Abstract
(From a German Correspondent)
DURING the past year some interesting observations have been published with reference to the alterations in animals through external influences. One series of these researches is by Weissmann, on the transformation of the Mexican Axolotl into an Amblystoma (Zeitschrift für Wissenschaftliche Zoologie, xxv., 1875, Supplement). It refers, of course, to a phenomenon which is not now new; but it includes a number of original experiments and observations, and is especially important for the conclusions drawn from these. The Axolotl (Siredon Mexicanus) and its allies in Mexico retain there, during life, in the natural state, the form and organisation of the larvæ of our Tritons; but, in artificially breeding them in Europe, they sometimes undergo a metamorphosis into an Amblystoma, i.e. an animal of the form of our fully developed Tritons. These peculiar departures from the natural behaviour of the Mexican Siredon, Weissrnann desired to produce artificially, and with this view he entrusted the breeding of five eight-day larvæ to a lady, Fraülein v. Chauvain. All five actually underwent the desired transformation, having been put for sixto eight months in water that was quite shallow, so that they were compelled frequently to leave the water, and become used to lung-breathing. Now, since, besides the Mexican Siredon species, which are never transformed in the natural state, there occur in the United States of North America quite similar animals, which, however, represent merely the temporary larva stage of various species of Amblystoma, the Mexican Siredon species have hitherto been regarded as forms that have remained at a lower stage of development, and, in the rare cases of metamorphosis by the action of changed conditions of life, have been incited to progression towards a higher stage. Weissmann, however, is now of a different opinion. He believes that the sudden and very remarkable transformation of the Siredon, which affects a whole series of organs, cannot be fully explained by the direct influence of changed conditions of life; and that should one see in such a transformation the leap-like (sprungweise) development of a new species or even genus, the hypothesis of a kind of life-force would be necessary. This teleological hypothesis should be avoided, according to Weissmann, and the transformation of the Siredon conceived as a not real but only apparent new formation of species, viz., as a reversion to a form which previously existed among the ancestors of the Siredon. Since the Perennibranchiata, at all events, represent the older form of the tailed amphibians, as it is indicated for the Amblystomas of North America in their Siredon-like larvae, all Siredons are to be regarded as the descendants of Amblystomas, which were permanently depressed to that older form, and in their occasional metamorphoses have realised a reversion to the second phylogenetic stage (Amblystoma). Such a conception Weissmann supports by the following reasoning:—The possibility of Siredon having come from Amblystoma is proved by the fact that we sometimes see Triton-larvæ, which attain the full size and sex-forms of an adult Triton without being transformed; now the Tritons and Amblystomas are very similar animals, and their larvæ are again extremely similar to the Siredon. But it is possible also to indicate the probable causes which forced the Amblystoma-like ancestors of the Siredon to reversion into the Perennibranchiate form. According to Humboldt's view, the high table-lands of Mexico were formerly covered with extensive lakes, and the evaporation of such large water-surfaces must then have produced a very moist atmosphere, which is necessary to the naked amphibia living on land. Consequently, Amblystoma forms could at that time live in Mexico quite well. With disappearance of the waters, however, came the present extreme dryness of the air on the Mexican highlands, which allows only the Amphibia living in water to survive, and is therefore probably the reason why the Amblystoma larvæ have gradually quite ceased leaving the water and being transformed, and thus have constituted the present Siredon species. If, then,the occasional transformation of Siredon to Amblystoma may be explained as a reversion, the necessity ceases of supposing for so sudden a change a special life force, which in Weissmann's opinion is necessary, should his theory be rejected.
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Science in Germany . Nature 14, 133 (1876). https://doi.org/10.1038/014133a0
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DOI: https://doi.org/10.1038/014133a0