Weismann tried to explain neoteny as cases of reversion to atavistic ancestral conditions, but this idea is based upon an assumption which is probably wrong. His idea necessitates the supposition that all the Amphibia were originally gill-breathing, aquatic, and limbless animals, and that every feature seen in a larva must necessarily indicate an ancestral phylogenetic stage. It is, on the contrary, much more probable that the external gills of the Urodela have been developed in adaptation to their embryonic and larval, essentially aquatic, life. Consequently the possession of such gills would be a secondary, and not, strictly speaking, an atavistic feature. Normal loss of these gills, exclusively pulmonary respiration, and preponderating terrestrial life characterise the final adult Amphibian. These cases of neoteny are therefore instances of more or less complete retardation, or of the retention, of partially larval conditions.
The whole problem is, however, by no means simple. Salamandra atra has become viviparous, and the whole metamorphosis takes place within the uterus; in fact, the young have an embryonic, but no larval period, if by the latter we understand the free swimming and still imperfect stage. Similarly, various Anura–for instance, Hylodes martinicensis–pass rapidly through their metamorphosis, and have suppressed the stage of free swimming tadpoles. On the other hand, in many newts, the duration of the larval period is much prolonged, and moreover is very subject to individual variation. In the Axolotl this larval period is continued until and after sexual maturity is reached. The extreme condition would then be represented by the Perennibranchiate genera. It may seem reasonable to look upon these as the youngest members of the Urodela, and the loss of the maxillae in the Sirenidae and Proteidae supports this idea. But it so happens that the majority of the most neotenic genera are more primitive in the composition of the skull and the vertebral column than the typically terrestrial and rapidly metamorphosing genera. Witness the amphicoelous vertebrae, the completeness of the pterygoids, the separate nature of the palatine bones, and the separate splenials, as mentioned in detail in the description of their skull.
We have therefore to conclude, first, that the various Perennibranchiate genera do not form a natural group, but are a heterogeneous assembly; secondly, that they have become Perennibranchiate at a phylogenetically old stage–in fact, that they are the oldest, and not the newest, members of the present Urodela. At the same time, it would be erroneous to suppose that the first Urodela were aquatic creatures, provided with a finny tail, with small, ill-developed lungs, and with epidermal sense organs. All these features are, on the contrary, directly correlated with aquatic life, and are larval acquisitions, not ancestral reminiscences. It would be equally wrong to allude to the absence of lungs in many newts as a piscine and therefore ancestral feature. The development of the typical pentadactyloid limb, the connexion of the pelvic girdle with the vertebral column, the development of the lungs, and absolute suppression of internal gills point without doubt to terrestrial creatures. What then, may we ask, were the first Amphibia like? and how about the external gills? They were undoubtedly akin to the less specialised Lepospondylous Stegocephali, in particular the gill-less Microsauri, and the various stages may perhaps be reconstructed as follows:–
(1) Terrestrial, with two pairs of pentadactyloid limbs; breathing by lungs only; with a fully developed apparatus of five pairs of gill-arches, which during the embryonic life perhaps still carried internal gills; with or without several pairs of gill-clefts. Reduction of the dermal armour and of the cutaneous scutes had taken place.
(2) Additional respiratory organs were developed by the embryo, in the shape of external gills; these were at first restricted to embryonic life (as in the existing Apoda), but were gradually used also during the aquatic life of the larva. These external gills, together with the lungs, have superseded the internal gills, of which there are now no traces either in Urodela or in Anura.
(3a) Some Urodeles, retaking to aquatic life, retained and further enlarged the external gills into more or less permanent organs (cf. also Siren, p. [136]).
(3b) The majority of Urodela hurried through the larval, aquatic stage, and some–e.g. Salamandra atra–became absolutely terrestrial. The possession of unusually long external gills by this species and by the Apoda indicate that these organs are essentially embryonic, not larval, features.
Regeneration.–Most Amphibia possess the faculty of regenerating mutilated or lost limbs. This takes place the more certainly and quickly the younger the animal. The amputation necessary to study these phenomena need not be experimental. Axolotls and other Urodelous larvae frequently maim each other fearfully, by biting off the gills or one or more limbs. The gills do not even require amputation. If the larvae are kept in stagnant water the gills often shrivel up or slough off and grow again. The same applies to the larvae of viviparous species, e.g. Salamandra atra, which, when cut out of the uterus and put into water, soon cast off their long, tender gills and produce a stronger set. In an Axolotl,[[38]] two years old, a hand was cut off. After four weeks there was a conical stump; after the sixth week this stump had two points; in the eleventh week three or four fingers were discernible, and a week later the complete hand. Frequently these creatures reproduce five instead of the normal four fingers. But the more proximal the cut, the more liable is the new limb to reproduce supernumerary fingers, or even extra hands and feet. Complete regeneration of the limb, cut off in the middle of the humerus, took place within five months.
Triton taeniatus, adult, reproduces cut fingers within five or six weeks, and if the hand be cut above the carpus, new finger-stumps appear in about one month. Götte has observed that an adult Proteus did not completely reproduce its whole leg until after eighteen months; and, according to Spallanzani, more than one year elapses before the limb, bones, and cartilages of Triton regain their normal strength.
The Anura are likewise capable of regenerating their limbs, the more readily the younger the specimens. For instance, in a tadpole of Rana temporaria, in which the fore-limbs were still hidden, the hind-limb, cut at the middle of the thigh, reproduced nineteen days later a knee, followed by a short two-toed stump. Ultimately the whole limb became completed. The tail of tadpoles regenerates very quickly and completely, even if it be cut off shortly before the final metamorphosis, when the tail would in any case be reduced. Metamorphosed Anura have almost entirely lost this faculty, but not absolutely. I myself have kept two specimens of Rana temporaria, which, when already adult, had each lost a hand at the wrist. First there was only the clean-cut stump with a scar, but within a year this changed into a four-cornered stump, and two of the protuberances developed a little further, reaching a length of about 4 mm. These specimens lived for four years without further changes.