170 Records of the Indian Museum. [Vol. XIY, 



manifested by the aquatic molluscs of a single district. Parallel 

 oases have been cited for comparison, not to support any one theory. 

 Four facts stand out prominent in reference to these MoUusca : — 



(1) That racial plasticity is a more common phenomenon among 



them than extreme individual variability, and that the 

 two are not necessarily correlated. 



(2) That both plasticity and individual variability are specific 



characters ; they may be almost absent for the time being 

 in a stable species, but may be either acquired or lost in 

 the course of evolution. 



(3) That in very few instances is it possible to detect any advan- 



tage that the race can have gained by its plasticity. 



(4) That the moulding forces, or the causes of plasticity, of great- 



est influence are not the same in all species, and that 

 apparently slight differences in environment are some- 

 times of greater practical moment than changes which 

 seem to be much greater. ^ 



I will deal with each of these points in some detail. 



(1) 



In most of the aquatic molluscs of the Inle district, individuals from 

 the same environment are very like one another. The main exceptions 

 are Planorhis velifer (of which two distinct varieties live together), 

 Melania baccata, Taia naticoides and Taia elitoralis. 



The case of Taia naticoides is a remarkable one, for the species is 

 both variable and plastic. Individuals from any one environment 

 differ greatly from one another, falling roughly into three groups, the 

 limits of which are, however, undefined. At the same time races from 

 different types of environment differ from one another, while retaining 

 their individual variability in slightly modified ways. It is, however, 

 to the most extreme variety of the species {carinata, Theobald) that all 

 the more highly modified forms of the genus are most closely related. 

 Indeed, one of these forms (intermedia) is little more than a fixed race 

 of this variety. T. intermedia is not, however, precisely speaking, a 

 mutation in the sense in which the term is used by most biologists, 

 because it is not descended, so far as can be seen from the evidence 

 available, from a single individual or group of individuals that have 

 departed suddenly from the normal type of the species. T. naticoides 

 has a fairly wide distribution, and the variety carinata always occurs 

 with the typical form. Moreover the transition between the two forms 

 is quite gradual. 



The circumstances of T. elitoralis are different in that it is only 

 known from a single locality and a single type of environment, but it also 

 seems to have given rise to a constant species very like itself, namely, 

 T. intha, which is at once the most highly specialized form and one of 

 the most constant forms in the aenus. 



On the other hand Taia theohaldi, probably the parent form of all 

 these species, is a constant species. So also, in a sense, is Hydrohioides 

 nassa, which has produced four races, each fairly constant in its own type 

 of environment. Further, H. nassa is derived from another constant 



