1918. J N. Annandale: Molluscs of the Inle Lake. 153 



tank in the Museum compound in Calcutta. The form may, indeed, 

 represent a distinct species, but it is in any case closely allied. The 

 different phases of L. shanensis probably resemble those of L. acuminata, 

 assuming the narrow form to be specifically identical, in this respect, 

 and there is no more reason to be certain that one is directly descended 

 from the other, than there would be reason to claim that the Baltic race 

 of L. auricularia was directly descended from the Pamir race, or the 

 deep-water Swiss forms from the Baltic race. It is convenient to arrange 

 the shells in a regular series, but it is not improbable that they represent 

 divergent lines from a common type, rather than a single line of descent 

 undergoing progressive modification. They may, therefore, be called 

 links in a chain of evolution only in quite a general sense. Details of 

 these instances of plasticity in Limnaea are different in the different 

 cases, but they have one important point in common. They provide 

 little evidence of individual variation. The individuals of each species 

 found in the same environment as a rule resemble one another closely, 

 but those from different environments differ. 



These facts, observed under natural conditions, receive considerable 

 additional interest from the experiments made by Semper, i by de 

 Varigny,^ by Whitfield^ and by Roszkowski* in aquaria. The investi- 

 gations of Semper and deVarigny were carried out independently and 

 mainly with the object of discovering the results of confinement on 

 Lirmiaea. Both experimentors agree that if individuals of this genus are 

 kept in small masses of water, the shells of their young, bred in captivity, 

 are dwarfed and more or less altered in shape. They found, moreover, 

 that the effect was cumulative from generation to generation. The chief 

 point in which they differed was that of the agency that produced 

 these changes, but it seems not improbable that the most important 

 factor in all cases was the products of metabolism, which contaminated 

 the water and did not diffuse equally through the whole of an aquarium 

 even when the individuals on which the experiment was being made 

 were confined merely by means of a piece of muslin covering the end of 

 an open tube. This factor was, therefore, essentially a chemical change 

 in the water. For our purpose it is not necessary to follow these inter- 

 esting experiments further. An excellent summary is given by Vernor 

 in his " Variation in Animals and Plants " (London : 1903). 



Whitfield, apparently by accident, obtained additional evidence of the 

 dwarfing and distorting of the shells of Limnaea through succeeding 

 generations in captivity. He discovered that confinement in an aquarium 

 resulted, after three generations, " in the production of a monoecious 

 animal from a dioecious one of the most perfect kind. Also in 

 changing the specific characters, as far as the foim of the shell can be 

 considered, to such an extent that when shown to a good working con- 

 chologist (Dr. James Lewis) he gave it as his opinion that they could 

 have no specific relations to each other." 



1 Arb. a. d. Zool. Inst, in Wurzhvrg, I, p. 137 (1874). 



2 Journ. de VAnai. et de la Physiol., p. 147 (1894). 



3 Biill. Awer. Mus. Nat. Hist., I. p. 29 (1882). 

 * Zool. Anz. XL, p. 375 (1912). 



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