176 Records of the Induin Museum. [Vol. XIV, 



mate parts, there can be no difficulty in claiming that change of condi 

 tions may ultimately affect the gonads in such a way that a new race 

 or species is evolved, for either the animal must become parthenogenetic 

 or else new combinations in the germ-plasm must be liable to occur. 

 The precise meaning of Whitfield's statement is, however, obscure, and 

 it cannot be applied to Limnaea mimetica, in which the genitalia are 

 quite normal and as usual hermaphrodite. I reproduce a figure (fig. 9) 

 prepared for me by Mr. Baini Prashad to illustrate this point. 



It does not seem to be possible to explain the extraordinary 

 development of the forms of Taia in the same way as that of the 

 genera already discussed. The history of the genus as we know it may 

 be summarized as follows : — Taia theohaldi, a constant species, lived and 

 lives in running water in mountainous districts. It gave rise, how we 

 do not know, to T. naticoides, an extremely variable species that lives 

 in ponds and marshes and the backwaters of streams. Some individuals 

 of T. naticoides closely resemble the parent form, while others depart from 

 it widely and represent a much higher degree of specialization. From 

 this extreme form of T. naticoides some ten or eleven other forms have 

 been derived, mostly if not solely in large lakes. Evolution, therefore, 

 seems to have taken place in this genus along somewhat peculiar 

 lines, but the highly specialized sculpture of the shell can hardly have 

 been more than a by-product of evolution.^ Dendy and Nicholson ^ 

 have recently shown that certain sponge-spicules of somewhat ela- 

 borate outline owe their peculiar shape to the mechanical forces 

 produced by the flow of water through the sponge. Dendy has also 

 pointed out, however, that advantage is taken of the modifications 

 thus produced, should they chance to be useful. The cases are o 

 course analogous, not homologous, for the forces, be they chemical oi 

 purely physical, to which a free-living mollusc is subjected in still 

 water cannot be the same as those that have moulded the spicules 

 into shape ; and even the analogy must not be pressed too far, for 

 we have evidence in Taia naticoides that at any rate the rudiments 

 of the peculiar sculpture of the shell may appear without the appli- 

 cation of any well-defined physical or chemical force. The resemblance 

 consists in this — that in both cases highly peculiar and elaborate 

 forms have been produced in the inanimate parts of living organisms 

 without any apparent utility in the first instance, but capable of 

 utilization and as it were of standardization. In the life-history of 

 Taia there seems to have been no economic need for the application 

 of these peculiarities, but they have become standardized by what 

 seems to have been a racial (or possibly a germinal) as distinct from an 

 individual selection ; Taia intermedia affords no evidence of the sur- 

 vival of the fittest individuals but suggests rather that all the indivi- 

 duals born in a certain locality and type of environment were 

 formed in accordance with a certain pattern that already existed, 

 with others, in the reproductive potentialities — the phrase is purposely 

 vasue — of T. naticoides, its immediate ancestor. 



1 Dendy, Journ. Qiieh-tt Mia: Club (2), XIII, p. 38 (1916). 



- Dendy, ibid., XIII, ])p. 1-16 (1917) ; JDendy and Nicholson, Proc. Roy. Soc, London 

 (B), LXXXIX, pp. 573-587. 



