392 T. Bland on the Geographical Distribution of Molhisca, 



distribution of Animals." '-'Notwithstanding," say the authors, 

 " the uniform nature of the watery element, the animals which 

 dwell in it are not dispersed at random ; and though the limits 

 of the marine may be less easily defined than those of terrestrial 

 faunas, still, marked differences between the animals of great 

 basins are not less observable. Properly to apprehend how ma- 

 rine animals may be distributed into local faunas, it must be 

 remembered that their residence is not in the high sea, but along 

 the coasts of continents, and on soundings." p. 191. And far- 

 ther, — "A very influential cause in the distribution of aquatic 

 animals is the depth of the water ; so that several zoological 

 zones, receding from the shore, may be defined, according to the 

 depth of the water; much in the same manner as we mark dif- 

 ferent zones at different elevations in ascending mountains. The 

 Mollusks and even the fishes found near the shore in shallow 

 water, differ, in general, from those living at the depth of twenty 

 or thirty feet, and these again are found to be different from those 

 which are met with at a greater depth." — p. 192. 



In the various numbers of his " Contributions to Co7icholo^y,^^ 

 (No. 1, published in Sept., 1849, and No. 10, the last, in Nov., 

 1851,) Professor Adams describes the curious local distribution 

 of the terrestrial shells of Jamaica, and corrects many errors with 

 regard to them. He found some species improperly attributed to 

 Jamaica, and other species peculiar to that island, referred to other 

 localities. It is now satisfactorily proved, that of the 357species, the 

 whole number of strictly terrestrial shells at present known to in- 

 habit Jamaica, not more than 10 species are found in other islands. 



In " Contributions^^ No. 4, Professor Adams remarks, " With 

 this extremely local distribution of the terrestrial Mollusca in the 

 West Indies may be associated the great fact of their geological 

 history, — that these islands have, since the later tertiary periods, 

 been in the process of elevation, — that they are the harbingers of 

 a future continent, unlike the groups in the Pacific, which are the 

 remains of ancient continents. Coincident with these two gene- 

 ral facts in the West Indies is also a third, — that their coral reefs 

 are all fringing, and that coral islands are wanting." 



In August. 1850, the Professor read, (at the meeting of the 

 American Association for the Advancement of Science,) a paper 

 '' On the nature and origin of the Species of the Terrestrial 

 Mollusca in the island of Jarnaica^''^ (republished in " Contribu- 

 tions^^ No. 10.) From this paper we make the following extract : 

 "Notwithstanding the difficulty of exploration in tropical regions, 

 the island of Jamaica presents remarkable facilities for the inves- 

 tigation 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 con- 

 ceivable grade of value, from varieties up to genera and families. 



