DISTKIBUTION OF DNIVALTES. 299 



characters of their shells ; nevertheless, both have been sub- 

 divided into a great number of dift'erent genera, and, as has 

 been proved by a study of the animals themselves, in many 

 cases this has been perfectly justified. But hitherto we do not 

 know the anatomy of more than a few dozen of these genera, 

 and so it is at least possible, if not probable, that a more exact 

 investigation of the animals may demonstrate precisely the re- 

 verse of any extensive uniformity in the structure of the species 

 placed in these genera. Thus we have no longer the right to 

 speak of the extensive distribution of the genera Melania, Pcdu- 

 dina, Anodonta, and Unio, and it is consequently superfluous to 



r ~ \ 



Fig. 77. — Two operculated fresh-water univalves, n, Melania ; b, Paludina. 



seek an explanation of a fact which, though it cannot be shown 

 to be false, is as yet ' not proven,' but, on the contrary, cannot be 

 brought into harmony with the few facts which are ascertained 

 and established. 



One more difficulty must here be briefly alluded to. 

 According to Darwin's views it might be expected that all 

 easily transportable kinds should show a greater uniformity of 

 species than those which are less protected against the perils of 

 a long journey. But the reverse is often the case with fresh- 

 water univalves. The species of Paludina and Melania have 

 an operculum which fits almost exactly into the mouth of the 

 shell, so that the animal would seem to be effectually protected ; 



