OF CONCHOLOGY. 197 



In the former Professor Adams notices, that since the pub- 

 lication of his previous remarks, he had received an article"^ 

 on the Geographical Distribution of Animals, by Professor 

 Agassiz, and adds: — "Following so eminent an authority and 

 safe guide, we need no longer hesitate to state the hypothesis, 

 which was prepared with the remarks above referred to, on 

 the sinuata group of HelicesJ'' 



The Professor then remarks as follows: — 



"The distribution of the terrestrial Molluscs in Jamaica 

 (and probably of all Molluscs in all parts of the world) is most 

 easily accounted for by the following hypothesis: that the 

 introduction of the existing races was efl'ected by the creation 

 of many individuals, and that they were modeled after certain 

 types, which were mostly local, and between which there ex- 

 isted, 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, &c." 



. . . "If these views are correct, we cannot retain the preva- 

 lent theory that species are natural groups, but that genera 

 and other groups are artificial. All groups are natural, so far 

 as they are founded on actual types. Any groups are artifi- 

 cial, or, to speak more correctly and intelligibly, are arbitrary, 

 in proportion as the differences between individuals, varieties, 

 species, and the successively m^re comprehensive groups, pre- 

 sent a series of an indefinite number of nearly equal degrees. 

 In proportion to the equality of the degrees are the points at 

 which we mark off species, &c., arbitrary." 



. . . "It is obvious that an illustration of this subject may 

 be found in the different species and varieties of mankind." 



With regard to the foregoing hypothesis as to the distribu- 

 tion of the terrestrial Molluscs, Professor Adams, in the latter 

 of the two papers above referred to, adds this additional ex- 

 planation : — 



"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, 



* The following extract from the article referred to, states the conclu- 

 sions of Agassiz: — "And this is the view which we take of the natural 

 distribution of animals, that they originated primitively over the whole 

 extent of their natural distribution ; that they originated there, not in pairs, 

 but in large numbers, in such proportions as suits their natural 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." ("Christian Examiner," Boston, March, 1850, p. 

 193.) 



