1865.] Contributions to Indian Malacology. 1()3 



to a single river and its feeders^ as is the ease, so far as is known, with. 

 U. olivarius, Lea. In other cases again, as in U. cceruleus, Lea, and 

 its allies, one form is found over a considerable area, as Bengal, and in 

 separate rivers, and is replaced at a distance, as in Scind and 

 Western India, by forms which may either be considered as distinct 

 species, or as local varieties, according to the value attached to specific 

 rank. In the intermediate country of Central India, we find interme- 

 diate forms. Now it is surely more philosophical to assume that we 

 are only partially acquainted with the phenomena attending the means 

 of distribution enjoyed by animals of low organisation, especially in 

 the young state,* than to arrogate to ourselves complete knowledge of 

 the subject, and to assert that no means of passage exist. If we sup- 

 pose that facilities for migration exist, or have existed, with which we 

 are unacquainted, all the facts above detailed are at once accounted for 

 in the simplest manner, whereas on the theory that the species were 

 originally created throughout the whole area, no explanation whatever 

 is afforded of the limitation of that area, no cause shewn why the 

 same species does not exist in other areas where the conditions are 

 equally favourable for its existence, and still less is any explanation 

 afforded of the gradual divergence of varieties at a distance from the 

 typical form. Let it be distinctly noted that the case of mollusks 

 and of other animals inhabiting fresh water is an exceptional one ; in 

 the vast majority of the members of the animal and vegetable king- 

 dom, the phenomena are far more strongly in favour of the theory 

 of specific centres. 



On another question, more especially treated in Mr. Theobald's 

 second paper, viz. : the impracticability of drawing a line between 

 species and varieties in many cases, I entirely coincide ; indeed in the 

 preceding pages will be found remarks upon the varieties of H. simi- 

 laris and its allies, and of H. rotatoria and its allies, similar in pur- 

 pose to those of Mr. Theobald. I must, however, object to the 

 practice of publishing names, whether of varieties or species, without 

 any description, or with such extremely inadequate details, as in the 

 case of Helix Arakanensis and H. geiton. I can only say that, 



* It should not be forgotten that the ciliated fry of the Unioriidae have very 

 considerable power of locomotion, and that even the adults are amongst bha 

 most vagrant of bivalve sheila. 



