64 AMPHIBIA CHAP. 



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 JVeotenie by Kollmaun ^ {v6o<;, young ; 

 Tetva, extend, stretch). He distinguished further between : — I. 

 Partial Neoteny, namely, simple retardation of the metamor- 

 phosis beyond the normal period, for instance, the wintering of 

 tadpoles of Felobates fuscus, Bomhinator pachypus, Pelodytes 

 iniAictattos, Alytes ohstetricans, Hyla arhorea, Sana esculenta, R. 

 Umporaria, Bvfo vulgaris, smd £. viridis: II. Total Neoteny, 

 where the animal retains its gills, but becomes sexually mature ; 

 hitherto observed in Urodela only, e.g. Triton vulgaris, T. alpes- 

 tris, T. cristatus, T. boscai, T. waltli and AmUy stoma. Inter- 

 mediate stages between these tvio 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 neotenio and overgrown specimens occur side 

 by side with others whicli 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. 



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 t'hat all the Amphibia were originally gill- 

 breathing, aquatic, and limbless animals, and that every feature 

 seen in a larva must necessarily indicate an ancestral phylo- 

 genetic stage. It is, on the contrary, much more probable that 



' Verh. Ges. Basel, vii. 1882, ji. 387. 



