254 BRITISH MOLLUSKS. 



on the general distribution over the globe of rnollusks. Only sixteen 

 of the British land rnollusks are adduced in evidence by Edward 

 Forbes, in his celebrated memoir (Mem. Geol. Surv., vol. i. p. 336), 

 while the freshwater species are not alluded to in any way, and the 

 idea of a distribution of individuals of Old World species iu the 

 New, and vice versa, is not allowed to either kiud. " Species of 

 opposite hemispheres," says the author "placed under similar con- 

 ditions, are representatives, and not identical." The distribution 

 of species over the globe, so far as I am able to gather from the 

 land and freshwater rnollusks, appears to require that we should 

 take for granted the doctrine of a plurality of progenitors for each 

 species ; and the term ' specific centre ' to indicate merely that point 

 of a geographical province, in which the species are most numerous 

 and come nearest to our notion of an ideal type of the group. 



The doctrine of more than one point of origin for a species, con- 

 sidered with reference to the typical character and distribution 

 of land and freshwater rnollusks, rests mainly on the following 

 propositions : — 



1. Land species with greater facilities of migration than freshwater 



species, are less widely and evenly diffused. 



2. Land and freshwater species of opposite hemispheres are not always 



representatives, hut are sometimes identical. 



3. The range of land and freshwater species over areas (zoological pro- 



vinces) indicated by uniformity of type, is not arrested by the 

 intervention of sea. 



§ 1- 



Land species with greater facilities of migration than freshwater species 

 are less tvidelg and evenly diffused. 



The doctrine of the migration of all the individuals of a species 

 from a single parent (or pair), involves the conclusion that species, 

 permanent, as I think, in their character, and immutable, diminish 

 in number in their march from the specific centre of a province 

 towards its confines. Out of many hundreds of land rnollusks 

 inhabiting the Caucasian province at its specific centre, only ninety 

 have reached the British Isles, of which thirty-five stop short of 

 Scotland, and nineteen of Ireland. Their progress northwards, it 

 may be argued, is arrested to a great extent by change of climate, 



