This discovery led to a series of observations and experiments, chiefly conducted by Marie von Chauvin, instigated thereto by Koelliker and by Camerano.[36] It was then found that many, if not most of the European Amphibia, both Urodela and Anura, {64}occasionally postpone their metamorphosis, and also that such Urodela sometimes become adult for all practical purposes, but retain their gills. This retardation, the retention of larval characters beyond the normal period, was called _Neotenie_ by Kollmann[37] (νέος, young; τείνω, extend, stretch). He distinguished further between:–I. Partial Neoteny, namely, simple retardation of the metamorphosis beyond the normal period, for instance, the wintering of tadpoles of _Pelobates fuscus_, _Bombinator pachypus_, _Pelodytes punctatus_, _Alytes obstetricans_, _Hyla arborea_, _Rana esculenta_, _R. temporaria_, _Bufo vulgaris_, and _B. viridis_: II. Total Neoteny, where the animal retains its gills, but becomes sexually mature; hitherto observed in Urodela only, e.g. _Triton vulgaris_, _T. alpestris_, _T. cristatus_, _T. boscai_, _T. waltli_ and _Amblystoma_. Intermediate stages between these two categories are not uncommon. A satisfactory explanation of the meaning of neoteny is beset with difficulties. Some authorities look upon the phenomenon simply as the result of adaptation to the surroundings, which make it advantageous for the creature to retain its larval features. Others think that the surroundings somehow or other retard or prevent the assumption of the adult characters. Undoubtedly there are many cases in which larvae have been reared in water-holes with steep walls, so that they could not change from aquatic to terrestrial life, and it stands to reason that abnormally forced and prolonged use of the gills and of the tail may stimulate these organs into further growth at the expense of the limbs and other organs which are intended for terrestrial life. But not unfrequently typical neotenic and overgrown specimens occur side by side with others which have completed their metamorphosis, and the same is true of larvae of newts which were reared, for experimental purposes, under exactly the same conditions–for instance, in a high-walled glass vessel.