242 Mr. E. Forbes on Freshwater Mollusca. 



which enables us at once to name its fatherland. American 

 forms and Asiatic forms differ ; we find the minor groups cen- 

 tralized, and we might colour our maps variously, according 

 to the centralization of those groups. Their colouring varies 

 with the climate, and specimens of the same species from dif- 

 ferent localities may be distinguished by variation of size. 



In the lacustrine and fluviatile genera of Acephala we see a 

 similar influence of climate at work. The groups of Naiades 

 and of Cyclades are concentrated in certain geographical lo- 

 calities, and the southern species are often more splendid in 

 form and colouring than the northern. The old and new con- 

 tinents have few or no species in common, and the freshwater 

 Acephala of the east and west are in most cases very differ- 

 ent. 



These facts may be stated generally in the form of two pro- 

 bable laws : — 



1st. The variations of form, specific and generic, are not 

 so dependent on climate in the freshwater pulmoniferous 

 Gasteropoda as in the freshwater pectinibranchous Gastero- 

 poda and Acephala. 



2nd. In a genus independent of climatal influence the ex- 

 tension of distribution is correspondent with the non-variation 

 of form, and vice versa in a genus subject to the climatal in- 

 fluence. 



The following inferences applicable to geology may be 

 drawn from these considerations :— 



1st. If these views be correct, and if the great differences 

 between the animals of the primaeval world and those of the 

 present depend on climatal conditions as is usually admitted, 

 the difference between the generic and subgeneric forms of the 

 pulmoniferous Mollusca in the ancient strata and those now 

 living, should not be nearly so great as that between the an- 

 cient and existing marine fauna. And so we find it. When 

 we look over a collection of fossil freshwater Pulmonifera we 

 are at once struck by the circumstance of the absence of ex- 

 tinct genera and of the near alliance between the fossil and 

 extinct forms. 



2nd. But there should be a difference either as to the dis- 

 tribution or as to the character and number of species be- 

 tween the pectinibranchous Gasteropoda and Acephala of the 

 present and of the fossiliferous past. Thus, as climatal 

 causes affect the distribution of their genera and species, if 

 Britain had a warmer climate in the early ages of the world, 

 these tribes should evidence it. And such is the case. Me- 

 lania and Melanopsis and Ampullaria counted numerous sub- 

 jects at one time in our lakes and rivers ; Paludina were once 



