118 LAND REGIONS. 



of latitude. Thomson has arrived at the same conclusion ; 

 which appears probable from the number of identical abyssal 

 species alread}^ obtained from far distant localities. 



That mollusks are able to subsist through a great range of 

 depth may be shown by the five species obtained by the 

 Porcupine at 4451 metres : — 



Dentalium candidum^ 149-2010-3200 m. 



Dacrydiwm vitreum, 54-502'7 m. 



Syndosmya nitida, 1-200 m. 



Neaera obesa, 36-13 m. 



Pecten fenestratus^ 91-451 m. 

 Terehratula Wyvillei lives between 1891 and 5300 metres. 

 Mr. W. H. Dall, from the study of the extensive dredgings 

 made by the " Blake " in the Grulf of Mexico (Bull. Mus. Comp. 

 Zool., vi, 1880), has been able to full}^ illustrate the range of 

 bathymetrical distribution ; the material studied embracing 

 nearly 500 species belonging to not quite 100 genera, and obtained 

 from depths varying from a few fathoms to 1920 fathoms. Man}^ 

 species range from 30 to over 800 fathoms ; a fact of extensive 

 distribution which had never been so clearly indicated before, 

 as most of the deep-sea expeditions omitted carrying their inves- 

 tigations continuously from the abyssal into the littoral regions. 

 Mr. Dall finds that, whilst many species have a limited vertical 

 range, a fair proportion extend from littoral to abyssal ; of these 

 about ten per cent, (in the Gulf of Mexico) belong to boreal or 

 cold-water forms. 



LAND REGIONS. 



Distribution of Land, and Fresh-ivater Shells. 



The boundaries of the ISTatural-History land-regions are more 

 distinctly marked, and have been more fully investigated, than 

 their counterparts in the sea. Almost every large island has its 

 own fauna and fiora ; almost every river system its peculiar 

 fresh-water fish and shells ; and mountain-cha^is like the Andes 

 appear to present impassable barriers to the " nations " of 

 animals and plants of either side. Exceptions, however, occur 

 which show that beyond this first generalization there exists a 

 higher law. The British Channel is not a barrier between two 

 provinces, nor is the Mediterranean ; and the Desert of Sahara- 

 separates only two portions of the same zoological region. In 

 these and other similar instances the " barrier " is of later date 

 than the surrounding fauna and flora. 



The specialization of island faunas is one of the most aston- 

 ishing facts connected with this subject. Almost every large 

 island, when surrounded by deep water, has its peculiar types of 



