnishes an explanation of the complete transformation of the whole structure, such as I have already alluded to, and which I will now enter into more closely.
The differences between the Paris Axolotl and its Amblystoma according to Duméril, Kölliker, and my own observations are as follow:—
1. The gills disappear; the gill-clefts close up, and of the branchial arches only the foremost remain, the posterior ones disappearing. At the same time the os hyoideum becomes changed (Duméril).
2. The dorsal crest completely disappears (Duméril).
3. The aquatic tail becomes transformed into one like that of the salamanders (Duméril), which, however, is not as in the salamander cylindrical, but somewhat compressed laterally (Weismann).
4. The skin becomes yellowish white, irregularly spotted on the sides and back (Duméril), whilst at the same time its former grey-black ground-colour changes into a shining greenish black (Weismann); it loses, moreover, the slimy secretion of the skin, and the cutaneous glands become insignificant (Kölliker).
5. The eyes become prominent and the pupils narrow (Kölliker), and eyelids capable of completely covering the eyes are formed; in Axolotl only a narrow annular fold surrounds the eyes, so that these cannot be closed (Weismann).
6. The toes become narrowed and lose their skin-like appendages (Kölliker), or more precisely, the half webs which connect the proximal extremities of the toes on all the feet (Weismann).
7. The teeth are disposed in this species, as in all other Amblystomæ, in transverse series; whilst in Axolotl, as in Triton larvæ, they are arranged at the sides of the jaw in the form of a curved arch-like band beset with several rows of teeth.[238] (Duméril. See his fig., loc. cit. p. 279).
8. In Axolotl the lower jaw, in addition to the teeth on the upper edge of the bone, also bears “de très petites dents disposées sur plusieurs rangs;” these last disappear after metamorphosis (Duméril). I will add that the persistent teeth belong to the os dentale of the lower jaw, and those that are shed to the os operculare.[239]