CONTRIBUTIONS TO CONCHOLOGY. 



No, 10. 



On the Nature and Origin of the Species of the Terrestrial 

 MoLLUSCA in the Island of Jamaica, B// C. B. Adams. 

 November, 1851. 



Note. — This article, and the one which follows, were read before the American 

 Association for the Promotion of Science, in New Haven, at the meeting in August, 

 1850, and were subsequently published in the Proceedings of the Association. 

 Having failed to obtain extra copies for distribution among conchological friends, 

 we reprint them with a few unimjDortant alterations. 



Notwithstanding the difficulty of exploration in tropical 

 regions, the Island of Jamaica presents remarkable facilities 

 for the investigation of subjects which are connected with the 

 geographical distribution of species. Among the terrestrial 

 shells, typical forms exist in great profusion. These forms 

 are of every conceivable grade of value, from varieties up to 

 genera and families. They have also a determinate geogra- 

 phical distribution. The facts on these subjects are even 

 more numerous than those which are expanded over the whole 

 temperate regions of North America. In this respect, there- 

 fore, the island is a miniature continent. Probably the same 

 is true of each of the larger Antilles. 



The following remarks will be restricted to certain conclu- 

 sions, which are suggested by these facts. The special de- 

 scription of the facts themselves is reserved for another oppor- 

 tunity. 



I. The Nature of the Species. 



1. Our first conclusion is this ; — that in many groups tlie 



species are distinguishable by types * only, and not by well 



* A type is the aggregate of distinctive characters. In order to be of specific 

 value it must perpetuate itself in succes&ive generations. 



No. 10—1. 



