Terrestrial Blollusca in the Island of Jamaica. 193 



according to certain types, that were mostly but not wholly 

 local, and which differed from each other unequally, as do the 

 existing varieties. 



The proof of this proposition is found in the geographical 

 distribution of the varieties. In the great majority of species, 

 the varieties are so distributed, that the space which is occu- 

 pied by one of them coincides with that of other two or 

 more. Now, if the circumstances of locality had produced the 

 local types by modifications of one original type of the species, 

 then all the varieties which inhabit a locality should have been 

 affected. In that case, all the varieties in any given place 

 would have the same geographical limits. But the contrary 

 more frequently occurs. Each variety has its own limits of 

 distribution. If a few coincide in the boundary of their pro- 

 vince, on the other hand one is often found to have an extent 

 of distribution, which is equal to that of two or more other 

 varieties. But such a geographical coincidence of one variety 

 with several other varieties is inconsistent with any other theory 

 than that of an original constitutional peculiarity of character 

 in each variety. This inference is confirmed by the occasional 

 intermingling in one locality of varieties, which differ from 

 each other as much as those which occupy distinct regions. 

 If then we assume the original independent creation of all the 

 varieties, each originally represented by at least several indivi- 

 duals, the facts of distribution become explicable with the 

 greatest facility. 



The same statements might be made respecting entire 

 species, and even groups of species and genera. Some are 

 very local, and others, more widely distributed, occupy the 

 ground of several local species. AVe have then indistinct varie- 

 ties, distinct varieties, doubtful species, good species, and groups 

 of species, and all the intermediate types, distributed in the 

 same manner. Now, the theory of unity of origin requires us 

 to believe that all the types which are comprised within one 



