Terrestrial Molhisca in the Island of Jamaica. 191 



The special investigation of varieties is rapidly filling up the 

 gaps which were once supposed to exist between species. Not 

 dissimilar is the case of the human species, which graduate 

 into each other in such a manner, that the fact is often used 

 as an argument for confounding all the races in one species. 

 Yet it is admitted that the differences between the human races 

 are much greater than between many distinct species of ani- 

 mals. 



Our conclusion is briefly expressed in the proposition, that 

 species are of the same nature as genera ; that is, are to be 

 founded on types, whether or not an impassable vacuum can 

 be found between the types. 



2. The second inference on the nature of the species and 

 higher groups, is this ; — that the natural types are not sus- 

 ceptible of being wholly comprehended in a few successive 

 ranks, in each of which all the types shall be of exactly equal 

 value ; but there is an indefinite series of types within types, 

 which are inequidistant. If in one group, as that of Helix sinu- 

 ata and the kindred species, it is practicable to establish sev- 

 eral species of a given value, in another, as that of Cylindrella 

 Maug-eri, it is impossible to find species of the same value. 

 We must either make numerous species of less value, or re- 

 gard the group as one species of greater value. The same 

 doctrine is illustrated also by the comparison of the genus 

 Helix, in which the number of distinct subtypes is very great, 

 with Succinea, in which subtypes are indistinguishable. 

 There is no mathematical or physical reason why the generic 

 form of Succinea should not have been repeated with a great 

 diversity of subtypes. Yet it is so slightly modified in the 

 species, that Dr. Pfeiffer has grouped them geographically. 

 On the contrary, Helix contains nearly one hundred types, 

 of all values intermediate between a generic and a specific 

 value. 



In the same manner numerous other genera, families, and 



