CHAP, i.] INTRODUCTION. 39 



and yet the geographical distribution of most of the 

 shallow- water species is well defined, and frequently 

 somewhat restricted. Unfortunately we know as yet 

 very little about the general distribution of marine 

 animals. Except along the coasts of Britain and 

 Scandinavia, a part of the North American coast, and 

 a part of the Mediterranean, we know absolutely 

 nothing beyond the shore zone, or at all events beyond 

 10 or 15 fathoms. What little we do know is con- 

 fined almost entirely to the mollusca, and is due, not 

 so much to scientific research as to the commercial 

 value which the acquisitive zeal of conchologists has 

 placed upon rare shells. It may be supposed, how- 

 ever, that the same laws which regulate the distri- 

 bution of littoral and sub-littoral mollusca, affect in 

 like manner that of shallow-water annulosa, echino- 

 derms, and ccelent crates ; indeed, from the scattered 

 observations which have been made on the distribu- 

 tion of these latter groups, it seems certain that such 

 is the case. 



Woodward 1 regarded the marine mollusca as occupy- 

 ing eighteen well-defined ' provinces,' fulfilling more 

 or less completely the condition of having at least one- 

 half of the species peculiar to the province. Edward 

 Porbes defined twenty-five such 'regions;' but it must 

 be remembered that in both cases at least three- fourths 

 of the number of areas defined were based upon the 

 most imperfect knowledge of the larger and more con- 

 spicuous shore shells only. It has been constantly 

 observed in the few cases confined entirely to the 

 shores of the North Atlantic and the Mediterranean, 



1 A Manual of the Mollusca. By S. P. Woodward. London, 1851. 

 P. 354. 



