1918.] N. Annandale : Molluscs of the Inle Lake. 171 



form, H. avarix, in which its most striking peculiarity (the varix) is 

 absent. 



(2) 

 In species such as Planorhis exustus neither individuals nor local 

 communities as a rule differ much one from another. A deficiency — 

 it is not of course an entire absence — in variation of all kinds is also well 

 exemplified in Taia theohaldi and Hydrobioides avarix. Yet these two 

 constant forms have both given rise to species that are both variable 

 individually and plastic : to T. naticoides, in which both individual 

 variation and plasticity are extreme, in the one case, to H. nassa, in which 

 plasticity is more marked than individual variability, in the other. 

 Lmmaea sJianensis, on the other hand, has proved itself plastic without 

 exhibiting individual variability ; we only know it as a plastic species 

 because of the discovery of shells of extinct phases ; each phase was 

 constant in its proper environment. Taia intha, a highly specialized 

 form descended from ancestors that were both plastic and variable 

 individually, has become in its owm proper habitat a constant species 

 and has apparently lost plasticity. That Limnaea ynimetica has done so 

 is proved by its continued existence as a modified form in conditions 

 totally different from those with which its modification must be corre- 

 lated ; for its resemblance to deep-water forms is not merely superficial 

 as in some of the instances cited on p. 174, but so detailed as to be almost 

 beyond dispute.^ Further, it is even possible that plasticity once lost 

 may in certain circumstances be regained. This is, however, more 

 difficult to demonstrate. The He-Ho living phase of T. naticoides may 

 conceivably be descended direct from T. intermedia and represent a 

 reversion to the variability of the ancestral form, but it seems on the 

 whole more probable that T. naticoides, even after giving rise to T. 

 intermedia, persisted in the neighbourhood unchanged, in a different 

 environment from its daughter form. 



(3) 



The species or groups of species that have exhibited greatest plais- 

 ticity are Limnaea andersoniana, L. shanensis, Melania tiibercvlata, 

 Hydrobioides avarix and nassa, and the species of Taia. 



On general grounds it is clear that two types of aquatic environment 

 are the most favourable, at any rate in tropical and subtropical climates, 

 for the type of plasticity that results in the evolution of peculiar species 

 and genera. They are small mountain streams and large lakes. This 

 is the case not only with molluscs but also with other groups of animals. 

 Many of the most highly modified Indian genera of fish and of aquatic 

 molluscs, as well as the most peculiar species of Batrachian and insect 

 larvae, live in small streams in the hills, e.g., Pseudecheneis of the family 

 Siluridae among the fish ; the almost neritiform Stomatodon of the family 

 Melaniidae among the molluscs ; and the tadpoles of such frogs and 



^ Compare the blind prawn Typhocaris galilea, which has all the characters of an 

 underground animal but now lives in an open fountain in which it has probably been 

 isolated by an earthquake. See Annandale and Kemp, Journ, As. Soc. Bengal, IX 

 (n. s.), p. 245 (1913). 



