404 T. Bland on the Geographical Distribution of Mollusca, 



ral mode of living, and the preservation of their species ; and that 

 the same species may have originated in different unconnected 

 parts of the more extensive circle of their distribution." — p. 192. 



Professor Agassiz enters upon the same subject in an article in 

 the July number of the Christian Examiner^ to which we also 

 refer our readers. 



Prof. Adams states in his Contributions^ No. 6, the conclusions 

 at which he had arrived from a careful study of the land shells 

 of Jamaica. 



'' The*distribution of the terrestrial Mollusks in Jamaica, (and 

 probably of all MoMusks in all parts of the world,) is most easily 

 accounted for by the following hypothesis: — that the introduc- 

 tion of the existing races was effected by the creation of many 

 individuals, and that they were modelled after certain types, 

 which were mostly local, and between which there existed, as at 

 the present day, unequal differences, from those which merely 

 distinguish individuals, to those of varieties, of species, of groups 

 of species, of genera," (fee. The author subsequently observes^ 

 ^' of course the doctrine of contemporaneous origin must have a 

 geological latitude." 



We add the explanatory statement from No. 10 : — " 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 occupied by one of them co- 

 incides with that of other two or more. Now, if the circum- 

 stances 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 geo- 

 graphical limits. But the contrary more frequently occurs. Each 

 variety has its own limits of distribution. If a few coincide in 

 the boundary of their province, 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 individu- 

 als, 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. We have then indistinct varieties, distinct 

 varieties, doubtful species, good species, and groups of species, 

 and all the intermediate types, distributed in the same manner." 



