COJNTRIBUTIONS TO CONCIIOLOGY. 



NO 6. 



Eemarks on the origin op the Terrestrial Molluscs 

 OF Jamaica. By C. B. Adams. March. 1850. 



Since the publication of the remarks on the distribution and differ- 

 ences of the types of the Terrestrial Molhiscs of Jamaica, as illus- 

 trated by the ^simiata' group of Helices, (p. 78,) we have received 

 an article * on the geographical distribution of animals by Professor 

 Agassiz. In this article a similar statement is made respecting the 

 distribution of the different types of lions. This case differs from 

 that of the Helices above-named, inasmuch as thei'e is no difficulty in 

 defining the species, because the difference between the specific type 

 and the types of its congeners is much greater than between the sub- 

 types comprehended within the species. But the obvious conclusion, 

 from the fact that different subtypes of one species exist in different 

 Zoological provinces associated severally with distinct species of other 

 genera, is boldly affirmed ; viz. plurality of origin, with the same 

 original differences which now exist. Following so eminent an au- 

 thority and safe a guide, we need nq longer hesitate to state the 

 hypothesis, which was prepared with the remarks above referred to, 

 on the ' sinuata ' group of Helices. 



The distribution of the terrestrial Molluscs in Jamaica, (and prob- 

 ably 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 effected by the creation of many individuals, and that they 

 were modeled after certain types, which were mostly local, and be- 

 tween 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, &c. Of course we do not 

 mean to assert that these differences proceed, from the less to the 

 greater, in a regular arithmetical progression. On the contrary, it is 

 because the spaces, which graduate the scale of differences, between 

 the types of any group are themselves unequal, that we are able to 



* Christian Examiner. March. 1850. Boston. 



