1864.] 



Proceedings of the Asiatic Society. 



343 



suggested, still require investigation before the inference drawn by 

 M. Agassiz can be admitted. — To take a case better capable of argu- 

 ment ; that of the Hill Mollusca of Southern India. It is an actual 

 fact that while certain of the species, as Helico Castra are common to 

 two or more isolated groups, others, such as the JDiplommatinas differ 

 on two hill groups, but are more closely allied to each other than to 

 their congeners on the Himalaya or elsewhere. This latter may be 

 regarded as a case in which specific variation has supervened since 

 that communication of conditions existed between the hill groups, 

 which has been inferred on geological grounds. The Streptacces differ 

 less than the Diplommatinas, and it is questionable whether on the 

 score of difference of external characters alone they should be treated as 

 species or varieties, so that here we have gradations of difference up 

 to actual identity. This is certainly in accordance with the view that 

 variation has supervened since separation, and is not accounted for 

 rationally by the assumption that each hill group is an original centre 

 of specific distribution. 



Mr. Theobald has much combated and ridiculed the idea of acci- 

 dental distribution by floating timber, &c, but now apparently admits 

 it as an occasional though rare phenomenon. It was never regarded 

 by Mr. Blanford as otherwise than exceptional, but there may be 

 other modes of distribution by transport, not yet known or fully 

 appreciated. In a paper lately transmitted to the Linnean Society, 

 Mr. Blanford had remarked upon certain facts of distribution of 

 Melanice and Paludomi which seemed to support Mr. Darwin's view 

 that birds are active unconscious agents of transport. The Melanice 

 and Paludomi of marshes, tanks, estuaries, &c, which are much fre- 

 quented by water fowl, are of extremely wide distribution. Those 

 of hill streams, which are not frequented by water fowl are of very 

 restricted range, and even in small areas, as in the hill region of Ceylon, 

 two adjacent streams not communicating were tenanted by forms so 

 different that they had in a great number of cases been described as 

 distinct, although as Mr. Blanford had shown by the comparison of 

 large numbers taken from a great variety of localities, they were 

 almost unquestionably mere varieties, that is, that the most diverse 

 forms were connected by intermediate gradations. How communication 

 originally took place can only be surmised, but the comparative 

 absence or rarity of communication had hare admitted of great local 



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