I have met with a third case of a similar kind mentioned by Leydig in his memoir, rich in interesting details, “on the tailed Amphibians of the Wurtemburg fauna.”[253] Schreibers, the former director of the Vienna Museum, also found “larvæ” of Tritons with well-developed gills, but of the size of the “adult male individuals,” and, as shown by anatomical investigation, with well “developed sexual organs,” the ovaria especially being distended with eggs.
It is thus established that species which long ago reached the salamander stage in phyletic development, may occasionally degenerate to the perennibranchiate stage. This fact obviously makes my conception of the Axolotl as a reversion form appear much less paradoxical—indeed, the cases of reversion in Triton are precisely analagous to the process which I suppose to have taken place in the Axolotl. We have only to substitute Amblystomas for Tritons, to imagine the pool in which De Filippi found his “sexually mature Triton larvæ” enlarged to the size of the Lake of Mexico, and to conceive the unknown, and perhaps here transitory, causes of the reversion to be permanent, and we have all that is necessary, so far as we at present know, for the restoration of the Axolotl; we obtain a perennibranchiate population of the lake.
It has not yet been determined whether the perennibranchiate form of the Triton actually prevailed permanently in De Filippi’s pool, since, so far as I know, this has not since been examined.
Let us, however, assume for an instant that this is really the case, and that there exists at that spot a colony of sexually reproductive perennibranchiate Tritons: should we wonder if a true Triton occasionally appeared among their progeny, or if we were able to induce the majority of the individuals of this brood to become metamorphosed into Tritons by keeping them in shallow water? According to my view this is precisely the case of the Mexican Axolotl.
I need not, however, restrict myself to this in order to support my hypothesis, but must also directly combat the view hitherto received, since the latter is in contradiction with facts.
Did there really exist in the Axolotl a tendency to sudden phyletic advancement, then one fact would remain quite incomprehensible, viz. the sterility of the Amblystomas.
Out of about thirty Amblystomas obtained by Duméril down to the year 1870, there was not one in a state of sexual maturity; neither copulation nor deposition of eggs took place, and the anatomical investigation of single specimens showed that the eggs were immature, and that the spermatozoa, although present, were without the undulating membrane characteristic of the salamanders, but were not devoid of all power of movement, only, as established by Quatrefages, were “incompletely motile.”[254]
So also the five Amblystomas about which I have been writing, show up to the present time no appearance of reproduction.
The objection raised by Sacc,[255] that the sterility of the Amblystomas bred from Axolotls is attributable to “bad nourishment,” is obviously of but little avail. How is it that the Axolotls, which are fed in a precisely similar manner, propagate so readily? Moreover, I am able to expressly assert that my Amblystomas were very well fed. It is true that they have as yet scarcely reached the age of two years, but the Axolotl propagates freely in the second year, and some of Duméril’s Amblystomas were five years old in 1870.
This fact of the sterility is strongly opposed to the idea that these Amblystomas are the regular precursors of the phyletically advancing genus Siredon.[256] I will by no means assert that my theory of reversion actually explains the sterility, but it is at least not directly opposed to it. Mere reversion forms may die off without propagating themselves; but a new form called forth by the action of a phyletic vital force should not be sterile, because this is the precise “aim” which the vital force had in view. The conception of a vital force comprises that of teleology.