Thus the supposition is not wanting in support, that peculiar conditions make it more difficult for the creature to obtain its food upon land than in the water, and this alone may have been sufficient to have induced it to acquire the habits of a purely aquatic existence, and thus to revert to the perennibranchiate or Ichthyodeous form. But enough of supposition. We must not complain that we are unable from afar to discover with precision the causes which compelled the Axolotl to abandon the Amblystoma stage, as long as we are not able to explain the much nearer cases of reversion in Filippi’s and Jullien’s Tritons; nevertheless, in these cases also, the causes affecting the whole colony of Tritons must be general, since--at least in the case noticed by Filippi--the greater majority of the individuals remained in the larval condition. Experiments with Triton larvæ could throw greater light upon this subject; it would have in the first place to be established whether reversion could be artificially induced, and if so, by what influences. From the previously mentioned experiments with butterflies, as well as from the results obtained with Axolotls, we should expect that in Tritons, reversion to the Ichthyodeous form would take place if we allowed the inciting cause, viz. the bathing of the gills and of the whole body with water, to act persistently, and at the same time withheld that influence under whose action the salamander form became developed, viz. the bathing of the gills, the skin, and the surfaces of the lungs with air. Old experiments of this kind are to be met with, but they were never carried on for a sufficient time to entirely allay the suspicion, that the specimens concerned would perhaps have undergone the ordinary metamorphosis if their existence had been prolonged. Thus, Schreibers[263] relates that “by confining tadpoles of the salamander found at large in their last stage of growth, under water by means of an arrangement (net?), and feeding them with finely chopped earthworms, he was able to keep them for several months--and indeed throughout the winter--in this condition, and in this way to forcibly defer their final change, and their transition from the tadpole stage to that of the perfected creature during this period.” It is not stated whether the animals finally underwent transformation, so that it cannot be decided whether we have here a case of reversion or simply one of retarded development. That metamorphosis may occur after a long period of time, is shown by experiments upon the tadpole of _Pelobates_ conducted by Professor Langer in Vienna.[264] The creatures were kept in deep water in such a manner that they were not able to land, and by this means three out of a large number of individuals had their metamorphosis delayed till the second summer; notwithstanding this, transformation then occurred.