OP CONCHOLOGY. 199 



come islands, and that islands by emergence become conti- 

 nents, does not affect the foregoing reasoning, because such 

 changes require an amount of time exceeding one geological 

 period, during which time there is change of faunce. 



"Such insular faunte as have been described in § 7 and § 13, 

 prove that the islands which they inhabit have been geographi- 

 cally separate since an era anterior to the introduction of the 

 existing species. But this conclusion does not depend on the 

 assumption that the species would have dispersed themselves 

 over several islands if they had not always been restrained 

 by water ; but on the fact that such small zoological provinces 

 exist nowhere on continents. 



" The occurrence of a very few identical species on different 

 islands, or on islands and the mainland, does not prove the 

 union of such land since the existence of the species, because 

 such a distribution may have been caused by the accidents of 

 dispersion, or by independent creation. 



"The occurrence of many species common to different 

 islands, in some parts of Polynesia, renders it probable that 

 such islands have constituted one island since the existence of 

 these species, 



"The frequent occurrence of analogues and the prevalence 

 of generic or subgeneric types in some islands do not prove 

 the former union of these islands, but may indicate their 

 greater proximity at a former period." 



Professor Adams, in a paper from which we have already 

 quoted, ("Contrib.," p. 50, 1849,) remarked: — 



" With this extremely local distribation 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 conti- 

 nents. Coincident with these two general facts in the West 

 Indies is also a third, — that their coral reefs are all fringing, 

 and that coral lagoon islands are wanting." 



It remains for us to notice the last, and, indeed, the most 

 complete and valuable work published by Professor Adams — • 

 his "Catalogue of Shells Collected at Panama." 



Professor Adams sailed from New York on the 13th of No- 

 vember, 1850, arrived at Panama on the 26th, and left it on 

 the 4th of January, 1851, having had scarcely more than five 

 weeks for making his collections; yet they amounted "to 38,920 

 specimens of 376 species of Gasteropoda, 2,860 specimens of 

 139 species of Acephala, and 50 specimens of 1 species of 



